Avoid Symbol to String conversions

compiler/rustc_builtin_macros/src/format.rs

@@ -1004,9 +1004,7 @@ fn lint_named_arguments_used_positionally(
                 node_id: ast::CRATE_NODE_ID,
                 lint_id: LintId::of(&NAMED_ARGUMENTS_USED_POSITIONALLY),
                 diagnostic: BuiltinLintDiagnostics::NamedArgumentUsedPositionally(
-                    arg_span,
-                    span,
-                    symbol.to_string(),
+                    arg_span, span, symbol,
                 ),
             });
         }

compiler/rustc_errors/src/diagnostic.rs

@@ -390,18 +390,17 @@ impl Diagnostic {
         expected: DiagnosticStyledString,
         found: DiagnosticStyledString,
     ) -> &mut Self {
-        let mut msg: Vec<_> =
-            vec![("required when trying to coerce from type `".to_string(), Style::NoStyle)];
+        let mut msg: Vec<_> = vec![("required when trying to coerce from type `", Style::NoStyle)];
         msg.extend(expected.0.iter().map(|x| match *x {
-            StringPart::Normal(ref s) => (s.to_owned(), Style::NoStyle),
-            StringPart::Highlighted(ref s) => (s.to_owned(), Style::Highlight),
+            StringPart::Normal(ref s) => (s.as_str(), Style::NoStyle),
+            StringPart::Highlighted(ref s) => (s.as_str(), Style::Highlight),
         }));
-        msg.push(("` to type '".to_string(), Style::NoStyle));
+        msg.push(("` to type '", Style::NoStyle));
         msg.extend(found.0.iter().map(|x| match *x {
-            StringPart::Normal(ref s) => (s.to_owned(), Style::NoStyle),
-            StringPart::Highlighted(ref s) => (s.to_owned(), Style::Highlight),
+            StringPart::Normal(ref s) => (s.as_str(), Style::NoStyle),
+            StringPart::Highlighted(ref s) => (s.as_str(), Style::Highlight),
         }));
-        msg.push(("`".to_string(), Style::NoStyle));
+        msg.push(("`", Style::NoStyle));
 
         // For now, just attach these as notes
         self.highlighted_note(msg);

compiler/rustc_lint_defs/src/lib.rs

@@ -467,7 +467,7 @@ pub enum BuiltinLintDiagnostics {
         /// If true, the lifetime will be fully elided.
         use_span: Option<(Span, bool)>,
     },
-    NamedArgumentUsedPositionally(Option<Span>, Span, String),
+    NamedArgumentUsedPositionally(Option<Span>, Span, Symbol),
 }
 
 /// Lints that are buffered up early on in the `Session` before the

compiler/rustc_resolve/src/diagnostics.rs

@@ -2603,9 +2603,9 @@ fn show_candidates(
                 .skip(1)
                 .all(|(_, descr, _, _)| descr == descr_first)
             {
-                descr_first.to_string()
+                descr_first
             } else {
-                "item".to_string()
+                "item"
             };
             let plural_descr =
                 if descr.ends_with("s") { format!("{}es", descr) } else { format!("{}s", descr) };