rustdoc: reduce allocations when generating tooltips

An attempt to reduce the perf regression in
https://github.com/rust-lang/rust/pull/108052#issuecomment-1430631861

2 files changed, 17 insertions, 10 deletions

diff --git a/src/librustdoc/html/format.rs b/src/librustdoc/html/format.rs
index 6bdd9db9bfa..0e4c5ed6836 100644
--- a/src/librustdoc/html/format.rs
+++ b/src/librustdoc/html/format.rs
@@ -772,14 +772,21 @@ pub(crate) fn link_tooltip(did: DefId, fragment: &Option<UrlFragment>, cx: &Cont
     let Some((fqp, shortty)) = cache.paths.get(&did)
         .or_else(|| cache.external_paths.get(&did))
         else { return String::new() };
-    let fqp = fqp.iter().map(|sym| sym.as_str()).join("::");
+    let mut buf = Buffer::new();
     if let &Some(UrlFragment::Item(id)) = fragment {
-        let name = cx.tcx().item_name(id);
-        let descr = cx.tcx().def_descr(id);
-        format!("{descr} {fqp}::{name}")
-    } else {
-        format!("{shortty} {fqp}")
+        write!(buf, "{} ", cx.tcx().def_descr(id));
+        for component in fqp {
+            write!(buf, "{component}::");
+        }
+        write!(buf, "{}", cx.tcx().item_name(id));
+    } else if !fqp.is_empty() {
+        let mut fqp_it = fqp.into_iter();
+        write!(buf, "{shortty} {}", fqp_it.next().unwrap());
+        for component in fqp_it {
+            write!(buf, "::{component}");
+        }
     }
+    buf.into_inner()
 }
 
 /// Used to render a [`clean::Path`].
diff --git a/src/librustdoc/html/markdown.rs b/src/librustdoc/html/markdown.rs
index 9ef0b501c08..b5a34814382 100644
--- a/src/librustdoc/html/markdown.rs
+++ b/src/librustdoc/html/markdown.rs
@@ -981,7 +981,7 @@ impl Markdown<'_> {
         let mut replacer = |broken_link: BrokenLink<'_>| {
             links
                 .iter()
-                .find(|link| link.original_text.as_str() == &*broken_link.reference)
+                .find(|link| &*link.original_text == &*broken_link.reference)
                 .map(|link| (link.href.as_str().into(), link.tooltip.as_str().into()))
         };
 
@@ -1064,7 +1064,7 @@ impl MarkdownSummaryLine<'_> {
         let mut replacer = |broken_link: BrokenLink<'_>| {
             links
                 .iter()
-                .find(|link| link.original_text.as_str() == &*broken_link.reference)
+                .find(|link| &*link.original_text == &*broken_link.reference)
                 .map(|link| (link.href.as_str().into(), link.tooltip.as_str().into()))
         };
 
@@ -1111,7 +1111,7 @@ fn markdown_summary_with_limit(
     let mut replacer = |broken_link: BrokenLink<'_>| {
         link_names
             .iter()
-            .find(|link| link.original_text.as_str() == &*broken_link.reference)
+            .find(|link| &*link.original_text == &*broken_link.reference)
             .map(|link| (link.href.as_str().into(), link.tooltip.as_str().into()))
     };
 
@@ -1192,7 +1192,7 @@ pub(crate) fn plain_text_summary(md: &str, link_names: &[RenderedLink]) -> Strin
     let mut replacer = |broken_link: BrokenLink<'_>| {
         link_names
             .iter()
-            .find(|link| link.original_text.as_str() == &*broken_link.reference)
+            .find(|link| &*link.original_text == &*broken_link.reference)
             .map(|link| (link.href.as_str().into(), link.tooltip.as_str().into()))
     };