revert part that causes perf regression

Author: yotamofek

src/librustdoc/html/format.rs

@@ -495,7 +495,12 @@ pub(crate) enum HrefError {
 // Panics if `syms` is empty.
 pub(crate) fn join_with_double_colon(syms: &[Symbol]) -> String {
     let mut s = String::with_capacity(estimate_item_path_byte_length(syms.len()));
-    write!(s, "{}", fmt::from_fn(|f| syms.iter().joined("::", f))).unwrap();
+    // NOTE: using `Joined::joined` here causes a noticeable perf regression
+    s.push_str(syms[0].as_str());
+    for sym in &syms[1..] {
+        s.push_str("::");
+        s.push_str(sym.as_str());
+    }
     s
 }