Fix some clippy perf lints

compiler/rustc_errors/src/emitter.rs

@@ -2362,11 +2362,7 @@ impl FileWithAnnotatedLines {
 
                 let label = label.as_ref().map(|m| {
                     normalize_whitespace(
-                        &emitter
-                            .translate_message(m, &args)
-                            .map_err(Report::new)
-                            .unwrap()
-                            .to_string(),
+                        &emitter.translate_message(m, &args).map_err(Report::new).unwrap(),
                     )
                 });
 

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -274,7 +274,7 @@ pub fn suggest_constraining_type_params<'a>(
                 span,
                 if span_to_replace.is_some() {
                     constraint.clone()
-                } else if constraint.starts_with("<") {
+                } else if constraint.starts_with('<') {
                     constraint.to_string()
                 } else if bound_list_non_empty {
                     format!(" + {constraint}")

compiler/rustc_resolve/src/diagnostics.rs

@@ -1545,7 +1545,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         let (span, sugg, post) = if let SuggestionTarget::SimilarlyNamed = suggestion.target
             && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
             && let Some(span) = suggestion.span
-            && let Some(candidate) = suggestion.candidate.as_str().strip_prefix("_")
+            && let Some(candidate) = suggestion.candidate.as_str().strip_prefix('_')
             && snippet == candidate
         {
             // When the suggested binding change would be from `x` to `_x`, suggest changing the

compiler/rustc_session/src/config.rs

@@ -2477,7 +2477,7 @@ pub fn parse_externs(
             let mut error = handler.early_struct_error(format!(
                 "crate name `{name}` passed to `--extern` is not a valid ASCII identifier"
             ));
-            let adjusted_name = name.replace("-", "_");
+            let adjusted_name = name.replace('-', "_");
             if crate::utils::is_ascii_ident(&adjusted_name) {
                 error.help(format!(
                     "consider replacing the dashes with underscores: `{adjusted_name}`"

src/librustdoc/clean/inline.rs

@@ -599,7 +599,7 @@ fn build_module_items(
                 let prim_ty = clean::PrimitiveType::from(p);
                 items.push(clean::Item {
                     name: None,
-                    attrs: Box::new(clean::Attributes::default()),
+                    attrs: Box::default(),
                     // We can use the item's `DefId` directly since the only information ever used
                     // from it is `DefId.krate`.
                     item_id: ItemId::DefId(did),

src/librustdoc/clean/types.rs

@@ -1327,6 +1327,7 @@ impl WherePredicate {
 pub(crate) enum GenericParamDefKind {
     Lifetime { outlives: ThinVec<Lifetime> },
     Type { did: DefId, bounds: ThinVec<GenericBound>, default: Option<Box<Type>>, synthetic: bool },
+    // Option<Box<String>> makes this type smaller than `Option<String>` would.
     Const { ty: Box<Type>, default: Option<Box<String>>, is_host_effect: bool },
 }
 

src/librustdoc/clean/utils.rs

@@ -588,7 +588,7 @@ pub(crate) fn has_doc_flag(tcx: TyCtxt<'_>, did: DefId, flag: Symbol) -> bool {
 /// Set by `bootstrap::Builder::doc_rust_lang_org_channel` in order to keep tests passing on beta/stable.
 pub(crate) const DOC_RUST_LANG_ORG_CHANNEL: &str = env!("DOC_RUST_LANG_ORG_CHANNEL");
 pub(crate) static DOC_CHANNEL: Lazy<&'static str> =
-    Lazy::new(|| DOC_RUST_LANG_ORG_CHANNEL.rsplit("/").filter(|c| !c.is_empty()).next().unwrap());
+    Lazy::new(|| DOC_RUST_LANG_ORG_CHANNEL.rsplit('/').filter(|c| !c.is_empty()).next().unwrap());
 
 /// Render a sequence of macro arms in a format suitable for displaying to the user
 /// as part of an item declaration.

src/librustdoc/html/render/write_shared.rs

@@ -356,15 +356,12 @@ if (typeof exports !== 'undefined') {exports.searchIndex = searchIndex};
 
             let content = format!(
                 "<h1>List of all crates</h1><ul class=\"all-items\">{}</ul>",
-                krates
-                    .iter()
-                    .map(|s| {
-                        format!(
-                            "<li><a href=\"{trailing_slash}index.html\">{s}</a></li>",
-                            trailing_slash = ensure_trailing_slash(s),
-                        )
-                    })
-                    .collect::<String>()
+                krates.iter().format_with("", |k, f| {
+                    f(&format_args!(
+                        "<li><a href=\"{trailing_slash}index.html\">{k}</a></li>",
+                        trailing_slash = ensure_trailing_slash(k),
+                    ))
+                })
             );
             let v = layout::render(&shared.layout, &page, "", content, &shared.style_files);
             shared.fs.write(dst, v)?;

src/librustdoc/passes/lint/redundant_explicit_links.rs

@@ -87,7 +87,7 @@ fn check_redundant_explicit_link<'md>(
                 let link_data = collect_link_data(&mut offset_iter);
 
                 if let Some(resolvable_link) = link_data.resolvable_link.as_ref() {
-                    if &link_data.display_link.replace("`", "") != resolvable_link {
+                    if &link_data.display_link.replace('`', "") != resolvable_link {
                         // Skips if display link does not match to actual
                         // resolvable link, usually happens if display link
                         // has several segments, e.g.