Auto merge of #96260 - Kobzol:rustdoc-idmap, r=petrochenkov

rustdoc: Optimize IdMap

Slightly optimizes `IdMap`, which is hot in `markdown_links` (context [here](https://github.com/rust-lang/rust/pull/96135#issuecomment-1103539052)). There are more improvements that can be made near this place, but this seemed like an easy win locally (although I tried it on top of https://github.com/rust-lang/rust/pull/94857, so let's see what happens without that PR).

r? `@petrochenkov`

3 files changed, 52 insertions, 46 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 687fbb24d9f..30175ae3561 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2531,9 +2531,9 @@ dependencies = [
 
 [[package]]
 name = "once_cell"
-version = "1.7.2"
+version = "1.10.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "af8b08b04175473088b46763e51ee54da5f9a164bc162f615b91bc179dbf15a3"
+checksum = "87f3e037eac156d1775da914196f0f37741a274155e34a0b7e427c35d2a2ecb9"
 
 [[package]]
 name = "opaque-debug"
@@ -4536,6 +4536,7 @@ dependencies = [
  "expect-test",
  "itertools",
  "minifier",
+ "once_cell",
  "pulldown-cmark",
  "rayon",
  "regex",
diff --git a/src/librustdoc/Cargo.toml b/src/librustdoc/Cargo.toml
index cc5c583bea8..21efd040663 100644
--- a/src/librustdoc/Cargo.toml
+++ b/src/librustdoc/Cargo.toml
@@ -22,6 +22,7 @@ regex = "1"
 rustdoc-json-types = { path = "../rustdoc-json-types" }
 tracing = "0.1"
 tracing-tree = "0.2.0"
+once_cell = "1.10.0"
 
 [dependencies.tracing-subscriber]
 version = "0.3.3"
diff --git a/src/librustdoc/html/markdown.rs b/src/librustdoc/html/markdown.rs
index b0f7836a834..9e76af98298 100644
--- a/src/librustdoc/html/markdown.rs
+++ b/src/librustdoc/html/markdown.rs
@@ -32,6 +32,7 @@ use rustc_middle::ty::TyCtxt;
 use rustc_span::edition::Edition;
 use rustc_span::Span;
 
+use once_cell::sync::Lazy;
 use std::borrow::Cow;
 use std::cell::RefCell;
 use std::collections::VecDeque;
@@ -1429,62 +1430,65 @@ crate fn rust_code_blocks(md: &str, extra_info: &ExtraInfo<'_>) -> Vec<RustCodeB
 
 #[derive(Clone, Default, Debug)]
 pub struct IdMap {
-    map: FxHashMap<String, usize>,
+    map: FxHashMap<Cow<'static, str>, usize>,
 }
 
-fn init_id_map() -> FxHashMap<String, usize> {
+// The map is pre-initialized and cloned each time to avoid reinitializing it repeatedly.
+static DEFAULT_ID_MAP: Lazy<FxHashMap<Cow<'static, str>, usize>> = Lazy::new(|| init_id_map());
+
+fn init_id_map() -> FxHashMap<Cow<'static, str>, usize> {
     let mut map = FxHashMap::default();
     // This is the list of IDs used in Javascript.
-    map.insert("help".to_owned(), 1);
+    map.insert("help".into(), 1);
     // This is the list of IDs used in HTML generated in Rust (including the ones
     // used in tera template files).
-    map.insert("mainThemeStyle".to_owned(), 1);
-    map.insert("themeStyle".to_owned(), 1);
-    map.insert("theme-picker".to_owned(), 1);
-    map.insert("theme-choices".to_owned(), 1);
-    map.insert("settings-menu".to_owned(), 1);
-    map.insert("help-button".to_owned(), 1);
-    map.insert("main-content".to_owned(), 1);
-    map.insert("search".to_owned(), 1);
-    map.insert("crate-search".to_owned(), 1);
-    map.insert("render-detail".to_owned(), 1);
-    map.insert("toggle-all-docs".to_owned(), 1);
-    map.insert("all-types".to_owned(), 1);
-    map.insert("default-settings".to_owned(), 1);
-    map.insert("rustdoc-vars".to_owned(), 1);
-    map.insert("sidebar-vars".to_owned(), 1);
-    map.insert("copy-path".to_owned(), 1);
-    map.insert("TOC".to_owned(), 1);
+    map.insert("mainThemeStyle".into(), 1);
+    map.insert("themeStyle".into(), 1);
+    map.insert("theme-picker".into(), 1);
+    map.insert("theme-choices".into(), 1);
+    map.insert("settings-menu".into(), 1);
+    map.insert("help-button".into(), 1);
+    map.insert("main-content".into(), 1);
+    map.insert("search".into(), 1);
+    map.insert("crate-search".into(), 1);
+    map.insert("render-detail".into(), 1);
+    map.insert("toggle-all-docs".into(), 1);
+    map.insert("all-types".into(), 1);
+    map.insert("default-settings".into(), 1);
+    map.insert("rustdoc-vars".into(), 1);
+    map.insert("sidebar-vars".into(), 1);
+    map.insert("copy-path".into(), 1);
+    map.insert("TOC".into(), 1);
     // This is the list of IDs used by rustdoc sections (but still generated by
     // rustdoc).
-    map.insert("fields".to_owned(), 1);
-    map.insert("variants".to_owned(), 1);
-    map.insert("implementors-list".to_owned(), 1);
-    map.insert("synthetic-implementors-list".to_owned(), 1);
-    map.insert("foreign-impls".to_owned(), 1);
-    map.insert("implementations".to_owned(), 1);
-    map.insert("trait-implementations".to_owned(), 1);
-    map.insert("synthetic-implementations".to_owned(), 1);
-    map.insert("blanket-implementations".to_owned(), 1);
-    map.insert("required-associated-types".to_owned(), 1);
-    map.insert("provided-associated-types".to_owned(), 1);
-    map.insert("provided-associated-consts".to_owned(), 1);
-    map.insert("required-associated-consts".to_owned(), 1);
-    map.insert("required-methods".to_owned(), 1);
-    map.insert("provided-methods".to_owned(), 1);
-    map.insert("implementors".to_owned(), 1);
-    map.insert("synthetic-implementors".to_owned(), 1);
-    map.insert("implementations-list".to_owned(), 1);
-    map.insert("trait-implementations-list".to_owned(), 1);
-    map.insert("synthetic-implementations-list".to_owned(), 1);
-    map.insert("blanket-implementations-list".to_owned(), 1);
-    map.insert("deref-methods".to_owned(), 1);
+    map.insert("fields".into(), 1);
+    map.insert("variants".into(), 1);
+    map.insert("implementors-list".into(), 1);
+    map.insert("synthetic-implementors-list".into(), 1);
+    map.insert("foreign-impls".into(), 1);
+    map.insert("implementations".into(), 1);
+    map.insert("trait-implementations".into(), 1);
+    map.insert("synthetic-implementations".into(), 1);
+    map.insert("blanket-implementations".into(), 1);
+    map.insert("required-associated-types".into(), 1);
+    map.insert("provided-associated-types".into(), 1);
+    map.insert("provided-associated-consts".into(), 1);
+    map.insert("required-associated-consts".into(), 1);
+    map.insert("required-methods".into(), 1);
+    map.insert("provided-methods".into(), 1);
+    map.insert("implementors".into(), 1);
+    map.insert("synthetic-implementors".into(), 1);
+    map.insert("implementations-list".into(), 1);
+    map.insert("trait-implementations-list".into(), 1);
+    map.insert("synthetic-implementations-list".into(), 1);
+    map.insert("blanket-implementations-list".into(), 1);
+    map.insert("deref-methods".into(), 1);
     map
 }
 
 impl IdMap {
     pub fn new() -> Self {
-        IdMap { map: init_id_map() }
+        IdMap { map: DEFAULT_ID_MAP.clone() }
     }
 
     crate fn derive<S: AsRef<str> + ToString>(&mut self, candidate: S) -> String {
@@ -1497,7 +1501,7 @@ impl IdMap {
             }
         };
 
-        self.map.insert(id.clone(), 1);
+        self.map.insert(id.clone().into(), 1);
         id
     }
 }