[DO NOT MERGE] Experiment with micro-optimizing push_str calls

Author: lolbinarycat

library/alloc/src/string.rs

@@ -1112,6 +1112,22 @@ impl String {
         self.vec.extend_from_slice(string.as_bytes())
     }
 
+    #[unstable(feature = "string_push_str_slice", issue = "none")]
+    #[inline]
+    pub fn push_str_slice(&mut self, slice: &[&str]) {
+        let additional = slice.iter().map(|x| x.len()).sum();
+        self.reserve(additional);
+        let (ptr, len, cap) = core::mem::take(self).into_raw_parts();
+        unsafe {
+            let mut dst = ptr.add(len);
+            for new in slice {
+                core::ptr::copy_nonoverlapping(new.as_ptr(), dst, new.len());
+                dst = dst.add(new.len());
+            }
+            *self = String::from_raw_parts(ptr, len + additional, cap);
+        }
+    }
+
     /// Copies elements from `src` range to the end of the string.
     ///
     /// # Panics

src/librustdoc/html/render/print_item.rs

@@ -2257,8 +2257,7 @@ pub(crate) fn compare_names(left: &str, right: &str) -> Ordering {
 
 pub(super) fn full_path(cx: &Context<'_>, item: &clean::Item) -> String {
     let mut s = join_path_syms(&cx.current);
-    s.push_str("::");
-    s.push_str(item.name.unwrap().as_str());
+    crate::push_str_slice(&mut s, &["::", item.name.unwrap().as_str()]);
     s
 }
 

src/librustdoc/lib.rs

@@ -18,6 +18,7 @@
 #![feature(rustc_private)]
 #![feature(test)]
 #![feature(trim_prefix_suffix)]
+#![feature(vec_into_raw_parts)]
 #![warn(rustc::internal)]
 // tidy-alphabetical-end
 
@@ -983,3 +984,19 @@ fn dump_feature_usage_metrics(tcxt: TyCtxt<'_>, metrics_dir: &Path) {
         tcxt.dcx().err(format!("cannot emit feature usage metrics: {error}"));
     }
 }
+
+#[inline]
+fn push_str_slice(s: &mut String, slice: &[&str]) {
+    use std::ptr::copy_nonoverlapping;
+    let additional = slice.iter().map(|x| x.len()).sum();
+    s.reserve(additional);
+    let (ptr, len, cap) = std::mem::take(s).into_raw_parts();
+    unsafe {
+        let mut dst = ptr.add(len);
+        for new in slice {
+            copy_nonoverlapping(new.as_ptr(), dst, new.len());
+            dst = dst.add(new.len());
+        }
+        *s = String::from_raw_parts(ptr, len + additional, cap);
+    }
+}

src/librustdoc/passes/collect_intra_doc_links.rs

@@ -223,17 +223,18 @@ impl UrlFragment {
                     DefKind::Field => {
                         let parent_id = tcx.parent(def_id);
                         if tcx.def_kind(parent_id) == DefKind::Variant {
-                            s.push_str("variant.");
-                            s.push_str(tcx.item_name(parent_id).as_str());
+                            crate::push_str_slice(
+                                s,
+                                &["variant.", tcx.item_name(parent_id).as_str()],
+                            );
                             ".field."
                         } else {
                             "structfield."
                         }
                     }
                     kind => bug!("unexpected associated item kind: {kind:?}"),
                 };
-                s.push_str(kind);
-                s.push_str(tcx.item_name(def_id).as_str());
+                crate::push_str_slice(s, &[kind, tcx.item_name(def_id).as_str()]);
             }
             UrlFragment::UserWritten(raw) => s.push_str(raw),
         }