Avoid some unnecessary symbol interner operations

src/librustc_resolve/build_reduced_graph.rs

@@ -305,7 +305,7 @@ impl<'a> Resolver<'a> {
                 }
 
                 // Empty groups `a::b::{}` are turned into synthetic `self` imports
-                // `a::b::c::{self as __dummy}`, so that their prefixes are correctly
+                // `a::b::c::{self as _}`, so that their prefixes are correctly
                 // resolved and checked for privacy/stability/etc.
                 if items.is_empty() && !empty_for_self(&prefix) {
                     let new_span = prefix[prefix.len() - 1].ident.span;
@@ -314,7 +314,7 @@ impl<'a> Resolver<'a> {
                             Ident::new(kw::SelfLower, new_span)
                         ),
                         kind: ast::UseTreeKind::Simple(
-                            Some(Ident::from_str_and_span("__dummy", new_span).gensym()),
+                            Some(Ident::new(kw::Underscore, new_span)),
                             ast::DUMMY_NODE_ID,
                             ast::DUMMY_NODE_ID,
                         ),

src/libsyntax/diagnostics/plugin.rs

@@ -121,19 +121,7 @@ pub fn expand_register_diagnostic<'cx>(ecx: &'cx mut ExtCtxt<'_>,
         }
     });
 
-    let span = span.apply_mark(ecx.current_expansion.mark);
-
-    let name = Ident::from_str_and_span(&format!("__register_diagnostic_{}", code), span).gensym();
-
-    MacEager::items(smallvec![
-        ecx.item_mod(
-            span,
-            span,
-            name,
-            vec![],
-            vec![],
-        )
-    ])
+    MacEager::items(smallvec![])
 }
 
 #[allow(deprecated)]

src/libsyntax/ext/tt/macro_rules.rs

@@ -249,8 +249,9 @@ pub fn compile(
     def: &ast::Item,
     edition: Edition
 ) -> SyntaxExtension {
-    let lhs_nm = ast::Ident::from_str("lhs").gensym();
-    let rhs_nm = ast::Ident::from_str("rhs").gensym();
+    let lhs_nm = ast::Ident::new(sym::lhs, def.span);
+    let rhs_nm = ast::Ident::new(sym::rhs, def.span);
+    let tt_spec = ast::Ident::new(sym::tt, def.span);
 
     // Parse the macro_rules! invocation
     let body = match def.node {
@@ -266,9 +267,9 @@ pub fn compile(
     let argument_gram = vec![
         quoted::TokenTree::Sequence(DelimSpan::dummy(), Lrc::new(quoted::SequenceRepetition {
             tts: vec![
-                quoted::TokenTree::MetaVarDecl(def.span, lhs_nm, ast::Ident::from_str("tt")),
+                quoted::TokenTree::MetaVarDecl(def.span, lhs_nm, tt_spec),
                 quoted::TokenTree::token(token::FatArrow, def.span),
-                quoted::TokenTree::MetaVarDecl(def.span, rhs_nm, ast::Ident::from_str("tt")),
+                quoted::TokenTree::MetaVarDecl(def.span, rhs_nm, tt_spec),
             ],
             separator: Some(Token::new(
                 if body.legacy { token::Semi } else { token::Comma }, def.span
@@ -1115,10 +1116,9 @@ fn has_legal_fragment_specifier(sess: &ParseSess,
                                 tok: &quoted::TokenTree) -> Result<(), String> {
     debug!("has_legal_fragment_specifier({:?})", tok);
     if let quoted::TokenTree::MetaVarDecl(_, _, ref frag_spec) = *tok {
-        let frag_name = frag_spec.as_str();
         let frag_span = tok.span();
-        if !is_legal_fragment_specifier(sess, features, attrs, &frag_name, frag_span) {
-            return Err(frag_name.to_string());
+        if !is_legal_fragment_specifier(sess, features, attrs, frag_spec.name, frag_span) {
+            return Err(frag_spec.to_string());
         }
     }
     Ok(())
@@ -1127,7 +1127,7 @@ fn has_legal_fragment_specifier(sess: &ParseSess,
 fn is_legal_fragment_specifier(_sess: &ParseSess,
                                _features: &Features,
                                _attrs: &[ast::Attribute],
-                               frag_name: &str,
+                               frag_name: Symbol,
                                _frag_span: Span) -> bool {
     /*
      * If new fragment specifiers are invented in nightly, `_sess`,
@@ -1136,9 +1136,9 @@ fn is_legal_fragment_specifier(_sess: &ParseSess,
      * this function.
      */
     match frag_name {
-        "item" | "block" | "stmt" | "expr" | "pat" | "lifetime" |
-        "path" | "ty" | "ident" | "meta" | "tt" | "vis" | "literal" |
-        "" => true,
+        sym::item | sym::block | sym::stmt | sym::expr | sym::pat |
+        sym::lifetime | sym::path | sym::ty | sym::ident | sym::meta | sym::tt |
+        sym::vis | sym::literal | kw::Invalid => true,
         _ => false,
     }
 }

src/libsyntax/test.rs

@@ -327,7 +327,7 @@ fn mk_main(cx: &mut TestCtxt<'_>) -> P<ast::Item> {
     //        }
     let sp = ignored_span(cx, DUMMY_SP);
     let ecx = &cx.ext_cx;
-    let test_id = ecx.ident_of("test").gensym();
+    let test_id = Ident::with_empty_ctxt(sym::test);
 
     // test::test_main_static(...)
     let mut test_runner = cx.test_runner.clone().unwrap_or(
@@ -350,7 +350,7 @@ fn mk_main(cx: &mut TestCtxt<'_>) -> P<ast::Item> {
     let test_extern_stmt = ecx.stmt_item(sp, ecx.item(sp,
         test_id,
         vec![],
-        ast::ItemKind::ExternCrate(Some(sym::test))
+        ast::ItemKind::ExternCrate(None)
     ));
 
     // pub fn main() { ... }

src/libsyntax_pos/symbol.rs

@@ -347,6 +347,7 @@ symbols! {
         label_break_value,
         lang,
         lang_items,
+        lhs,
         lib,
         lifetime,
         link,
@@ -505,6 +506,7 @@ symbols! {
         result,
         Result,
         Return,
+        rhs,
         rlib,
         rt,
         rtm_target_feature,