avoid some &str to String conversions

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -597,16 +597,16 @@ impl UseSpans<'_> {
     }
 
     /// Describe the span associated with a use of a place.
-    pub(super) fn describe(&self) -> String {
+    pub(super) fn describe(&self) -> &str {
         match *self {
             UseSpans::ClosureUse { generator_kind, .. } => {
                 if generator_kind.is_some() {
-                    " in generator".to_string()
+                    " in generator"
                 } else {
-                    " in closure".to_string()
+                    " in closure"
                 }
             }
-            _ => String::new(),
+            _ => "",
         }
     }
 

compiler/rustc_middle/src/mir/mod.rs

@@ -1929,12 +1929,10 @@ impl BorrowKind {
         }
     }
 
-    pub fn describe_mutability(&self) -> String {
+    pub fn describe_mutability(&self) -> &str {
         match *self {
-            BorrowKind::Shared | BorrowKind::Shallow | BorrowKind::Unique => {
+            BorrowKind::Shared | BorrowKind::Shallow | BorrowKind::Unique => "immutable",
-                "immutable".to_string()
+            BorrowKind::Mut { .. } => "mutable",
-            }
-            BorrowKind::Mut { .. } => "mutable".to_string(),
         }
     }
 }

compiler/rustc_mir_transform/src/check_packed_ref.rs

@@ -39,13 +39,11 @@ fn unsafe_derive_on_repr_packed(tcx: TyCtxt<'_>, def_id: LocalDefId) {
         let message = if tcx.generics_of(def_id).own_requires_monomorphization() {
             "`#[derive]` can't be used on a `#[repr(packed)]` struct with \
              type or const parameters (error E0133)"
-                .to_string()
         } else {
             "`#[derive]` can't be used on a `#[repr(packed)]` struct that \
              does not derive Copy (error E0133)"
-                .to_string()
         };
-        lint.build(&message).emit();
+        lint.build(message).emit();
     });
 }
 

compiler/rustc_resolve/src/diagnostics.rs

@@ -928,10 +928,7 @@ impl<'a> Resolver<'a> {
                     "generic parameters with a default cannot use \
                                                 forward declared identifiers"
                 );
-                err.span_label(
+                err.span_label(span, "defaulted generic parameters cannot be forward declared");
-                    span,
-                    "defaulted generic parameters cannot be forward declared".to_string(),
-                );
                 err
             }
             ResolutionError::ParamInTyOfConstParam(name) => {

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -349,10 +349,8 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
 
             err.code(rustc_errors::error_code!(E0424));
             err.span_label(span, match source {
-                PathSource::Pat => "`self` value is a keyword and may not be bound to variables or shadowed"
+                PathSource::Pat => "`self` value is a keyword and may not be bound to variables or shadowed",
-                                   .to_string(),
+                _ => "`self` value is a keyword only available in methods with a `self` parameter",
-                _ => "`self` value is a keyword only available in methods with a `self` parameter"
-                     .to_string(),
             });
             if let Some((fn_kind, span)) = &self.diagnostic_metadata.current_function {
                 // The current function has a `self' parameter, but we were unable to resolve

compiler/rustc_span/src/hygiene.rs

@@ -629,8 +629,7 @@ pub fn debug_hygiene_data(verbose: bool) -> String {
         if verbose {
             format!("{:#?}", data)
         } else {
-            let mut s = String::from("");
+            let mut s = String::from("Expansions:");
-            s.push_str("Expansions:");
             let mut debug_expn_data = |(id, expn_data): (&ExpnId, &ExpnData)| {
                 s.push_str(&format!(
                     "\n{:?}: parent: {:?}, call_site_ctxt: {:?}, def_site_ctxt: {:?}, kind: {:?}",

compiler/rustc_trait_selection/src/traits/specialize/mod.rs

@@ -406,7 +406,7 @@ fn report_conflicting_impls(
         let mut err = err.build(&msg);
         match tcx.span_of_impl(overlap.with_impl) {
             Ok(span) => {
-                err.span_label(span, "first implementation here".to_string());
+                err.span_label(span, "first implementation here");
 
                 err.span_label(
                     impl_span,

compiler/rustc_typeck/src/check/_match.rs

@@ -263,7 +263,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 } else if let ExprKind::Block(block, _) = &then_expr.kind
                     && let Some(expr) = &block.expr
                 {
-                    err.span_label(expr.span, "found here".to_string());
+                    err.span_label(expr.span, "found here");
                 }
                 err.note("`if` expressions without `else` evaluate to `()`");
                 err.help("consider adding an `else` block that evaluates to the expected type");

compiler/rustc_typeck/src/check/demand.rs

@@ -317,9 +317,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 .tcx
                                 .is_diagnostic_item(sym::Result, expected_adt.did())
                             {
-                                vec!["Ok(())".to_string()]
+                                vec!["Ok(())"]
                             } else if self.tcx.is_diagnostic_item(sym::Option, expected_adt.did()) {
-                                vec!["None".to_string(), "Some(())".to_string()]
+                                vec!["None", "Some(())"]
                             } else {
                                 return;
                             };

compiler/rustc_typeck/src/check/op.rs

@@ -565,9 +565,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 .is_ok()
             {
                 let (variable_snippet, applicability) = if !fn_sig.inputs().is_empty() {
-                    ("( /* arguments */ )".to_string(), Applicability::HasPlaceholders)
+                    ("( /* arguments */ )", Applicability::HasPlaceholders)
                 } else {
-                    ("()".to_string(), Applicability::MaybeIncorrect)
+                    ("()", Applicability::MaybeIncorrect)
                 };
 
                 err.span_suggestion_verbose(

compiler/rustc_typeck/src/hir_wf_check.rs

@@ -86,7 +86,7 @@ fn diagnostic_hir_wf_check<'tcx>(
 
                 let errors = fulfill.select_all_or_error(&infcx);
                 if !errors.is_empty() {
-                    tracing::debug!("Wf-check got errors for {:?}: {:?}", ty, errors);
+                    debug!("Wf-check got errors for {:?}: {:?}", ty, errors);
                     for error in errors {
                         if error.obligation.predicate == self.predicate {
                             // Save the cause from the greatest depth - this corresponds

compiler/rustc_typeck/src/lib.rs

@@ -252,7 +252,7 @@ fn check_main_fn_ty(tcx: TyCtxt<'_>, main_def_id: DefId) {
         let mut diag =
             struct_span_err!(tcx.sess, generics_param_span.unwrap_or(main_span), E0131, "{}", msg);
         if let Some(generics_param_span) = generics_param_span {
-            let label = "`main` cannot have generic parameters".to_string();
+            let label = "`main` cannot have generic parameters";
             diag.span_label(generics_param_span, label);
         }
         diag.emit();
@@ -307,8 +307,7 @@ fn check_main_fn_ty(tcx: TyCtxt<'_>, main_def_id: DefId) {
         let return_ty_span = main_fn_return_type_span(tcx, main_def_id).unwrap_or(main_span);
         if !return_ty.bound_vars().is_empty() {
             let msg = "`main` function return type is not allowed to have generic \
-                    parameters"
+                    parameters";
-                .to_owned();
             struct_span_err!(tcx.sess, return_ty_span, E0131, "{}", msg).emit();
             error = true;
         }

compiler/rustc_typeck/src/structured_errors/wrong_number_of_generic_args.rs

@@ -126,8 +126,8 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
         }
     }
 
-    fn kind(&self) -> String {
+    fn kind(&self) -> &str {
-        if self.missing_lifetimes() { "lifetime".to_string() } else { "generic".to_string() }
+        if self.missing_lifetimes() { "lifetime" } else { "generic" }
     }
 
     fn num_provided_args(&self) -> usize {