Auto merge of #81398 - bugadani:rustdoc-perf, r=GuillaumeGomez

rustdoc tweaking

* Reuse memory
* simplify `next_def_id`, avoid multiple hashing and unnecessary lookups
* remove `all_fake_def_ids`, use the global map instead (probably not a good step toward parallelization, though...)
* convert `add_deref_target` to iterative implementation
* use `ArrayVec` where we know the max number of elements
* minor touchups here and there
* avoid building temporary vectors that get appended to other vectors

At most places I may or may not be doing the compiler's job is this PR.

Cargo.lock

@@ -4386,6 +4386,7 @@ dependencies = [
 name = "rustdoc"
 version = "0.0.0"
 dependencies = [
+ "arrayvec",
  "expect-test",
  "itertools 0.9.0",
  "minifier",

src/librustdoc/Cargo.toml

@@ -8,6 +8,7 @@ edition = "2018"
 path = "lib.rs"
 
 [dependencies]
+arrayvec = { version = "0.5.1", default-features = false }
 pulldown-cmark = { version = "0.8", default-features = false }
 minifier = "0.0.33"
 rayon = { version = "0.3.0", package = "rustc-rayon" }

src/librustdoc/clean/inline.rs

@@ -56,7 +56,7 @@ crate fn try_inline(
     let kind = match res {
         Res::Def(DefKind::Trait, did) => {
             record_extern_fqn(cx, did, clean::TypeKind::Trait);
-            ret.extend(build_impls(cx, Some(parent_module), did, attrs));
+            build_impls(cx, Some(parent_module), did, attrs, &mut ret);
             clean::TraitItem(build_external_trait(cx, did))
         }
         Res::Def(DefKind::Fn, did) => {
@@ -65,27 +65,27 @@ crate fn try_inline(
         }
         Res::Def(DefKind::Struct, did) => {
             record_extern_fqn(cx, did, clean::TypeKind::Struct);
-            ret.extend(build_impls(cx, Some(parent_module), did, attrs));
+            build_impls(cx, Some(parent_module), did, attrs, &mut ret);
             clean::StructItem(build_struct(cx, did))
         }
         Res::Def(DefKind::Union, did) => {
             record_extern_fqn(cx, did, clean::TypeKind::Union);
-            ret.extend(build_impls(cx, Some(parent_module), did, attrs));
+            build_impls(cx, Some(parent_module), did, attrs, &mut ret);
             clean::UnionItem(build_union(cx, did))
         }
         Res::Def(DefKind::TyAlias, did) => {
             record_extern_fqn(cx, did, clean::TypeKind::Typedef);
-            ret.extend(build_impls(cx, Some(parent_module), did, attrs));
+            build_impls(cx, Some(parent_module), did, attrs, &mut ret);
             clean::TypedefItem(build_type_alias(cx, did), false)
         }
         Res::Def(DefKind::Enum, did) => {
             record_extern_fqn(cx, did, clean::TypeKind::Enum);
-            ret.extend(build_impls(cx, Some(parent_module), did, attrs));
+            build_impls(cx, Some(parent_module), did, attrs, &mut ret);
             clean::EnumItem(build_enum(cx, did))
         }
         Res::Def(DefKind::ForeignTy, did) => {
             record_extern_fqn(cx, did, clean::TypeKind::Foreign);
-            ret.extend(build_impls(cx, Some(parent_module), did, attrs));
+            build_impls(cx, Some(parent_module), did, attrs, &mut ret);
             clean::ForeignTypeItem
         }
         // Never inline enum variants but leave them shown as re-exports.
@@ -133,10 +133,7 @@ crate fn try_inline_glob(
     res: Res,
     visited: &mut FxHashSet<DefId>,
 ) -> Option<Vec<clean::Item>> {
-    if res == Res::Err {
-        return None;
-    }
-    let did = res.def_id();
+    let did = res.opt_def_id()?;
     if did.is_local() {
         return None;
     }
@@ -280,16 +277,14 @@ crate fn build_impls(
     parent_module: Option<DefId>,
     did: DefId,
     attrs: Option<Attrs<'_>>,
-) -> Vec<clean::Item> {
+    ret: &mut Vec<clean::Item>,
+) {
     let tcx = cx.tcx;
-    let mut impls = Vec::new();
 
     // for each implementation of an item represented by `did`, build the clean::Item for that impl
     for &did in tcx.inherent_impls(did).iter() {
-        build_impl(cx, parent_module, did, attrs, &mut impls);
+        build_impl(cx, parent_module, did, attrs, ret);
     }
-
-    impls
 }
 
 /// `parent_module` refers to the parent of the re-export, not the original item

src/librustdoc/clean/types.rs

@@ -8,6 +8,7 @@ use std::rc::Rc;
 use std::sync::Arc;
 use std::{slice, vec};
 
+use arrayvec::ArrayVec;
 use rustc_ast::attr;
 use rustc_ast::util::comments::beautify_doc_string;
 use rustc_ast::{self as ast, AttrStyle};
@@ -16,7 +17,7 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_feature::UnstableFeatures;
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, Res};
-use rustc_hir::def_id::{CrateNum, DefId};
+use rustc_hir::def_id::{CrateNum, DefId, DefIndex};
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::Mutability;
 use rustc_index::vec::IndexVec;
@@ -28,7 +29,6 @@ use rustc_span::symbol::{kw, sym, Ident, Symbol, SymbolStr};
 use rustc_span::{self, FileName, Loc};
 use rustc_target::abi::VariantIdx;
 use rustc_target::spec::abi::Abi;
-use smallvec::{smallvec, SmallVec};
 
 use crate::clean::cfg::Cfg;
 use crate::clean::external_path;
@@ -45,7 +45,7 @@ use self::ItemKind::*;
 use self::SelfTy::*;
 use self::Type::*;
 
-thread_local!(crate static MAX_DEF_ID: RefCell<FxHashMap<CrateNum, DefId>> = Default::default());
+thread_local!(crate static MAX_DEF_IDX: RefCell<FxHashMap<CrateNum, DefIndex>> = Default::default());
 
 #[derive(Clone, Debug)]
 crate struct Crate {
@@ -293,8 +293,8 @@ impl Item {
     ///
     /// [`next_def_id()`]: DocContext::next_def_id()
     crate fn is_fake(&self) -> bool {
-        MAX_DEF_ID.with(|m| {
-            m.borrow().get(&self.def_id.krate).map(|id| self.def_id >= *id).unwrap_or(false)
+        MAX_DEF_IDX.with(|m| {
+            m.borrow().get(&self.def_id.krate).map(|&idx| idx <= self.def_id.index).unwrap_or(false)
         })
     }
 }
@@ -1539,12 +1539,12 @@ impl PrimitiveType {
         }
     }
 
-    crate fn impls(&self, tcx: TyCtxt<'_>) -> &'static SmallVec<[DefId; 4]> {
+    crate fn impls(&self, tcx: TyCtxt<'_>) -> &'static ArrayVec<[DefId; 4]> {
         Self::all_impls(tcx).get(self).expect("missing impl for primitive type")
     }
 
-    crate fn all_impls(tcx: TyCtxt<'_>) -> &'static FxHashMap<PrimitiveType, SmallVec<[DefId; 4]>> {
-        static CELL: OnceCell<FxHashMap<PrimitiveType, SmallVec<[DefId; 4]>>> = OnceCell::new();
+    crate fn all_impls(tcx: TyCtxt<'_>) -> &'static FxHashMap<PrimitiveType, ArrayVec<[DefId; 4]>> {
+        static CELL: OnceCell<FxHashMap<PrimitiveType, ArrayVec<[DefId; 4]>>> = OnceCell::new();
 
         CELL.get_or_init(move || {
             use self::PrimitiveType::*;
@@ -1568,7 +1568,7 @@ impl PrimitiveType {
             }
 
             let single = |a: Option<DefId>| a.into_iter().collect();
-            let both = |a: Option<DefId>, b: Option<DefId>| -> SmallVec<_> {
+            let both = |a: Option<DefId>, b: Option<DefId>| -> ArrayVec<_> {
                 a.into_iter().chain(b).collect()
             };
 
@@ -1601,8 +1601,8 @@ impl PrimitiveType {
                         .collect()
                 },
                 Array => single(lang_items.array_impl()),
-                Tuple => smallvec![],
-                Unit => smallvec![],
+                Tuple => ArrayVec::new(),
+                Unit => ArrayVec::new(),
                 RawPointer => {
                     lang_items
                         .const_ptr_impl()
@@ -1612,9 +1612,9 @@ impl PrimitiveType {
                         .chain(lang_items.mut_slice_ptr_impl())
                         .collect()
                 },
-                Reference => smallvec![],
-                Fn => smallvec![],
-                Never => smallvec![],
+                Reference => ArrayVec::new(),
+                Fn => ArrayVec::new(),
+                Never => ArrayVec::new(),
             }
         })
     }

src/librustdoc/clean/utils.rs

@@ -322,21 +322,15 @@ crate fn build_deref_target_impls(cx: &DocContext<'_>, items: &[Item], ret: &mut
             ItemKind::TypedefItem(ref t, true) => &t.type_,
             _ => continue,
         };
-        let primitive = match *target {
-            ResolvedPath { did, .. } if did.is_local() => continue,
-            ResolvedPath { did, .. } => {
-                ret.extend(inline::build_impls(cx, None, did, None));
-                continue;
-            }
-            _ => match target.primitive_type() {
-                Some(prim) => prim,
-                None => continue,
-            },
-        };
-        for &did in primitive.impls(tcx) {
-            if !did.is_local() {
+
+        if let Some(prim) = target.primitive_type() {
+            for &did in prim.impls(tcx).iter().filter(|did| !did.is_local()) {
                 inline::build_impl(cx, None, did, None, ret);
             }
+        } else if let ResolvedPath { did, .. } = *target {
+            if !did.is_local() {
+                inline::build_impls(cx, None, did, None, ret);
+            }
         }
     }
 }

src/librustdoc/config.rs

@@ -474,7 +474,7 @@ impl Options {
         };
 
         let mut id_map = html::markdown::IdMap::new();
-        id_map.populate(html::render::initial_ids());
+        id_map.populate(&html::render::INITIAL_IDS);
         let external_html = match ExternalHtml::load(
             &matches.opt_strs("html-in-header"),
             &matches.opt_strs("html-before-content"),

src/librustdoc/core.rs

@@ -24,12 +24,15 @@ use rustc_span::source_map;
 use rustc_span::symbol::sym;
 use rustc_span::DUMMY_SP;
 
-use std::cell::{Cell, RefCell};
 use std::mem;
 use std::rc::Rc;
+use std::{
+    cell::{Cell, RefCell},
+    collections::hash_map::Entry,
+};
 
 use crate::clean;
-use crate::clean::{AttributesExt, MAX_DEF_ID};
+use crate::clean::{AttributesExt, MAX_DEF_IDX};
 use crate::config::{Options as RustdocOptions, RenderOptions};
 use crate::config::{OutputFormat, RenderInfo};
 use crate::formats::cache::Cache;
@@ -63,8 +66,7 @@ crate struct DocContext<'tcx> {
     crate ct_substs: RefCell<FxHashMap<DefId, clean::Constant>>,
     /// Table synthetic type parameter for `impl Trait` in argument position -> bounds
     crate impl_trait_bounds: RefCell<FxHashMap<ImplTraitParam, Vec<clean::GenericBound>>>,
-    crate fake_def_ids: RefCell<FxHashMap<CrateNum, DefId>>,
-    crate all_fake_def_ids: RefCell<FxHashSet<DefId>>,
+    crate fake_def_ids: RefCell<FxHashMap<CrateNum, DefIndex>>,
     /// Auto-trait or blanket impls processed so far, as `(self_ty, trait_def_id)`.
     // FIXME(eddyb) make this a `ty::TraitRef<'tcx>` set.
     crate generated_synthetics: RefCell<FxHashSet<(Ty<'tcx>, DefId)>>,
@@ -138,37 +140,38 @@ impl<'tcx> DocContext<'tcx> {
     /// [`Debug`]: std::fmt::Debug
     /// [`clean::Item`]: crate::clean::types::Item
     crate fn next_def_id(&self, crate_num: CrateNum) -> DefId {
-        let start_def_id = {
-            let num_def_ids = if crate_num == LOCAL_CRATE {
-                self.tcx.hir().definitions().def_path_table().num_def_ids()
-            } else {
-                self.enter_resolver(|r| r.cstore().num_def_ids(crate_num))
-            };
-
-            DefId { krate: crate_num, index: DefIndex::from_usize(num_def_ids) }
-        };
-
         let mut fake_ids = self.fake_def_ids.borrow_mut();
 
-        let def_id = *fake_ids.entry(crate_num).or_insert(start_def_id);
-        fake_ids.insert(
-            crate_num,
-            DefId { krate: crate_num, index: DefIndex::from(def_id.index.index() + 1) },
-        );
+        let def_index = match fake_ids.entry(crate_num) {
+            Entry::Vacant(e) => {
+                let num_def_idx = {
+                    let num_def_idx = if crate_num == LOCAL_CRATE {
+                        self.tcx.hir().definitions().def_path_table().num_def_ids()
+                    } else {
+                        self.enter_resolver(|r| r.cstore().num_def_ids(crate_num))
+                    };
 
-        MAX_DEF_ID.with(|m| {
-            m.borrow_mut().entry(def_id.krate).or_insert(start_def_id);
-        });
+                    DefIndex::from_usize(num_def_idx)
+                };
 
-        self.all_fake_def_ids.borrow_mut().insert(def_id);
+                MAX_DEF_IDX.with(|m| {
+                    m.borrow_mut().insert(crate_num, num_def_idx);
+                });
+                e.insert(num_def_idx)
+            }
+            Entry::Occupied(e) => e.into_mut(),
+        };
+        *def_index = DefIndex::from(*def_index + 1);
 
-        def_id
+        DefId { krate: crate_num, index: *def_index }
     }
 
     /// Like `hir().local_def_id_to_hir_id()`, but skips calling it on fake DefIds.
     /// (This avoids a slice-index-out-of-bounds panic.)
     crate fn as_local_hir_id(&self, def_id: DefId) -> Option<HirId> {
-        if self.all_fake_def_ids.borrow().contains(&def_id) {
+        if MAX_DEF_IDX.with(|m| {
+            m.borrow().get(&def_id.krate).map(|&idx| idx <= def_id.index).unwrap_or(false)
+        }) {
             None
         } else {
             def_id.as_local().map(|def_id| self.tcx.hir().local_def_id_to_hir_id(def_id))
@@ -517,7 +520,6 @@ crate fn run_global_ctxt(
         ct_substs: Default::default(),
         impl_trait_bounds: Default::default(),
         fake_def_ids: Default::default(),
-        all_fake_def_ids: Default::default(),
         generated_synthetics: Default::default(),
         auto_traits: tcx
             .all_traits(LOCAL_CRATE)

src/librustdoc/formats/item_type.rs

@@ -158,6 +158,6 @@ impl ItemType {
 
 impl fmt::Display for ItemType {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{}", self.as_str())
+        f.write_str(self.as_str())
     }
 }

src/librustdoc/html/escape.rs

@@ -16,23 +16,20 @@ impl<'a> fmt::Display for Escape<'a> {
         let Escape(s) = *self;
         let pile_o_bits = s;
         let mut last = 0;
-        for (i, ch) in s.bytes().enumerate() {
-            match ch as char {
-                '<' | '>' | '&' | '\'' | '"' => {
-                    fmt.write_str(&pile_o_bits[last..i])?;
-                    let s = match ch as char {
-                        '>' => "&gt;",
-                        '<' => "&lt;",
-                        '&' => "&amp;",
-                        '\'' => "&#39;",
-                        '"' => "&quot;",
-                        _ => unreachable!(),
-                    };
-                    fmt.write_str(s)?;
-                    last = i + 1;
-                }
-                _ => {}
-            }
+        for (i, ch) in s.char_indices() {
+            let s = match ch {
+                '>' => "&gt;",
+                '<' => "&lt;",
+                '&' => "&amp;",
+                '\'' => "&#39;",
+                '"' => "&quot;",
+                _ => continue,
+            };
+            fmt.write_str(&pile_o_bits[last..i])?;
+            fmt.write_str(s)?;
+            // NOTE: we only expect single byte characters here - which is fine as long as we
+            // only match single byte characters
+            last = i + 1;
         }
 
         if last < s.len() {

src/librustdoc/html/highlight.rs

@@ -7,7 +7,7 @@
 
 use crate::html::escape::Escape;
 
-use std::fmt::{Display, Write};
+use std::fmt::Display;
 use std::iter::Peekable;
 
 use rustc_lexer::{LiteralKind, TokenKind};
@@ -15,16 +15,18 @@ use rustc_span::edition::Edition;
 use rustc_span::symbol::Symbol;
 use rustc_span::with_default_session_globals;
 
+use super::format::Buffer;
+
 /// Highlights `src`, returning the HTML output.
 crate fn render_with_highlighting(
-    src: String,
+    src: &str,
+    out: &mut Buffer,
     class: Option<&str>,
     playground_button: Option<&str>,
     tooltip: Option<(Option<Edition>, &str)>,
     edition: Edition,
-) -> String {
+) {
     debug!("highlighting: ================\n{}\n==============", src);
-    let mut out = String::with_capacity(src.len());
     if let Some((edition_info, class)) = tooltip {
         write!(
             out,
@@ -35,23 +37,19 @@ crate fn render_with_highlighting(
             } else {
                 String::new()
             },
-        )
-        .unwrap();
+        );
     }
 
-    write_header(&mut out, class);
-    write_code(&mut out, &src, edition);
-    write_footer(&mut out, playground_button);
-
-    out
+    write_header(out, class);
+    write_code(out, &src, edition);
+    write_footer(out, playground_button);
 }
 
-fn write_header(out: &mut String, class: Option<&str>) {
-    write!(out, "<div class=\"example-wrap\"><pre class=\"rust {}\">\n", class.unwrap_or_default())
-        .unwrap()
+fn write_header(out: &mut Buffer, class: Option<&str>) {
+    write!(out, "<div class=\"example-wrap\"><pre class=\"rust {}\">\n", class.unwrap_or_default());
 }
 
-fn write_code(out: &mut String, src: &str, edition: Edition) {
+fn write_code(out: &mut Buffer, src: &str, edition: Edition) {
     // This replace allows to fix how the code source with DOS backline characters is displayed.
     let src = src.replace("\r\n", "\n");
     Classifier::new(&src, edition).highlight(&mut |highlight| {
@@ -63,8 +61,8 @@ fn write_code(out: &mut String, src: &str, edition: Edition) {
     });
 }
 
-fn write_footer(out: &mut String, playground_button: Option<&str>) {
-    write!(out, "</pre>{}</div>\n", playground_button.unwrap_or_default()).unwrap()
+fn write_footer(out: &mut Buffer, playground_button: Option<&str>) {
+    write!(out, "</pre>{}</div>\n", playground_button.unwrap_or_default());
 }
 
 /// How a span of text is classified. Mostly corresponds to token kinds.
@@ -331,13 +329,13 @@ impl<'a> Classifier<'a> {
 
 /// Called when we start processing a span of text that should be highlighted.
 /// The `Class` argument specifies how it should be highlighted.
-fn enter_span(out: &mut String, klass: Class) {
-    write!(out, "<span class=\"{}\">", klass.as_html()).unwrap()
+fn enter_span(out: &mut Buffer, klass: Class) {
+    write!(out, "<span class=\"{}\">", klass.as_html());
 }
 
 /// Called at the end of a span of highlighted text.
-fn exit_span(out: &mut String) {
-    write!(out, "</span>").unwrap()
+fn exit_span(out: &mut Buffer) {
+    out.write_str("</span>");
 }
 
 /// Called for a span of text. If the text should be highlighted differently
@@ -351,10 +349,10 @@ fn exit_span(out: &mut String) {
 /// ```
 /// The latter can be thought of as a shorthand for the former, which is more
 /// flexible.
-fn string<T: Display>(out: &mut String, text: T, klass: Option<Class>) {
+fn string<T: Display>(out: &mut Buffer, text: T, klass: Option<Class>) {
     match klass {
-        None => write!(out, "{}", text).unwrap(),
-        Some(klass) => write!(out, "<span class=\"{}\">{}</span>", klass.as_html(), text).unwrap(),
+        None => write!(out, "{}", text),
+        Some(klass) => write!(out, "<span class=\"{}\">{}</span>", klass.as_html(), text),
     }
 }
 

src/librustdoc/html/highlight/tests.rs

@@ -1,4 +1,5 @@
 use super::write_code;
+use crate::html::format::Buffer;
 use expect_test::expect_file;
 use rustc_span::edition::Edition;
 
@@ -18,9 +19,9 @@ const STYLE: &str = r#"
 fn test_html_highlighting() {
     let src = include_str!("fixtures/sample.rs");
     let html = {
-        let mut out = String::new();
+        let mut out = Buffer::new();
         write_code(&mut out, src, Edition::Edition2018);
-        format!("{}<pre><code>{}</code></pre>\n", STYLE, out)
+        format!("{}<pre><code>{}</code></pre>\n", STYLE, out.into_inner())
     };
     expect_file!["fixtures/sample.html"].assert_eq(&html);
 }
@@ -30,7 +31,7 @@ fn test_dos_backline() {
     let src = "pub fn foo() {\r\n\
     println!(\"foo\");\r\n\
 }\r\n";
-    let mut html = String::new();
+    let mut html = Buffer::new();
     write_code(&mut html, src, Edition::Edition2018);
-    expect_file!["fixtures/dos_line.html"].assert_eq(&html);
+    expect_file!["fixtures/dos_line.html"].assert_eq(&html.into_inner());
 }

src/librustdoc/html/layout.rs

@@ -114,7 +114,6 @@ crate fn render<T: Print, S: Print>(
     {after_content}\
     <div id=\"rustdoc-vars\" data-root-path=\"{root_path}\" data-current-crate=\"{krate}\"></div>
     <script src=\"{static_root_path}main{suffix}.js\"></script>\
-    {static_extra_scripts}\
     {extra_scripts}\
     <script defer src=\"{root_path}search-index{suffix}.js\"></script>\
 </body>\
@@ -135,22 +134,23 @@ crate fn render<T: Print, S: Print>(
         root_path = page.root_path,
         css_class = page.css_class,
         logo = {
-            let p = format!("{}{}", page.root_path, layout.krate);
-            let p = ensure_trailing_slash(&p);
             if layout.logo.is_empty() {
                 format!(
-                    "<a href='{path}index.html'>\
+                    "<a href='{root}{path}index.html'>\
                      <div class='logo-container rust-logo'>\
                      <img src='{static_root_path}rust-logo{suffix}.png' alt='logo'></div></a>",
-                    path = p,
+                    root = page.root_path,
+                    path = ensure_trailing_slash(&layout.krate),
                     static_root_path = static_root_path,
                     suffix = page.resource_suffix
                 )
             } else {
                 format!(
-                    "<a href='{}index.html'>\
-                     <div class='logo-container'><img src='{}' alt='logo'></div></a>",
-                    p, layout.logo
+                    "<a href='{root}{path}index.html'>\
+                     <div class='logo-container'><img src='{logo}' alt='logo'></div></a>",
+                    root = page.root_path,
+                    path = ensure_trailing_slash(&layout.krate),
+                    logo = layout.logo
                 )
             }
         },
@@ -194,7 +194,7 @@ crate fn render<T: Print, S: Print>(
             ))
             .collect::<String>(),
         suffix = page.resource_suffix,
-        static_extra_scripts = page
+        extra_scripts = page
             .static_extra_scripts
             .iter()
             .map(|e| {
@@ -204,17 +204,13 @@ crate fn render<T: Print, S: Print>(
                     extra_script = e
                 )
             })
-            .collect::<String>(),
-        extra_scripts = page
-            .extra_scripts
-            .iter()
-            .map(|e| {
+            .chain(page.extra_scripts.iter().map(|e| {
                 format!(
                     "<script src=\"{root_path}{extra_script}.js\"></script>",
                     root_path = page.root_path,
                     extra_script = e
                 )
-            })
+            }))
             .collect::<String>(),
         filter_crates = if layout.generate_search_filter {
             "<select id=\"crate-search\">\

src/librustdoc/html/markdown.rs

@@ -41,6 +41,8 @@ use pulldown_cmark::{
     html, BrokenLink, CodeBlockKind, CowStr, Event, LinkType, Options, Parser, Tag,
 };
 
+use super::format::Buffer;
+
 #[cfg(test)]
 mod tests;
 
@@ -235,9 +237,7 @@ impl<'a, I: Iterator<Item = Event<'a>>> Iterator for CodeBlocks<'_, 'a, I> {
         }
         let lines = origtext.lines().filter_map(|l| map_line(l).for_html());
         let text = lines.collect::<Vec<Cow<'_, str>>>().join("\n");
-        // insert newline to clearly separate it from the
-        // previous block so we can shorten the html output
-        let mut s = String::from("\n");
+
         let playground_button = self.playground.as_ref().and_then(|playground| {
             let krate = &playground.crate_name;
             let url = &playground.url;
@@ -298,8 +298,13 @@ impl<'a, I: Iterator<Item = Event<'a>>> Iterator for CodeBlocks<'_, 'a, I> {
             None
         };
 
-        s.push_str(&highlight::render_with_highlighting(
-            text,
+        // insert newline to clearly separate it from the
+        // previous block so we can shorten the html output
+        let mut s = Buffer::new();
+        s.push_str("\n");
+        highlight::render_with_highlighting(
+            &text,
+            &mut s,
             Some(&format!(
                 "rust-example-rendered{}",
                 if let Some((_, class)) = tooltip { format!(" {}", class) } else { String::new() }
@@ -307,8 +312,8 @@ impl<'a, I: Iterator<Item = Event<'a>>> Iterator for CodeBlocks<'_, 'a, I> {
             playground_button.as_deref(),
             tooltip,
             edition,
-        ));
-        Some(Event::Html(s.into()))
+        );
+        Some(Event::Html(s.into_inner().into()))
     }
 }
 
@@ -1332,7 +1337,7 @@ impl IdMap {
         IdMap { map: init_id_map() }
     }
 
-    crate fn populate<I: IntoIterator<Item = String>>(&mut self, ids: I) {
+    crate fn populate<I: IntoIterator<Item = S>, S: AsRef<str> + ToString>(&mut self, ids: I) {
         for id in ids {
             let _ = self.derive(id);
         }
@@ -1342,11 +1347,11 @@ impl IdMap {
         self.map = init_id_map();
     }
 
-    crate fn derive(&mut self, candidate: String) -> String {
-        let id = match self.map.get_mut(&candidate) {
-            None => candidate,
+    crate fn derive<S: AsRef<str> + ToString>(&mut self, candidate: S) -> String {
+        let id = match self.map.get_mut(candidate.as_ref()) {
+            None => candidate.to_string(),
             Some(a) => {
-                let id = format!("{}-{}", candidate, *a);
+                let id = format!("{}-{}", candidate.as_ref(), *a);
                 *a += 1;
                 id
             }

src/librustdoc/html/sources.rs

@@ -86,7 +86,7 @@ impl SourceCollector<'_, '_> {
             return Ok(());
         }
 
-        let mut contents = match fs::read_to_string(&p) {
+        let contents = match fs::read_to_string(&p) {
             Ok(contents) => contents,
             Err(e) => {
                 return Err(Error::new(e, &p));
@@ -94,9 +94,7 @@ impl SourceCollector<'_, '_> {
         };
 
         // Remove the utf-8 BOM if any
-        if contents.starts_with('\u{feff}') {
-            contents.drain(..3);
-        }
+        let contents = if contents.starts_with('\u{feff}') { &contents[3..] } else { &contents };
 
         // Create the intermediate directories
         let mut cur = self.dst.clone();
@@ -171,7 +169,7 @@ where
 
 /// Wrapper struct to render the source code of a file. This will do things like
 /// adding line numbers to the left-hand side.
-fn print_src(buf: &mut Buffer, s: String, edition: Edition) {
+fn print_src(buf: &mut Buffer, s: &str, edition: Edition) {
     let lines = s.lines().count();
     let mut cols = 0;
     let mut tmp = lines;
@@ -179,10 +177,10 @@ fn print_src(buf: &mut Buffer, s: String, edition: Edition) {
         cols += 1;
         tmp /= 10;
     }
-    write!(buf, "<pre class=\"line-numbers\">");
+    buf.write_str("<pre class=\"line-numbers\">");
     for i in 1..=lines {
         write!(buf, "<span id=\"{0}\">{0:1$}</span>\n", i, cols);
     }
-    write!(buf, "</pre>");
-    write!(buf, "{}", highlight::render_with_highlighting(s, None, None, None, edition));
+    buf.write_str("</pre>");
+    highlight::render_with_highlighting(s, buf, None, None, None, edition);
 }

src/librustdoc/passes/collect_trait_impls.rs

@@ -56,12 +56,12 @@ crate fn collect_trait_impls(krate: Crate, cx: &DocContext<'_>) -> Crate {
 
     // `tcx.crates()` doesn't include the local crate, and `tcx.all_trait_implementations`
     // doesn't work with it anyway, so pull them from the HIR map instead
+    let mut extra_attrs = Vec::new();
     for &trait_did in cx.tcx.all_traits(LOCAL_CRATE).iter() {
         for &impl_node in cx.tcx.hir().trait_impls(trait_did) {
-            let impl_did = cx.tcx.hir().local_def_id(impl_node);
+            let impl_did = cx.tcx.hir().local_def_id(impl_node).to_def_id();
             cx.tcx.sess.prof.generic_activity("build_local_trait_impl").run(|| {
-                let mut extra_attrs = Vec::new();
-                let mut parent = cx.tcx.parent(impl_did.to_def_id());
+                let mut parent = cx.tcx.parent(impl_did);
                 while let Some(did) = parent {
                     extra_attrs.extend(
                         cx.tcx
@@ -79,13 +79,8 @@ crate fn collect_trait_impls(krate: Crate, cx: &DocContext<'_>) -> Crate {
                     );
                     parent = cx.tcx.parent(did);
                 }
-                inline::build_impl(
-                    cx,
-                    None,
-                    impl_did.to_def_id(),
-                    Some(&extra_attrs),
-                    &mut new_items,
-                );
+                inline::build_impl(cx, None, impl_did, Some(&extra_attrs), &mut new_items);
+                extra_attrs.clear();
             });
         }
     }
@@ -137,25 +132,28 @@ crate fn collect_trait_impls(krate: Crate, cx: &DocContext<'_>) -> Crate {
         }
     }
 
-    new_items.retain(|it| {
-        if let ImplItem(Impl { ref for_, ref trait_, ref blanket_impl, .. }) = *it.kind {
-            cleaner.keep_impl(for_)
-                || trait_.as_ref().map_or(false, |t| cleaner.keep_impl(t))
-                || blanket_impl.is_some()
-        } else {
-            true
-        }
-    });
-
-    if let Some(ref mut it) = krate.module {
+    let items = if let Some(ref mut it) = krate.module {
         if let ModuleItem(Module { ref mut items, .. }) = *it.kind {
-            items.extend(synth.impls);
-            items.extend(new_items);
+            items
         } else {
             panic!("collect-trait-impls can't run");
         }
     } else {
         panic!("collect-trait-impls can't run");
+    };
+
+    items.extend(synth.impls);
+    for it in new_items.drain(..) {
+        if let ImplItem(Impl { ref for_, ref trait_, ref blanket_impl, .. }) = *it.kind {
+            if !(cleaner.keep_impl(for_)
+                || trait_.as_ref().map_or(false, |t| cleaner.keep_impl(t))
+                || blanket_impl.is_some())
+            {
+                continue;
+            }
+        }
+
+        items.push(it);
     }
 
     krate