check_doc_keyword: don't alloc string for emptiness check

check_doc_alias_value: get argument as Symbol to prevent needless string convertions

check_doc_attrs: don't alloc vec, iterate over slice. Vec introduced in #83149, but no perf run posted on merge

replace as_str() check with symbol check

get_single_str_from_tts: don't prealloc string

trivial string to str replace

LifetimeScopeForPath::NonElided use Vec<Symbol> instead of Vec<String>

AssertModuleSource use BTreeSet<Symbol> instead of BTreeSet<String>

CrateInfo.crate_name replace FxHashMap<CrateNum, String> with FxHashMap<CrateNum, Symbol>

compiler/rustc_passes/src/stability.rs

@@ -696,7 +696,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
             hir::ItemKind::ExternCrate(_) => {
                 // compiler-generated `extern crate` items have a dummy span.
                 // `std` is still checked for the `restricted-std` feature.
-                if item.span.is_dummy() && item.ident.as_str() != "std" {
+                if item.span.is_dummy() && item.ident.name != sym::std {
                     return;
                 }