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`

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/rustc_resolve/src/macros.rs b/compiler/rustc_resolve/src/macros.rs
index df5c16a9375..d8a7bcbfff9 100644
--- a/compiler/rustc_resolve/src/macros.rs
+++ b/compiler/rustc_resolve/src/macros.rs
@@ -827,7 +827,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                 if !is_allowed(feature) && !allowed_by_implication {
                     let lint_buffer = &mut self.lint_buffer;
                     let soft_handler =
-                        |lint, span, msg: &_| lint_buffer.buffer_lint(lint, node_id, span, msg);
+                        |lint, span, msg: String| lint_buffer.buffer_lint(lint, node_id, span, msg);
                     stability::report_unstable(
                         self.tcx.sess,
                         feature,
@@ -846,7 +846,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             let (message, lint) = stability::deprecation_message_and_lint(depr, "macro", &path);
             stability::early_report_deprecation(
                 &mut self.lint_buffer,
-                &message,
+                message,
                 depr.suggestion,
                 lint,
                 span,