Estimate path length instead of hardcoding 64 bytes

src/librustdoc/html/format.rs

@@ -30,6 +30,7 @@ use crate::formats::item_type::ItemType;
 use crate::html::escape::Escape;
 use crate::html::render::Context;
 
+use super::url_parts_builder::estimate_item_path_byte_length;
 use super::url_parts_builder::UrlPartsBuilder;
 
 crate trait Print {
@@ -505,8 +506,7 @@ crate enum HrefError {
 
 // Panics if `syms` is empty.
 crate fn join_with_double_colon(syms: &[Symbol]) -> String {
-    // 64 bytes covers 99.9%+ of cases.
-    let mut s = String::with_capacity(64);
+    let mut s = String::with_capacity(estimate_item_path_byte_length(syms.len()));
     s.push_str(&syms[0].as_str());
     for sym in &syms[1..] {
         s.push_str("::");

src/librustdoc/html/url_parts_builder.rs

@@ -110,6 +110,14 @@ impl UrlPartsBuilder {
 /// This is intentionally on the lower end to avoid overallocating.
 const AVG_PART_LENGTH: usize = 5;
 
+/// Estimate the number of bytes in an item's path, based on how many segments it has.
+///
+/// **Note:** This is only to be used with, e.g., [`String::with_capacity()`];
+/// the return value is just a rough estimate.
+crate const fn estimate_item_path_byte_length(segment_count: usize) -> usize {
+    AVG_PART_LENGTH * segment_count
+}
+
 impl<'a> FromIterator<&'a str> for UrlPartsBuilder {
     fn from_iter<T: IntoIterator<Item = &'a str>>(iter: T) -> Self {
         let iter = iter.into_iter();