Auto merge of #111748 - nnethercote:Cow-DiagnosticMessage, r=WaffleLapkin

Use `Cow` in `{D,Subd}iagnosticMessage`.

Each of `{D,Subd}iagnosticMessage::{Str,Eager}` has a comment:
```
// FIXME(davidtwco): can a `Cow<'static, str>` be used here?
```
This commit answers that question in the affirmative. It's not the most compelling change ever, but it might be worth merging.

This requires changing the `impl<'a> From<&'a str>` impls to `impl From<&'static str>`, which involves a bunch of knock-on changes that require/result in call sites being a little more precise about exactly what kind of string they use to create errors, and not just `&str`. This will result in fewer unnecessary allocations, though this will not have any notable perf effects given that these are error paths.

Note that I was lazy within Clippy, using `to_string` in a few places to preserve the existing string imprecision. I could have used `impl Into<{D,Subd}iagnosticMessage>` in various places as is done in the compiler, but that would have required changes to *many* call sites (mostly changing `&format("...")` to `format!("...")`) which didn't seem worthwhile.

r? `@WaffleLapkin`

2 files changed, 9 insertions, 7 deletions

diff --git a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
index 283a9ed3388..e33330e259c 100644
--- a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
+++ b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
@@ -25,6 +25,7 @@ use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _;
 use rustc_trait_selection::traits::{
     self, ObligationCause, ObligationCauseCode, ObligationCtxt, Reveal,
 };
+use std::borrow::Cow;
 use std::iter;
 
 /// Checks that a method from an impl conforms to the signature of
@@ -684,7 +685,7 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
                 &cause,
                 hir.get_if_local(impl_m.def_id)
                     .and_then(|node| node.fn_decl())
-                    .map(|decl| (decl.output.span(), "return type in trait".to_owned())),
+                    .map(|decl| (decl.output.span(), Cow::from("return type in trait"))),
                 Some(infer::ValuePairs::Terms(ExpectedFound {
                     expected: trait_return_ty.into(),
                     found: impl_return_ty.into(),
@@ -963,7 +964,7 @@ fn report_trait_method_mismatch<'tcx>(
     infcx.err_ctxt().note_type_err(
         &mut diag,
         &cause,
-        trait_err_span.map(|sp| (sp, "type in trait".to_owned())),
+        trait_err_span.map(|sp| (sp, Cow::from("type in trait"))),
         Some(infer::ValuePairs::Sigs(ExpectedFound { expected: trait_sig, found: impl_sig })),
         terr,
         false,
@@ -1731,7 +1732,7 @@ pub(super) fn compare_impl_const_raw(
         infcx.err_ctxt().note_type_err(
             &mut diag,
             &cause,
-            trait_c_span.map(|span| (span, "type in trait".to_owned())),
+            trait_c_span.map(|span| (span, Cow::from("type in trait"))),
             Some(infer::ValuePairs::Terms(ExpectedFound {
                 expected: trait_ty.into(),
                 found: impl_ty.into(),
diff --git a/compiler/rustc_hir_analysis/src/check/intrinsic.rs b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
index e8785235c83..fbd4a577d68 100644
--- a/compiler/rustc_hir_analysis/src/check/intrinsic.rs
+++ b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
@@ -121,10 +121,11 @@ pub fn intrinsic_operation_unsafety(tcx: TyCtxt<'_>, intrinsic_id: DefId) -> hir
     if has_safe_attr != is_in_list {
         tcx.sess.struct_span_err(
             tcx.def_span(intrinsic_id),
-            DiagnosticMessage::Str(format!(
-                    "intrinsic safety mismatch between list of intrinsics within the compiler and core library intrinsics for intrinsic `{}`",
-                    tcx.item_name(intrinsic_id)
-        ))).emit();
+            DiagnosticMessage::from(format!(
+                "intrinsic safety mismatch between list of intrinsics within the compiler and core library intrinsics for intrinsic `{}`",
+                tcx.item_name(intrinsic_id)
+            )
+        )).emit();
     }
 
     is_in_list