use clean::AttributesExt;

use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_hir as hir;
use rustc_hir::def::CtorKind;
use rustc_hir::def_id::DefId;
use rustc_middle::middle::stability;
use rustc_middle::span_bug;
use rustc_middle::ty::layout::LayoutError;
use rustc_middle::ty::{Adt, TyCtxt};
use rustc_span::hygiene::MacroKind;
use rustc_span::symbol::{kw, sym, Symbol};
use rustc_target::abi::{Layout, Primitive, TagEncoding, Variants};
use std::cmp::Ordering;
use std::fmt;
use std::rc::Rc;

use super::{
    collect_paths_for_type, document, ensure_trailing_slash, item_ty_to_section,
    notable_traits_decl, render_assoc_item, render_assoc_items, render_attributes_in_code,
    render_attributes_in_pre, render_impl, render_stability_since_raw, write_srclink,
    AssocItemLink, Context, ImplRenderingParameters,
};
use crate::clean;
use crate::config::ModuleSorting;
use crate::formats::item_type::ItemType;
use crate::formats::{AssocItemRender, Impl, RenderMode};
use crate::html::escape::Escape;
use crate::html::format::{
    join_with_double_colon, print_abi_with_space, print_constness_with_space, print_where_clause,
    Buffer, Ending, PrintWithSpace,
};
use crate::html::highlight;
use crate::html::layout::Page;
use crate::html::markdown::{HeadingOffset, MarkdownSummaryLine};
use crate::html::url_parts_builder::UrlPartsBuilder;

use askama::Template;
use itertools::Itertools;

const ITEM_TABLE_OPEN: &str = "<div class=\"item-table\">";
const ITEM_TABLE_CLOSE: &str = "</div>";
const ITEM_TABLE_ROW_OPEN: &str = "<div class=\"item-row\">";
const ITEM_TABLE_ROW_CLOSE: &str = "</div>";

// A component in a `use` path, like `string` in std::string::ToString
struct PathComponent {
    path: String,
    name: Symbol,
}

#[derive(Template)]
#[template(path = "print_item.html")]
struct ItemVars<'a> {
    page: &'a Page<'a>,
    static_root_path: &'a str,
    typ: &'a str,
    name: &'a str,
    item_type: &'a str,
    path_components: Vec<PathComponent>,
    stability_since_raw: &'a str,
    src_href: Option<&'a str>,
}

/// Calls `print_where_clause` and returns `true` if a `where` clause was generated.
fn print_where_clause_and_check<'a, 'tcx: 'a>(
    buffer: &mut Buffer,
    gens: &'a clean::Generics,
    cx: &'a Context<'tcx>,
) -> bool {
    let len_before = buffer.len();
    write!(buffer, "{}", print_where_clause(gens, cx, 0, Ending::Newline));
    len_before != buffer.len()
}

pub(super) fn print_item(
    cx: &mut Context<'_>,
    item: &clean::Item,
    buf: &mut Buffer,
    page: &Page<'_>,
) {
    debug_assert!(!item.is_stripped());
    let typ = match *item.kind {
        clean::ModuleItem(_) => {
            if item.is_crate() {
                "Crate "
            } else {
                "Module "
            }
        }
        clean::FunctionItem(..) | clean::ForeignFunctionItem(..) => "Function ",
        clean::TraitItem(..) => "Trait ",
        clean::StructItem(..) => "Struct ",
        clean::UnionItem(..) => "Union ",
        clean::EnumItem(..) => "Enum ",
        clean::TypedefItem(..) => "Type Definition ",
        clean::MacroItem(..) => "Macro ",
        clean::ProcMacroItem(ref mac) => match mac.kind {
            MacroKind::Bang => "Macro ",
            MacroKind::Attr => "Attribute Macro ",
            MacroKind::Derive => "Derive Macro ",
        },
        clean::PrimitiveItem(..) => "Primitive Type ",
        clean::StaticItem(..) | clean::ForeignStaticItem(..) => "Static ",
        clean::ConstantItem(..) => "Constant ",
        clean::ForeignTypeItem => "Foreign Type ",
        clean::KeywordItem => "Keyword ",
        clean::OpaqueTyItem(..) => "Opaque Type ",
        clean::TraitAliasItem(..) => "Trait Alias ",
        _ => {
            // We don't generate pages for any other type.
            unreachable!();
        }
    };
    let mut stability_since_raw = Buffer::new();
    render_stability_since_raw(
        &mut stability_since_raw,
        item.stable_since(cx.tcx()),
        item.const_stability(cx.tcx()),
        None,
        None,
    );
    let stability_since_raw: String = stability_since_raw.into_inner();

    // Write source tag
    //
    // When this item is part of a `crate use` in a downstream crate, the
    // source link in the downstream documentation will actually come back to
    // this page, and this link will be auto-clicked. The `id` attribute is
    // used to find the link to auto-click.
    let src_href =
        if cx.include_sources && !item.is_primitive() { cx.src_href(item) } else { None };

    let path_components = if item.is_primitive() || item.is_keyword() {
        vec![]
    } else {
        let cur = &cx.current;
        let amt = if item.is_mod() { cur.len() - 1 } else { cur.len() };
        cur.iter()
            .enumerate()
            .take(amt)
            .map(|(i, component)| PathComponent {
                path: "../".repeat(cur.len() - i - 1),
                name: *component,
            })
            .collect()
    };

    let item_vars = ItemVars {
        page,
        static_root_path: page.get_static_root_path(),
        typ,
        name: item.name.as_ref().unwrap().as_str(),
        item_type: &item.type_().to_string(),
        path_components,
        stability_since_raw: &stability_since_raw,
        src_href: src_href.as_deref(),
    };

    item_vars.render_into(buf).unwrap();

    match &*item.kind {
        clean::ModuleItem(ref m) => item_module(buf, cx, item, &m.items),
        clean::FunctionItem(ref f) | clean::ForeignFunctionItem(ref f) => {
            item_function(buf, cx, item, f)
        }
        clean::TraitItem(ref t) => item_trait(buf, cx, item, t),
        clean::StructItem(ref s) => item_struct(buf, cx, item, s),
        clean::UnionItem(ref s) => item_union(buf, cx, item, s),
        clean::EnumItem(ref e) => item_enum(buf, cx, item, e),
        clean::TypedefItem(ref t) => item_typedef(buf, cx, item, t),
        clean::MacroItem(ref m) => item_macro(buf, cx, item, m),
        clean::ProcMacroItem(ref m) => item_proc_macro(buf, cx, item, m),
        clean::PrimitiveItem(_) => item_primitive(buf, cx, item),
        clean::StaticItem(ref i) | clean::ForeignStaticItem(ref i) => item_static(buf, cx, item, i),
        clean::ConstantItem(ref c) => item_constant(buf, cx, item, c),
        clean::ForeignTypeItem => item_foreign_type(buf, cx, item),
        clean::KeywordItem => item_keyword(buf, cx, item),
        clean::OpaqueTyItem(ref e) => item_opaque_ty(buf, cx, item, e),
        clean::TraitAliasItem(ref ta) => item_trait_alias(buf, cx, item, ta),
        _ => {
            // We don't generate pages for any other type.
            unreachable!();
        }
    }
}

/// For large structs, enums, unions, etc, determine whether to hide their fields
fn should_hide_fields(n_fields: usize) -> bool {
    n_fields > 12
}

fn toggle_open(w: &mut Buffer, text: impl fmt::Display) {
    write!(
        w,
        "<details class=\"rustdoc-toggle type-contents-toggle\">\
            <summary class=\"hideme\">\
                <span>Show {}</span>\
            </summary>",
        text
    );
}

fn toggle_close(w: &mut Buffer) {
    w.write_str("</details>");
}

fn item_module(w: &mut Buffer, cx: &mut Context<'_>, item: &clean::Item, items: &[clean::Item]) {
    document(w, cx, item, None, HeadingOffset::H2);

    let mut indices = (0..items.len()).filter(|i| !items[*i].is_stripped()).collect::<Vec<usize>>();

    // the order of item types in the listing
    fn reorder(ty: ItemType) -> u8 {
        match ty {
            ItemType::ExternCrate => 0,
            ItemType::Import => 1,
            ItemType::Primitive => 2,
            ItemType::Module => 3,
            ItemType::Macro => 4,
            ItemType::Struct => 5,
            ItemType::Enum => 6,
            ItemType::Constant => 7,
            ItemType::Static => 8,
            ItemType::Trait => 9,
            ItemType::Function => 10,
            ItemType::Typedef => 12,
            ItemType::Union => 13,
            _ => 14 + ty as u8,
        }
    }

    fn cmp(
        i1: &clean::Item,
        i2: &clean::Item,
        idx1: usize,
        idx2: usize,
        tcx: TyCtxt<'_>,
    ) -> Ordering {
        let ty1 = i1.type_();
        let ty2 = i2.type_();
        if item_ty_to_section(ty1) != item_ty_to_section(ty2)
            || (ty1 != ty2 && (ty1 == ItemType::ExternCrate || ty2 == ItemType::ExternCrate))
        {
            return (reorder(ty1), idx1).cmp(&(reorder(ty2), idx2));
        }
        let s1 = i1.stability(tcx).as_ref().map(|s| s.level);
        let s2 = i2.stability(tcx).as_ref().map(|s| s.level);
        if let (Some(a), Some(b)) = (s1, s2) {
            match (a.is_stable(), b.is_stable()) {
                (true, true) | (false, false) => {}
                (false, true) => return Ordering::Less,
                (true, false) => return Ordering::Greater,
            }
        }
        let lhs = i1.name.unwrap_or(kw::Empty);
        let rhs = i2.name.unwrap_or(kw::Empty);
        compare_names(lhs.as_str(), rhs.as_str())
    }

    match cx.shared.module_sorting {
        ModuleSorting::Alphabetical => {
            indices.sort_by(|&i1, &i2| cmp(&items[i1], &items[i2], i1, i2, cx.tcx()));
        }
        ModuleSorting::DeclarationOrder => {}
    }
    // This call is to remove re-export duplicates in cases such as:
    //
    // ```
    // pub(crate) mod foo {
    //     pub(crate) mod bar {
    //         pub(crate) trait Double { fn foo(); }
    //     }
    // }
    //
    // pub(crate) use foo::bar::*;
    // pub(crate) use foo::*;
    // ```
    //
    // `Double` will appear twice in the generated docs.
    //
    // FIXME: This code is quite ugly and could be improved. Small issue: DefId
    // can be identical even if the elements are different (mostly in imports).
    // So in case this is an import, we keep everything by adding a "unique id"
    // (which is the position in the vector).
    indices.dedup_by_key(|i| {
        (
            items[*i].item_id,
            if items[*i].name.is_some() { Some(full_path(cx, &items[*i])) } else { None },
            items[*i].type_(),
            if items[*i].is_import() { *i } else { 0 },
        )
    });

    debug!("{:?}", indices);
    let mut last_section = None;

    for &idx in &indices {
        let myitem = &items[idx];
        if myitem.is_stripped() {
            continue;
        }

        let my_section = item_ty_to_section(myitem.type_());
        if Some(my_section) != last_section {
            if last_section.is_some() {
                w.write_str(ITEM_TABLE_CLOSE);
            }
            last_section = Some(my_section);
            write!(
                w,
                "<h2 id=\"{id}\" class=\"small-section-header\">\
                    <a href=\"#{id}\">{name}</a>\
                 </h2>{}",
                ITEM_TABLE_OPEN,
                id = cx.derive_id(my_section.id().to_owned()),
                name = my_section.name(),
            );
        }

        match *myitem.kind {
            clean::ExternCrateItem { ref src } => {
                use crate::html::format::anchor;

                w.write_str(ITEM_TABLE_ROW_OPEN);
                match *src {
                    Some(src) => write!(
                        w,
                        "<div class=\"item-left\"><code>{}extern crate {} as {};",
                        myitem.visibility.print_with_space(myitem.item_id, cx),
                        anchor(myitem.item_id.expect_def_id(), src, cx),
                        myitem.name.unwrap(),
                    ),
                    None => write!(
                        w,
                        "<div class=\"item-left\"><code>{}extern crate {};",
                        myitem.visibility.print_with_space(myitem.item_id, cx),
                        anchor(myitem.item_id.expect_def_id(), myitem.name.unwrap(), cx),
                    ),
                }
                w.write_str("</code></div>");
                w.write_str(ITEM_TABLE_ROW_CLOSE);
            }

            clean::ImportItem(ref import) => {
                let (stab, stab_tags) = if let Some(import_def_id) = import.source.did {
                    let ast_attrs = cx.tcx().get_attrs_unchecked(import_def_id);
                    let import_attrs = Box::new(clean::Attributes::from_ast(ast_attrs));

                    // Just need an item with the correct def_id and attrs
                    let import_item = clean::Item {
                        item_id: import_def_id.into(),
                        attrs: import_attrs,
                        cfg: ast_attrs.cfg(cx.tcx(), &cx.cache().hidden_cfg),
                        ..myitem.clone()
                    };

                    let stab = import_item.stability_class(cx.tcx());
                    let stab_tags = Some(extra_info_tags(&import_item, item, cx.tcx()));
                    (stab, stab_tags)
                } else {
                    (None, None)
                };

                let add = if stab.is_some() { " " } else { "" };

                w.write_str(ITEM_TABLE_ROW_OPEN);
                let id = match import.kind {
                    clean::ImportKind::Simple(s) => {
                        format!(" id=\"{}\"", cx.derive_id(format!("reexport.{}", s)))
                    }
                    clean::ImportKind::Glob => String::new(),
                };
                write!(
                    w,
                    "<div class=\"item-left {stab}{add}import-item\"{id}>\
                         <code>{vis}{imp}</code>\
                     </div>\
                     <div class=\"item-right docblock-short\">{stab_tags}</div>",
                    stab = stab.unwrap_or_default(),
                    vis = myitem.visibility.print_with_space(myitem.item_id, cx),
                    imp = import.print(cx),
                    stab_tags = stab_tags.unwrap_or_default(),
                );
                w.write_str(ITEM_TABLE_ROW_CLOSE);
            }

            _ => {
                if myitem.name.is_none() {
                    continue;
                }

                let unsafety_flag = match *myitem.kind {
                    clean::FunctionItem(_) | clean::ForeignFunctionItem(_)
                        if myitem.fn_header(cx.tcx()).unwrap().unsafety
                            == hir::Unsafety::Unsafe =>
                    {
                        "<a title=\"unsafe function\" href=\"#\"><sup>⚠</sup></a>"
                    }
                    _ => "",
                };

                let stab = myitem.stability_class(cx.tcx());
                let add = if stab.is_some() { " " } else { "" };

                let visibility_emoji = match myitem.visibility {
                    clean::Visibility::Restricted(_) => {
                        "<span title=\"Restricted Visibility\">&nbsp;🔒</span> "
                    }
                    _ => "",
                };

                let doc_value = myitem.doc_value().unwrap_or_default();
                w.write_str(ITEM_TABLE_ROW_OPEN);
                write!(
                    w,
                    "<div class=\"item-left {stab}{add}module-item\">\
                        <a class=\"{class}\" href=\"{href}\" title=\"{title}\">{name}</a>\
                        {visibility_emoji}\
                        {unsafety_flag}\
                        {stab_tags}\
                     </div>\
                     <div class=\"item-right docblock-short\">{docs}</div>",
                    name = myitem.name.unwrap(),
                    visibility_emoji = visibility_emoji,
                    stab_tags = extra_info_tags(myitem, item, cx.tcx()),
                    docs = MarkdownSummaryLine(&doc_value, &myitem.links(cx)).into_string(),
                    class = myitem.type_(),
                    add = add,
                    stab = stab.unwrap_or_default(),
                    unsafety_flag = unsafety_flag,
                    href = item_path(myitem.type_(), myitem.name.unwrap().as_str()),
                    title = [full_path(cx, myitem), myitem.type_().to_string()]
                        .iter()
                        .filter_map(|s| if !s.is_empty() { Some(s.as_str()) } else { None })
                        .collect::<Vec<_>>()
                        .join(" "),
                );
                w.write_str(ITEM_TABLE_ROW_CLOSE);
            }
        }
    }

    if last_section.is_some() {
        w.write_str(ITEM_TABLE_CLOSE);
    }
}

/// Render the stability, deprecation and portability tags that are displayed in the item's summary
/// at the module level.
fn extra_info_tags(item: &clean::Item, parent: &clean::Item, tcx: TyCtxt<'_>) -> String {
    let mut tags = String::new();

    fn tag_html(class: &str, title: &str, contents: &str) -> String {
        format!(r#"<span class="stab {}" title="{}">{}</span>"#, class, Escape(title), contents)
    }

    // The trailing space after each tag is to space it properly against the rest of the docs.
    if let Some(depr) = &item.deprecation(tcx) {
        let mut message = "Deprecated";
        if !stability::deprecation_in_effect(depr) {
            message = "Deprecation planned";
        }
        tags += &tag_html("deprecated", "", message);
    }

    // The "rustc_private" crates are permanently unstable so it makes no sense
    // to render "unstable" everywhere.
    if item.stability(tcx).as_ref().map(|s| s.is_unstable() && s.feature != sym::rustc_private)
        == Some(true)
    {
        tags += &tag_html("unstable", "", "Experimental");
    }

    let cfg = match (&item.cfg, parent.cfg.as_ref()) {
        (Some(cfg), Some(parent_cfg)) => cfg.simplify_with(parent_cfg),
        (cfg, _) => cfg.as_deref().cloned(),
    };

    debug!("Portability {:?} - {:?} = {:?}", item.cfg, parent.cfg, cfg);
    if let Some(ref cfg) = cfg {
        tags += &tag_html("portability", &cfg.render_long_plain(), &cfg.render_short_html());
    }

    tags
}

fn item_function(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, f: &clean::Function) {
    let header = it.fn_header(cx.tcx()).expect("printing a function which isn't a function");
    let constness = print_constness_with_space(&header.constness, it.const_stability(cx.tcx()));
    let unsafety = header.unsafety.print_with_space();
    let abi = print_abi_with_space(header.abi).to_string();
    let asyncness = header.asyncness.print_with_space();
    let visibility = it.visibility.print_with_space(it.item_id, cx).to_string();
    let name = it.name.unwrap();

    let generics_len = format!("{:#}", f.generics.print(cx)).len();
    let header_len = "fn ".len()
        + visibility.len()
        + constness.len()
        + asyncness.len()
        + unsafety.len()
        + abi.len()
        + name.as_str().len()
        + generics_len;

    wrap_into_docblock(w, |w| {
        wrap_item(w, "fn", |w| {
            render_attributes_in_pre(w, it, "");
            w.reserve(header_len);
            write!(
                w,
                "{vis}{constness}{asyncness}{unsafety}{abi}fn \
                 {name}{generics}{decl}{notable_traits}{where_clause}",
                vis = visibility,
                constness = constness,
                asyncness = asyncness,
                unsafety = unsafety,
                abi = abi,
                name = name,
                generics = f.generics.print(cx),
                where_clause = print_where_clause(&f.generics, cx, 0, Ending::Newline),
                decl = f.decl.full_print(header_len, 0, header.asyncness, cx),
                notable_traits = notable_traits_decl(&f.decl, cx),
            );
        });
    });
    document(w, cx, it, None, HeadingOffset::H2)
}

fn item_trait(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, t: &clean::Trait) {
    let bounds = bounds(&t.bounds, false, cx);
    let required_types = t.items.iter().filter(|m| m.is_ty_associated_type()).collect::<Vec<_>>();
    let provided_types = t.items.iter().filter(|m| m.is_associated_type()).collect::<Vec<_>>();
    let required_consts = t.items.iter().filter(|m| m.is_ty_associated_const()).collect::<Vec<_>>();
    let provided_consts = t.items.iter().filter(|m| m.is_associated_const()).collect::<Vec<_>>();
    let required_methods = t.items.iter().filter(|m| m.is_ty_method()).collect::<Vec<_>>();
    let provided_methods = t.items.iter().filter(|m| m.is_method()).collect::<Vec<_>>();
    let count_types = required_types.len() + provided_types.len();
    let count_consts = required_consts.len() + provided_consts.len();
    let count_methods = required_methods.len() + provided_methods.len();
    let must_implement_one_of_functions =
        cx.tcx().trait_def(t.def_id).must_implement_one_of.clone();

    // Output the trait definition
    wrap_into_docblock(w, |w| {
        wrap_item(w, "trait", |w| {
            render_attributes_in_pre(w, it, "");
            write!(
                w,
                "{}{}{}trait {}{}{}",
                it.visibility.print_with_space(it.item_id, cx),
                t.unsafety(cx.tcx()).print_with_space(),
                if t.is_auto(cx.tcx()) { "auto " } else { "" },
                it.name.unwrap(),
                t.generics.print(cx),
                bounds
            );

            if !t.generics.where_predicates.is_empty() {
                write!(w, "{}", print_where_clause(&t.generics, cx, 0, Ending::Newline));
            } else {
                w.write_str(" ");
            }

            if t.items.is_empty() {
                w.write_str("{ }");
            } else {
                // FIXME: we should be using a derived_id for the Anchors here
                w.write_str("{\n");
                let mut toggle = false;

                // If there are too many associated types, hide _everything_
                if should_hide_fields(count_types) {
                    toggle = true;
                    toggle_open(
                        w,
                        format_args!(
                            "{} associated items",
                            count_types + count_consts + count_methods
                        ),
                    );
                }
                for types in [&required_types, &provided_types] {
                    for t in types {
                        render_assoc_item(
                            w,
                            t,
                            AssocItemLink::Anchor(None),
                            ItemType::Trait,
                            cx,
                            RenderMode::Normal,
                        );
                        w.write_str(";\n");
                    }
                }
                // If there are too many associated constants, hide everything after them
                // We also do this if the types + consts is large because otherwise we could
                // render a bunch of types and _then_ a bunch of consts just because both were
                // _just_ under the limit
                if !toggle && should_hide_fields(count_types + count_consts) {
                    toggle = true;
                    toggle_open(
                        w,
                        format_args!(
                            "{} associated constant{} and {} method{}",
                            count_consts,
                            pluralize(count_consts),
                            count_methods,
                            pluralize(count_methods),
                        ),
                    );
                }
                if count_types != 0 && (count_consts != 0 || count_methods != 0) {
                    w.write_str("\n");
                }
                for consts in [&required_consts, &provided_consts] {
                    for c in consts {
                        render_assoc_item(
                            w,
                            c,
                            AssocItemLink::Anchor(None),
                            ItemType::Trait,
                            cx,
                            RenderMode::Normal,
                        );
                        w.write_str(";\n");
                    }
                }
                if !toggle && should_hide_fields(count_methods) {
                    toggle = true;
                    toggle_open(w, format_args!("{} methods", count_methods));
                }
                if count_consts != 0 && count_methods != 0 {
                    w.write_str("\n");
                }
                for (pos, m) in required_methods.iter().enumerate() {
                    render_assoc_item(
                        w,
                        m,
                        AssocItemLink::Anchor(None),
                        ItemType::Trait,
                        cx,
                        RenderMode::Normal,
                    );
                    w.write_str(";\n");

                    if pos < required_methods.len() - 1 {
                        w.write_str("<span class=\"item-spacer\"></span>");
                    }
                }
                if !required_methods.is_empty() && !provided_methods.is_empty() {
                    w.write_str("\n");
                }
                for (pos, m) in provided_methods.iter().enumerate() {
                    render_assoc_item(
                        w,
                        m,
                        AssocItemLink::Anchor(None),
                        ItemType::Trait,
                        cx,
                        RenderMode::Normal,
                    );
                    match *m.kind {
                        clean::MethodItem(ref inner, _)
                            if !inner.generics.where_predicates.is_empty() =>
                        {
                            w.write_str(",\n    { ... }\n");
                        }
                        _ => {
                            w.write_str(" { ... }\n");
                        }
                    }

                    if pos < provided_methods.len() - 1 {
                        w.write_str("<span class=\"item-spacer\"></span>");
                    }
                }
                if toggle {
                    toggle_close(w);
                }
                w.write_str("}");
            }
        });
    });

    // Trait documentation
    document(w, cx, it, None, HeadingOffset::H2);

    fn write_small_section_header(w: &mut Buffer, id: &str, title: &str, extra_content: &str) {
        write!(
            w,
            "<h2 id=\"{0}\" class=\"small-section-header\">\
                {1}<a href=\"#{0}\" class=\"anchor\"></a>\
             </h2>{2}",
            id, title, extra_content
        )
    }

    fn trait_item(w: &mut Buffer, cx: &mut Context<'_>, m: &clean::Item, t: &clean::Item) {
        let name = m.name.unwrap();
        info!("Documenting {} on {:?}", name, t.name);
        let item_type = m.type_();
        let id = cx.derive_id(format!("{}.{}", item_type, name));
        let mut content = Buffer::empty_from(w);
        document(&mut content, cx, m, Some(t), HeadingOffset::H5);
        let toggled = !content.is_empty();
        if toggled {
            write!(w, "<details class=\"rustdoc-toggle\" open><summary>");
        }
        write!(w, "<div id=\"{}\" class=\"method has-srclink\">", id);
        write!(w, "<div class=\"rightside\">");

        let has_stability = render_stability_since(w, m, t, cx.tcx());
        if has_stability {
            w.write_str(" · ");
        }
        write_srclink(cx, m, w);
        write!(w, "</div>");
        write!(w, "<h4 class=\"code-header\">");
        render_assoc_item(
            w,
            m,
            AssocItemLink::Anchor(Some(&id)),
            ItemType::Impl,
            cx,
            RenderMode::Normal,
        );
        w.write_str("</h4>");
        w.write_str("</div>");
        if toggled {
            write!(w, "</summary>");
            w.push_buffer(content);
            write!(w, "</details>");
        }
    }

    if !required_types.is_empty() {
        write_small_section_header(
            w,
            "required-associated-types",
            "Required Associated Types",
            "<div class=\"methods\">",
        );
        for t in required_types {
            trait_item(w, cx, t, it);
        }
        w.write_str("</div>");
    }
    if !provided_types.is_empty() {
        write_small_section_header(
            w,
            "provided-associated-types",
            "Provided Associated Types",
            "<div class=\"methods\">",
        );
        for t in provided_types {
            trait_item(w, cx, t, it);
        }
        w.write_str("</div>");
    }

    if !required_consts.is_empty() {
        write_small_section_header(
            w,
            "required-associated-consts",
            "Required Associated Constants",
            "<div class=\"methods\">",
        );
        for t in required_consts {
            trait_item(w, cx, t, it);
        }
        w.write_str("</div>");
    }
    if !provided_consts.is_empty() {
        write_small_section_header(
            w,
            "provided-associated-consts",
            "Provided Associated Constants",
            "<div class=\"methods\">",
        );
        for t in provided_consts {
            trait_item(w, cx, t, it);
        }
        w.write_str("</div>");
    }

    // Output the documentation for each function individually
    if !required_methods.is_empty() || must_implement_one_of_functions.is_some() {
        write_small_section_header(
            w,
            "required-methods",
            "Required Methods",
            "<div class=\"methods\">",
        );

        if let Some(list) = must_implement_one_of_functions.as_deref() {
            write!(
                w,
                "<div class=\"stab must_implement\">At least one of the `{}` methods is required.</div>",
                list.iter().join("`, `")
            );
        }

        for m in required_methods {
            trait_item(w, cx, m, it);
        }
        w.write_str("</div>");
    }
    if !provided_methods.is_empty() {
        write_small_section_header(
            w,
            "provided-methods",
            "Provided Methods",
            "<div class=\"methods\">",
        );
        for m in provided_methods {
            trait_item(w, cx, m, it);
        }
        w.write_str("</div>");
    }

    // If there are methods directly on this trait object, render them here.
    render_assoc_items(w, cx, it, it.item_id.expect_def_id(), AssocItemRender::All);

    let cloned_shared = Rc::clone(&cx.shared);
    let cache = &cloned_shared.cache;
    let mut extern_crates = FxHashSet::default();
    if let Some(implementors) = cache.implementors.get(&it.item_id.expect_def_id()) {
        // The DefId is for the first Type found with that name. The bool is
        // if any Types with the same name but different DefId have been found.
        let mut implementor_dups: FxHashMap<Symbol, (DefId, bool)> = FxHashMap::default();
        for implementor in implementors {
            if let Some(did) = implementor.inner_impl().for_.without_borrowed_ref().def_id(cache) &&
                !did.is_local() {
                extern_crates.insert(did.krate);
            }
            match implementor.inner_impl().for_.without_borrowed_ref() {
                clean::Type::Path { ref path } if !path.is_assoc_ty() => {
                    let did = path.def_id();
                    let &mut (prev_did, ref mut has_duplicates) =
                        implementor_dups.entry(path.last()).or_insert((did, false));
                    if prev_did != did {
                        *has_duplicates = true;
                    }
                }
                _ => {}
            }
        }

        let (local, foreign) =
            implementors.iter().partition::<Vec<_>, _>(|i| i.is_on_local_type(cx));

        let (mut synthetic, mut concrete): (Vec<&&Impl>, Vec<&&Impl>) =
            local.iter().partition(|i| i.inner_impl().kind.is_auto());

        synthetic.sort_by(|a, b| compare_impl(a, b, cx));
        concrete.sort_by(|a, b| compare_impl(a, b, cx));

        if !foreign.is_empty() {
            write_small_section_header(w, "foreign-impls", "Implementations on Foreign Types", "");

            for implementor in foreign {
                let provided_methods = implementor.inner_impl().provided_trait_methods(cx.tcx());
                let assoc_link =
                    AssocItemLink::GotoSource(implementor.impl_item.item_id, &provided_methods);
                render_impl(
                    w,
                    cx,
                    implementor,
                    it,
                    assoc_link,
                    RenderMode::Normal,
                    None,
                    &[],
                    ImplRenderingParameters {
                        show_def_docs: false,
                        show_default_items: false,
                        show_non_assoc_items: true,
                        toggle_open_by_default: false,
                    },
                );
            }
        }

        write_small_section_header(
            w,
            "implementors",
            "Implementors",
            "<div class=\"item-list\" id=\"implementors-list\">",
        );
        for implementor in concrete {
            render_implementor(cx, implementor, it, w, &implementor_dups, &[]);
        }
        w.write_str("</div>");

        if t.is_auto(cx.tcx()) {
            write_small_section_header(
                w,
                "synthetic-implementors",
                "Auto implementors",
                "<div class=\"item-list\" id=\"synthetic-implementors-list\">",
            );
            for implementor in synthetic {
                render_implementor(
                    cx,
                    implementor,
                    it,
                    w,
                    &implementor_dups,
                    &collect_paths_for_type(implementor.inner_impl().for_.clone(), cache),
                );
            }
            w.write_str("</div>");
        }
    } else {
        // even without any implementations to write in, we still want the heading and list, so the
        // implementors javascript file pulled in below has somewhere to write the impls into
        write_small_section_header(
            w,
            "implementors",
            "Implementors",
            "<div class=\"item-list\" id=\"implementors-list\"></div>",
        );

        if t.is_auto(cx.tcx()) {
            write_small_section_header(
                w,
                "synthetic-implementors",
                "Auto implementors",
                "<div class=\"item-list\" id=\"synthetic-implementors-list\"></div>",
            );
        }
    }

    // Include implementors in crates that depend on the current crate.
    //
    // This is complicated by the way rustdoc is invoked, which is basically
    // the same way rustc is invoked: it gets called, one at a time, for each
    // crate. When building the rustdocs for the current crate, rustdoc can
    // see crate metadata for its dependencies, but cannot see metadata for its
    // dependents.
    //
    // To make this work, we generate a "hook" at this stage, and our
    // dependents can "plug in" to it when they build. For simplicity's sake,
    // it's [JSONP]: a JavaScript file with the data we need (and can parse),
    // surrounded by a tiny wrapper that the Rust side ignores, but allows the
    // JavaScript side to include without having to worry about Same Origin
    // Policy. The code for *that* is in `write_shared.rs`.
    //
    // This is further complicated by `#[doc(inline)]`. We want all copies
    // of an inlined trait to reference the same JS file, to address complex
    // dependency graphs like this one (lower crates depend on higher crates):
    //
    // ```text
    //  --------------------------------------------
    //  |            crate A: trait Foo            |
    //  --------------------------------------------
    //      |                               |
    //  --------------------------------    |
    //  | crate B: impl A::Foo for Bar |    |
    //  --------------------------------    |
    //      |                               |
    //  ---------------------------------------------
    //  | crate C: #[doc(inline)] use A::Foo as Baz |
    //  |          impl Baz for Quux                |
    //  ---------------------------------------------
    // ```
    //
    // Basically, we want `C::Baz` and `A::Foo` to show the same set of
    // impls, which is easier if they both treat `/implementors/A/trait.Foo.js`
    // as the Single Source of Truth.
    //
    // We also want the `impl Baz for Quux` to be written to
    // `trait.Foo.js`. However, when we generate plain HTML for `C::Baz`,
    // we're going to want to generate plain HTML for `impl Baz for Quux` too,
    // because that'll load faster, and it's better for SEO. And we don't want
    // the same impl to show up twice on the same page.
    //
    // To make this work, the implementors JS file has a structure kinda
    // like this:
    //
    // ```js
    // JSONP({
    // "B": {"impl A::Foo for Bar"},
    // "C": {"impl Baz for Quux"},
    // });
    // ```
    //
    // First of all, this means we can rebuild a crate, and it'll replace its own
    // data if something changes. That is, `rustdoc` is idempotent. The other
    // advantage is that we can list the crates that get included in the HTML,
    // and ignore them when doing the JavaScript-based part of rendering.
    // So C's HTML will have something like this:
    //
    // ```html
    // <script type="text/javascript" src="/implementors/A/trait.Foo.js"
    //     data-ignore-extern-crates="A,B" async></script>
    // ```
    //
    // And, when the JS runs, anything in data-ignore-extern-crates is known
    // to already be in the HTML, and will be ignored.
    //
    // [JSONP]: https://en.wikipedia.org/wiki/JSONP
    let mut js_src_path: UrlPartsBuilder = std::iter::repeat("..")
        .take(cx.current.len())
        .chain(std::iter::once("implementors"))
        .collect();
    if let Some(did) = it.item_id.as_def_id() &&
        let get_extern = { || cache.external_paths.get(&did).map(|s| s.0.clone()) } &&
        let Some(fqp) = cache.exact_paths.get(&did).cloned().or_else(get_extern) {
        js_src_path.extend(fqp[..fqp.len() - 1].iter().copied());
        js_src_path.push_fmt(format_args!("{}.{}.js", it.type_(), fqp.last().unwrap()));
    } else {
        js_src_path.extend(cx.current.iter().copied());
        js_src_path.push_fmt(format_args!("{}.{}.js", it.type_(), it.name.unwrap()));
    }
    let extern_crates = extern_crates
        .into_iter()
        .map(|cnum| cx.shared.tcx.crate_name(cnum).to_string())
        .collect::<Vec<_>>()
        .join(",");
    write!(
        w,
        "<script type=\"text/javascript\" src=\"{src}\" data-ignore-extern-crates=\"{extern_crates}\" async></script>",
        src = js_src_path.finish(),
    );
}

fn item_trait_alias(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, t: &clean::TraitAlias) {
    wrap_into_docblock(w, |w| {
        wrap_item(w, "trait-alias", |w| {
            render_attributes_in_pre(w, it, "");
            write!(
                w,
                "trait {}{}{} = {};",
                it.name.unwrap(),
                t.generics.print(cx),
                print_where_clause(&t.generics, cx, 0, Ending::Newline),
                bounds(&t.bounds, true, cx)
            );
        });
    });

    document(w, cx, it, None, HeadingOffset::H2);

    // Render any items associated directly to this alias, as otherwise they
    // won't be visible anywhere in the docs. It would be nice to also show
    // associated items from the aliased type (see discussion in #32077), but
    // we need #14072 to make sense of the generics.
    render_assoc_items(w, cx, it, it.item_id.expect_def_id(), AssocItemRender::All)
}

fn item_opaque_ty(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, t: &clean::OpaqueTy) {
    wrap_into_docblock(w, |w| {
        wrap_item(w, "opaque", |w| {
            render_attributes_in_pre(w, it, "");
            write!(
                w,
                "type {}{}{where_clause} = impl {bounds};",
                it.name.unwrap(),
                t.generics.print(cx),
                where_clause = print_where_clause(&t.generics, cx, 0, Ending::Newline),
                bounds = bounds(&t.bounds, false, cx),
            );
        });
    });

    document(w, cx, it, None, HeadingOffset::H2);

    // Render any items associated directly to this alias, as otherwise they
    // won't be visible anywhere in the docs. It would be nice to also show
    // associated items from the aliased type (see discussion in #32077), but
    // we need #14072 to make sense of the generics.
    render_assoc_items(w, cx, it, it.item_id.expect_def_id(), AssocItemRender::All)
}

fn item_typedef(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, t: &clean::Typedef) {
    fn write_content(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, t: &clean::Typedef) {
        wrap_item(w, "typedef", |w| {
            render_attributes_in_pre(w, it, "");
            write!(w, "{}", it.visibility.print_with_space(it.item_id, cx));
            write!(
                w,
                "type {}{}{where_clause} = {type_};",
                it.name.unwrap(),
                t.generics.print(cx),
                where_clause = print_where_clause(&t.generics, cx, 0, Ending::Newline),
                type_ = t.type_.print(cx),
            );
        });
    }

    wrap_into_docblock(w, |w| write_content(w, cx, it, t));

    document(w, cx, it, None, HeadingOffset::H2);

    let def_id = it.item_id.expect_def_id();
    // Render any items associated directly to this alias, as otherwise they
    // won't be visible anywhere in the docs. It would be nice to also show
    // associated items from the aliased type (see discussion in #32077), but
    // we need #14072 to make sense of the generics.
    render_assoc_items(w, cx, it, def_id, AssocItemRender::All);
    document_type_layout(w, cx, def_id);
}

fn item_union(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, s: &clean::Union) {
    wrap_into_docblock(w, |w| {
        wrap_item(w, "union", |w| {
            render_attributes_in_pre(w, it, "");
            render_union(w, it, Some(&s.generics), &s.fields, "", cx);
        });
    });

    document(w, cx, it, None, HeadingOffset::H2);

    let mut fields = s
        .fields
        .iter()
        .filter_map(|f| match *f.kind {
            clean::StructFieldItem(ref ty) => Some((f, ty)),
            _ => None,
        })
        .peekable();
    if fields.peek().is_some() {
        write!(
            w,
            "<h2 id=\"fields\" class=\"fields small-section-header\">\
                Fields<a href=\"#fields\" class=\"anchor\"></a>\
            </h2>"
        );
        for (field, ty) in fields {
            let name = field.name.expect("union field name");
            let id = format!("{}.{}", ItemType::StructField, name);
            write!(
                w,
                "<span id=\"{id}\" class=\"{shortty} small-section-header\">\
                     <a href=\"#{id}\" class=\"anchor field\"></a>\
                     <code>{name}: {ty}</code>\
                 </span>",
                id = id,
                name = name,
                shortty = ItemType::StructField,
                ty = ty.print(cx),
            );
            if let Some(stability_class) = field.stability_class(cx.tcx()) {
                write!(w, "<span class=\"stab {stab}\"></span>", stab = stability_class);
            }
            document(w, cx, field, Some(it), HeadingOffset::H3);
        }
    }
    let def_id = it.item_id.expect_def_id();
    render_assoc_items(w, cx, it, def_id, AssocItemRender::All);
    document_type_layout(w, cx, def_id);
}

fn print_tuple_struct_fields(w: &mut Buffer, cx: &Context<'_>, s: &[clean::Item]) {
    for (i, ty) in s.iter().enumerate() {
        if i > 0 {
            w.write_str(",&nbsp;");
        }
        match *ty.kind {
            clean::StrippedItem(box clean::StructFieldItem(_)) => w.write_str("_"),
            clean::StructFieldItem(ref ty) => write!(w, "{}", ty.print(cx)),
            _ => unreachable!(),
        }
    }
}

fn item_enum(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, e: &clean::Enum) {
    let count_variants = e.variants().count();
    wrap_into_docblock(w, |w| {
        wrap_item(w, "enum", |w| {
            render_attributes_in_pre(w, it, "");
            write!(
                w,
                "{}enum {}{}",
                it.visibility.print_with_space(it.item_id, cx),
                it.name.unwrap(),
                e.generics.print(cx),
            );
            if !print_where_clause_and_check(w, &e.generics, cx) {
                // If there wasn't a `where` clause, we add a whitespace.
                w.write_str(" ");
            }

            let variants_stripped = e.has_stripped_entries();
            if count_variants == 0 && !variants_stripped {
                w.write_str("{}");
            } else {
                w.write_str("{\n");
                let toggle = should_hide_fields(count_variants);
                if toggle {
                    toggle_open(w, format_args!("{} variants", count_variants));
                }
                for v in e.variants() {
                    w.write_str("    ");
                    let name = v.name.unwrap();
                    match *v.kind {
                        clean::VariantItem(ref var) => match var {
                            clean::Variant::CLike => write!(w, "{}", name),
                            clean::Variant::Tuple(ref s) => {
                                write!(w, "{}(", name);
                                print_tuple_struct_fields(w, cx, s);
                                w.write_str(")");
                            }
                            clean::Variant::Struct(ref s) => {
                                render_struct(
                                    w,
                                    v,
                                    None,
                                    s.struct_type,
                                    &s.fields,
                                    "    ",
                                    false,
                                    cx,
                                );
                            }
                        },
                        _ => unreachable!(),
                    }
                    w.write_str(",\n");
                }

                if variants_stripped {
                    w.write_str("    // some variants omitted\n");
                }
                if toggle {
                    toggle_close(w);
                }
                w.write_str("}");
            }
        });
    });

    document(w, cx, it, None, HeadingOffset::H2);

    if count_variants != 0 {
        write!(
            w,
            "<h2 id=\"variants\" class=\"variants small-section-header\">\
                Variants{}<a href=\"#variants\" class=\"anchor\"></a>\
            </h2>",
            document_non_exhaustive_header(it)
        );
        document_non_exhaustive(w, it);
        for variant in e.variants() {
            let id = cx.derive_id(format!("{}.{}", ItemType::Variant, variant.name.unwrap()));
            write!(
                w,
                "<h3 id=\"{id}\" class=\"variant small-section-header\">\
                    <a href=\"#{id}\" class=\"anchor field\"></a>\
                    <code>{name}",
                id = id,
                name = variant.name.unwrap()
            );
            if let clean::VariantItem(clean::Variant::Tuple(ref s)) = *variant.kind {
                w.write_str("(");
                print_tuple_struct_fields(w, cx, s);
                w.write_str(")");
            }
            w.write_str("</code>");
            render_stability_since(w, variant, it, cx.tcx());
            w.write_str("</h3>");

            use crate::clean::Variant;

            let heading_and_fields = match &*variant.kind {
                clean::VariantItem(Variant::Struct(s)) => Some(("Fields", &s.fields)),
                // Documentation on tuple variant fields is rare, so to reduce noise we only emit
                // the section if at least one field is documented.
                clean::VariantItem(Variant::Tuple(fields))
                    if fields.iter().any(|f| f.doc_value().is_some()) =>
                {
                    Some(("Tuple Fields", fields))
                }
                _ => None,
            };

            if let Some((heading, fields)) = heading_and_fields {
                let variant_id =
                    cx.derive_id(format!("{}.{}.fields", ItemType::Variant, variant.name.unwrap()));
                write!(w, "<div class=\"sub-variant\" id=\"{id}\">", id = variant_id);
                write!(w, "<h4>{heading}</h4>", heading = heading);
                document_non_exhaustive(w, variant);
                for field in fields {
                    match *field.kind {
                        clean::StrippedItem(box clean::StructFieldItem(_)) => {}
                        clean::StructFieldItem(ref ty) => {
                            let id = cx.derive_id(format!(
                                "variant.{}.field.{}",
                                variant.name.unwrap(),
                                field.name.unwrap()
                            ));
                            write!(
                                w,
                                "<div class=\"sub-variant-field\">\
                                 <span id=\"{id}\" class=\"variant small-section-header\">\
                                     <a href=\"#{id}\" class=\"anchor field\"></a>\
                                     <code>{f}:&nbsp;{t}</code>\
                                 </span>",
                                id = id,
                                f = field.name.unwrap(),
                                t = ty.print(cx)
                            );
                            document(w, cx, field, Some(variant), HeadingOffset::H5);
                            write!(w, "</div>");
                        }
                        _ => unreachable!(),
                    }
                }
                w.write_str("</div>");
            }

            document(w, cx, variant, Some(it), HeadingOffset::H4);
        }
    }
    let def_id = it.item_id.expect_def_id();
    render_assoc_items(w, cx, it, def_id, AssocItemRender::All);
    document_type_layout(w, cx, def_id);
}

fn item_macro(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, t: &clean::Macro) {
    wrap_into_docblock(w, |w| {
        highlight::render_with_highlighting(
            &t.source,
            w,
            Some("macro"),
            None,
            None,
            it.span(cx.tcx()).inner().edition(),
            None,
            None,
            None,
        );
    });
    document(w, cx, it, None, HeadingOffset::H2)
}

fn item_proc_macro(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, m: &clean::ProcMacro) {
    wrap_into_docblock(w, |w| {
        let name = it.name.expect("proc-macros always have names");
        match m.kind {
            MacroKind::Bang => {
                wrap_item(w, "macro", |w| {
                    write!(w, "{}!() {{ /* proc-macro */ }}", name);
                });
            }
            MacroKind::Attr => {
                wrap_item(w, "attr", |w| {
                    write!(w, "#[{}]", name);
                });
            }
            MacroKind::Derive => {
                wrap_item(w, "derive", |w| {
                    write!(w, "#[derive({})]", name);
                    if !m.helpers.is_empty() {
                        w.push_str("\n{\n");
                        w.push_str("    // Attributes available to this derive:\n");
                        for attr in &m.helpers {
                            writeln!(w, "    #[{}]", attr);
                        }
                        w.push_str("}\n");
                    }
                });
            }
        }
    });
    document(w, cx, it, None, HeadingOffset::H2)
}

fn item_primitive(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item) {
    document(w, cx, it, None, HeadingOffset::H2);
    render_assoc_items(w, cx, it, it.item_id.expect_def_id(), AssocItemRender::All)
}

fn item_constant(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, c: &clean::Constant) {
    wrap_into_docblock(w, |w| {
        wrap_item(w, "const", |w| {
            render_attributes_in_code(w, it);

            write!(
                w,
                "{vis}const {name}: {typ}",
                vis = it.visibility.print_with_space(it.item_id, cx),
                name = it.name.unwrap(),
                typ = c.type_.print(cx),
            );

            // FIXME: The code below now prints
            //            ` = _; // 100i32`
            //        if the expression is
            //            `50 + 50`
            //        which looks just wrong.
            //        Should we print
            //            ` = 100i32;`
            //        instead?

            let value = c.value(cx.tcx());
            let is_literal = c.is_literal(cx.tcx());
            let expr = c.expr(cx.tcx());
            if value.is_some() || is_literal {
                write!(w, " = {expr};", expr = Escape(&expr));
            } else {
                w.write_str(";");
            }

            if !is_literal {
                if let Some(value) = &value {
                    let value_lowercase = value.to_lowercase();
                    let expr_lowercase = expr.to_lowercase();

                    if value_lowercase != expr_lowercase
                        && value_lowercase.trim_end_matches("i32") != expr_lowercase
                    {
                        write!(w, " // {value}", value = Escape(value));
                    }
                }
            }
        });
    });

    document(w, cx, it, None, HeadingOffset::H2)
}

fn item_struct(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, s: &clean::Struct) {
    wrap_into_docblock(w, |w| {
        wrap_item(w, "struct", |w| {
            render_attributes_in_code(w, it);
            render_struct(w, it, Some(&s.generics), s.struct_type, &s.fields, "", true, cx);
        });
    });

    document(w, cx, it, None, HeadingOffset::H2);

    let mut fields = s
        .fields
        .iter()
        .filter_map(|f| match *f.kind {
            clean::StructFieldItem(ref ty) => Some((f, ty)),
            _ => None,
        })
        .peekable();
    if let CtorKind::Fictive | CtorKind::Fn = s.struct_type {
        if fields.peek().is_some() {
            write!(
                w,
                "<h2 id=\"fields\" class=\"fields small-section-header\">\
                     {}{}<a href=\"#fields\" class=\"anchor\"></a>\
                 </h2>",
                if let CtorKind::Fictive = s.struct_type { "Fields" } else { "Tuple Fields" },
                document_non_exhaustive_header(it)
            );
            document_non_exhaustive(w, it);
            for (index, (field, ty)) in fields.enumerate() {
                let field_name =
                    field.name.map_or_else(|| index.to_string(), |sym| sym.as_str().to_string());
                let id = cx.derive_id(format!("{}.{}", ItemType::StructField, field_name));
                write!(
                    w,
                    "<span id=\"{id}\" class=\"{item_type} small-section-header\">\
                         <a href=\"#{id}\" class=\"anchor field\"></a>\
                         <code>{name}: {ty}</code>\
                     </span>",
                    item_type = ItemType::StructField,
                    id = id,
                    name = field_name,
                    ty = ty.print(cx)
                );
                document(w, cx, field, Some(it), HeadingOffset::H3);
            }
        }
    }
    let def_id = it.item_id.expect_def_id();
    render_assoc_items(w, cx, it, def_id, AssocItemRender::All);
    document_type_layout(w, cx, def_id);
}

fn item_static(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, s: &clean::Static) {
    wrap_into_docblock(w, |w| {
        wrap_item(w, "static", |w| {
            render_attributes_in_code(w, it);
            write!(
                w,
                "{vis}static {mutability}{name}: {typ}",
                vis = it.visibility.print_with_space(it.item_id, cx),
                mutability = s.mutability.print_with_space(),
                name = it.name.unwrap(),
                typ = s.type_.print(cx)
            );
        });
    });
    document(w, cx, it, None, HeadingOffset::H2)
}

fn item_foreign_type(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item) {
    wrap_into_docblock(w, |w| {
        wrap_item(w, "foreigntype", |w| {
            w.write_str("extern {\n");
            render_attributes_in_code(w, it);
            write!(
                w,
                "    {}type {};\n}}",
                it.visibility.print_with_space(it.item_id, cx),
                it.name.unwrap(),
            );
        });
    });

    document(w, cx, it, None, HeadingOffset::H2);

    render_assoc_items(w, cx, it, it.item_id.expect_def_id(), AssocItemRender::All)
}

fn item_keyword(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item) {
    document(w, cx, it, None, HeadingOffset::H2)
}

/// Compare two strings treating multi-digit numbers as single units (i.e. natural sort order).
pub(crate) fn compare_names(mut lhs: &str, mut rhs: &str) -> Ordering {
    /// Takes a non-numeric and a numeric part from the given &str.
    fn take_parts<'a>(s: &mut &'a str) -> (&'a str, &'a str) {
        let i = s.find(|c: char| c.is_ascii_digit());
        let (a, b) = s.split_at(i.unwrap_or(s.len()));
        let i = b.find(|c: char| !c.is_ascii_digit());
        let (b, c) = b.split_at(i.unwrap_or(b.len()));
        *s = c;
        (a, b)
    }

    while !lhs.is_empty() || !rhs.is_empty() {
        let (la, lb) = take_parts(&mut lhs);
        let (ra, rb) = take_parts(&mut rhs);
        // First process the non-numeric part.
        match la.cmp(ra) {
            Ordering::Equal => (),
            x => return x,
        }
        // Then process the numeric part, if both sides have one (and they fit in a u64).
        if let (Ok(ln), Ok(rn)) = (lb.parse::<u64>(), rb.parse::<u64>()) {
            match ln.cmp(&rn) {
                Ordering::Equal => (),
                x => return x,
            }
        }
        // Then process the numeric part again, but this time as strings.
        match lb.cmp(rb) {
            Ordering::Equal => (),
            x => return x,
        }
    }

    Ordering::Equal
}

pub(super) fn full_path(cx: &Context<'_>, item: &clean::Item) -> String {
    let mut s = join_with_double_colon(&cx.current);
    s.push_str("::");
    s.push_str(item.name.unwrap().as_str());
    s
}

pub(super) fn item_path(ty: ItemType, name: &str) -> String {
    match ty {
        ItemType::Module => format!("{}index.html", ensure_trailing_slash(name)),
        _ => format!("{}.{}.html", ty, name),
    }
}

fn bounds(t_bounds: &[clean::GenericBound], trait_alias: bool, cx: &Context<'_>) -> String {
    let mut bounds = String::new();
    if !t_bounds.is_empty() {
        if !trait_alias {
            bounds.push_str(": ");
        }
        for (i, p) in t_bounds.iter().enumerate() {
            if i > 0 {
                bounds.push_str(" + ");
            }
            bounds.push_str(&p.print(cx).to_string());
        }
    }
    bounds
}

fn wrap_into_docblock<F>(w: &mut Buffer, f: F)
where
    F: FnOnce(&mut Buffer),
{
    w.write_str("<div class=\"docblock item-decl\">");
    f(w);
    w.write_str("</div>")
}

fn wrap_item<F>(w: &mut Buffer, item_name: &str, f: F)
where
    F: FnOnce(&mut Buffer),
{
    w.write_fmt(format_args!("<pre class=\"rust {}\"><code>", item_name));
    f(w);
    w.write_str("</code></pre>");
}

fn render_stability_since(
    w: &mut Buffer,
    item: &clean::Item,
    containing_item: &clean::Item,
    tcx: TyCtxt<'_>,
) -> bool {
    render_stability_since_raw(
        w,
        item.stable_since(tcx),
        item.const_stability(tcx),
        containing_item.stable_since(tcx),
        containing_item.const_stable_since(tcx),
    )
}

fn compare_impl<'a, 'b>(lhs: &'a &&Impl, rhs: &'b &&Impl, cx: &Context<'_>) -> Ordering {
    let lhss = format!("{}", lhs.inner_impl().print(false, cx));
    let rhss = format!("{}", rhs.inner_impl().print(false, cx));

    // lhs and rhs are formatted as HTML, which may be unnecessary
    compare_names(&lhss, &rhss)
}

fn render_implementor(
    cx: &mut Context<'_>,
    implementor: &Impl,
    trait_: &clean::Item,
    w: &mut Buffer,
    implementor_dups: &FxHashMap<Symbol, (DefId, bool)>,
    aliases: &[String],
) {
    // If there's already another implementor that has the same abridged name, use the
    // full path, for example in `std::iter::ExactSizeIterator`
    let use_absolute = match implementor.inner_impl().for_ {
        clean::Type::Path { ref path, .. }
        | clean::BorrowedRef { type_: box clean::Type::Path { ref path, .. }, .. }
            if !path.is_assoc_ty() =>
        {
            implementor_dups[&path.last()].1
        }
        _ => false,
    };
    render_impl(
        w,
        cx,
        implementor,
        trait_,
        AssocItemLink::Anchor(None),
        RenderMode::Normal,
        Some(use_absolute),
        aliases,
        ImplRenderingParameters {
            show_def_docs: false,
            show_default_items: false,
            show_non_assoc_items: false,
            toggle_open_by_default: false,
        },
    );
}

fn render_union(
    w: &mut Buffer,
    it: &clean::Item,
    g: Option<&clean::Generics>,
    fields: &[clean::Item],
    tab: &str,
    cx: &Context<'_>,
) {
    write!(w, "{}union {}", it.visibility.print_with_space(it.item_id, cx), it.name.unwrap(),);

    let where_displayed = g
        .map(|g| {
            write!(w, "{}", g.print(cx));
            print_where_clause_and_check(w, g, cx)
        })
        .unwrap_or(false);

    // If there wasn't a `where` clause, we add a whitespace.
    if !where_displayed {
        w.write_str(" ");
    }

    write!(w, "{{\n{}", tab);
    let count_fields =
        fields.iter().filter(|f| matches!(*f.kind, clean::StructFieldItem(..))).count();
    let toggle = should_hide_fields(count_fields);
    if toggle {
        toggle_open(w, format_args!("{} fields", count_fields));
    }

    for field in fields {
        if let clean::StructFieldItem(ref ty) = *field.kind {
            write!(
                w,
                "    {}{}: {},\n{}",
                field.visibility.print_with_space(field.item_id, cx),
                field.name.unwrap(),
                ty.print(cx),
                tab
            );
        }
    }

    if it.has_stripped_entries().unwrap() {
        write!(w, "    /* private fields */\n{}", tab);
    }
    if toggle {
        toggle_close(w);
    }
    w.write_str("}");
}

fn render_struct(
    w: &mut Buffer,
    it: &clean::Item,
    g: Option<&clean::Generics>,
    ty: CtorKind,
    fields: &[clean::Item],
    tab: &str,
    structhead: bool,
    cx: &Context<'_>,
) {
    write!(
        w,
        "{}{}{}",
        it.visibility.print_with_space(it.item_id, cx),
        if structhead { "struct " } else { "" },
        it.name.unwrap()
    );
    if let Some(g) = g {
        write!(w, "{}", g.print(cx))
    }
    match ty {
        CtorKind::Fictive => {
            let where_diplayed = g.map(|g| print_where_clause_and_check(w, g, cx)).unwrap_or(false);

            // If there wasn't a `where` clause, we add a whitespace.
            if !where_diplayed {
                w.write_str(" {");
            } else {
                w.write_str("{");
            }
            let count_fields =
                fields.iter().filter(|f| matches!(*f.kind, clean::StructFieldItem(..))).count();
            let has_visible_fields = count_fields > 0;
            let toggle = should_hide_fields(count_fields);
            if toggle {
                toggle_open(w, format_args!("{} fields", count_fields));
            }
            for field in fields {
                if let clean::StructFieldItem(ref ty) = *field.kind {
                    write!(
                        w,
                        "\n{}    {}{}: {},",
                        tab,
                        field.visibility.print_with_space(field.item_id, cx),
                        field.name.unwrap(),
                        ty.print(cx),
                    );
                }
            }

            if has_visible_fields {
                if it.has_stripped_entries().unwrap() {
                    write!(w, "\n{}    /* private fields */", tab);
                }
                write!(w, "\n{}", tab);
            } else if it.has_stripped_entries().unwrap() {
                write!(w, " /* private fields */ ");
            }
            if toggle {
                toggle_close(w);
            }
            w.write_str("}");
        }
        CtorKind::Fn => {
            w.write_str("(");
            for (i, field) in fields.iter().enumerate() {
                if i > 0 {
                    w.write_str(", ");
                }
                match *field.kind {
                    clean::StrippedItem(box clean::StructFieldItem(..)) => write!(w, "_"),
                    clean::StructFieldItem(ref ty) => {
                        write!(
                            w,
                            "{}{}",
                            field.visibility.print_with_space(field.item_id, cx),
                            ty.print(cx),
                        )
                    }
                    _ => unreachable!(),
                }
            }
            w.write_str(")");
            if let Some(g) = g {
                write!(w, "{}", print_where_clause(g, cx, 0, Ending::NoNewline));
            }
            // We only want a ";" when we are displaying a tuple struct, not a variant tuple struct.
            if structhead {
                w.write_str(";");
            }
        }
        CtorKind::Const => {
            // Needed for PhantomData.
            if let Some(g) = g {
                write!(w, "{}", print_where_clause(g, cx, 0, Ending::NoNewline));
            }
            w.write_str(";");
        }
    }
}

fn document_non_exhaustive_header(item: &clean::Item) -> &str {
    if item.is_non_exhaustive() { " (Non-exhaustive)" } else { "" }
}

fn document_non_exhaustive(w: &mut Buffer, item: &clean::Item) {
    if item.is_non_exhaustive() {
        write!(
            w,
            "<details class=\"rustdoc-toggle non-exhaustive\">\
                 <summary class=\"hideme\"><span>{}</span></summary>\
                 <div class=\"docblock\">",
            {
                if item.is_struct() {
                    "This struct is marked as non-exhaustive"
                } else if item.is_enum() {
                    "This enum is marked as non-exhaustive"
                } else if item.is_variant() {
                    "This variant is marked as non-exhaustive"
                } else {
                    "This type is marked as non-exhaustive"
                }
            }
        );

        if item.is_struct() {
            w.write_str(
                "Non-exhaustive structs could have additional fields added in future. \
                 Therefore, non-exhaustive structs cannot be constructed in external crates \
                 using the traditional <code>Struct { .. }</code> syntax; cannot be \
                 matched against without a wildcard <code>..</code>; and \
                 struct update syntax will not work.",
            );
        } else if item.is_enum() {
            w.write_str(
                "Non-exhaustive enums could have additional variants added in future. \
                 Therefore, when matching against variants of non-exhaustive enums, an \
                 extra wildcard arm must be added to account for any future variants.",
            );
        } else if item.is_variant() {
            w.write_str(
                "Non-exhaustive enum variants could have additional fields added in future. \
                 Therefore, non-exhaustive enum variants cannot be constructed in external \
                 crates and cannot be matched against.",
            );
        } else {
            w.write_str(
                "This type will require a wildcard arm in any match statements or constructors.",
            );
        }

        w.write_str("</div></details>");
    }
}

fn document_type_layout(w: &mut Buffer, cx: &Context<'_>, ty_def_id: DefId) {
    fn write_size_of_layout(w: &mut Buffer, layout: Layout<'_>, tag_size: u64) {
        if layout.abi().is_unsized() {
            write!(w, "(unsized)");
        } else {
            let bytes = layout.size().bytes() - tag_size;
            write!(w, "{size} byte{pl}", size = bytes, pl = if bytes == 1 { "" } else { "s" },);
        }
    }

    if !cx.shared.show_type_layout {
        return;
    }

    writeln!(
        w,
        "<h2 id=\"layout\" class=\"small-section-header\"> \
        Layout<a href=\"#layout\" class=\"anchor\"></a></h2>"
    );
    writeln!(w, "<div class=\"docblock\">");

    let tcx = cx.tcx();
    let param_env = tcx.param_env(ty_def_id);
    let ty = tcx.type_of(ty_def_id);
    match tcx.layout_of(param_env.and(ty)) {
        Ok(ty_layout) => {
            writeln!(
                w,
                "<div class=\"warning\"><p><strong>Note:</strong> Most layout information is \
                 <strong>completely unstable</strong> and may even differ between compilations. \
                 The only exception is types with certain <code>repr(...)</code> attributes. \
                 Please see the Rust Reference’s \
                 <a href=\"https://doc.rust-lang.org/reference/type-layout.html\">“Type Layout”</a> \
                 chapter for details on type layout guarantees.</p></div>"
            );
            w.write_str("<p><strong>Size:</strong> ");
            write_size_of_layout(w, ty_layout.layout, 0);
            writeln!(w, "</p>");
            if let Variants::Multiple { variants, tag, tag_encoding, .. } =
                &ty_layout.layout.variants()
            {
                if !variants.is_empty() {
                    w.write_str(
                        "<p><strong>Size for each variant:</strong></p>\
                            <ul>",
                    );

                    let Adt(adt, _) = ty_layout.ty.kind() else {
                        span_bug!(tcx.def_span(ty_def_id), "not an adt")
                    };

                    let tag_size = if let TagEncoding::Niche { .. } = tag_encoding {
                        0
                    } else if let Primitive::Int(i, _) = tag.primitive() {
                        i.size().bytes()
                    } else {
                        span_bug!(tcx.def_span(ty_def_id), "tag is neither niche nor int")
                    };

                    for (index, layout) in variants.iter_enumerated() {
                        let name = adt.variant(index).name;
                        write!(w, "<li><code>{name}</code>: ", name = name);
                        write_size_of_layout(w, *layout, tag_size);
                        writeln!(w, "</li>");
                    }
                    w.write_str("</ul>");
                }
            }
        }
        // This kind of layout error can occur with valid code, e.g. if you try to
        // get the layout of a generic type such as `Vec<T>`.
        Err(LayoutError::Unknown(_)) => {
            writeln!(
                w,
                "<p><strong>Note:</strong> Unable to compute type layout, \
                 possibly due to this type having generic parameters. \
                 Layout can only be computed for concrete, fully-instantiated types.</p>"
            );
        }
        // This kind of error probably can't happen with valid code, but we don't
        // want to panic and prevent the docs from building, so we just let the
        // user know that we couldn't compute the layout.
        Err(LayoutError::SizeOverflow(_)) => {
            writeln!(
                w,
                "<p><strong>Note:</strong> Encountered an error during type layout; \
                 the type was too big.</p>"
            );
        }
        Err(LayoutError::NormalizationFailure(_, _)) => {
            writeln!(
                w,
                "<p><strong>Note:</strong> Encountered an error during type layout; \
                the type failed to be normalized.</p>"
            )
        }
    }

    writeln!(w, "</div>");
}

fn pluralize(count: usize) -> &'static str {
    if count > 1 { "s" } else { "" }
}
