Use the length limit as the initial capacity

The length limit turns out to be a surprisingly good heuristic for
initial allocation size. See here for more details [1].

[1]: https://github.com/rust-lang/rust/pull/88173#discussion_r692531631

src/librustdoc/html/length_limit.rs

@@ -29,8 +29,18 @@ pub(super) struct HtmlWithLimit {
 impl HtmlWithLimit {
     /// Create a new buffer, with a limit of `length_limit`.
     pub(super) fn new(length_limit: usize) -> Self {
+        let buf = if length_limit > 1000 {
+            // If the length limit is really large, don't preallocate tons of memory.
+            String::new()
+        } else {
+            // The length limit is actually a good heuristic for initial allocation size.
+            // Measurements showed that using it as the initial capacity ended up using less memory
+            // than `String::new`.
+            // See https://github.com/rust-lang/rust/pull/88173#discussion_r692531631 for more.
+            String::with_capacity(length_limit)
+        };
         Self {
-            buf: String::new(),
+            buf,
             len: 0,
             limit: length_limit,
             unclosed_tags: Vec::new(),

src/librustdoc/html/markdown.rs

@@ -1096,7 +1096,6 @@ fn markdown_summary_with_limit(
     let p = Parser::new_with_broken_link_callback(md, opts(), Some(&mut replacer));
     let mut p = LinkReplacer::new(p, link_names);
 
-    // FIXME: capacity
     let mut buf = HtmlWithLimit::new(length_limit);
     let mut stopped_early = false;
     p.try_for_each(|event| {