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`

4 files changed, 22 insertions, 26 deletions

diff --git a/compiler/rustc_errors/src/diagnostic.rs b/compiler/rustc_errors/src/diagnostic.rs
index 29c692128bc..488f2d67ee5 100644
--- a/compiler/rustc_errors/src/diagnostic.rs
+++ b/compiler/rustc_errors/src/diagnostic.rs
@@ -352,14 +352,9 @@ impl Diagnostic {
 
     /// Labels all the given spans with the provided label.
     /// See [`Self::span_label()`] for more information.
-    pub fn span_labels(
-        &mut self,
-        spans: impl IntoIterator<Item = Span>,
-        label: impl AsRef<str>,
-    ) -> &mut Self {
-        let label = label.as_ref();
+    pub fn span_labels(&mut self, spans: impl IntoIterator<Item = Span>, label: &str) -> &mut Self {
         for span in spans {
-            self.span_label(span, label);
+            self.span_label(span, label.to_string());
         }
         self
     }
@@ -394,17 +389,18 @@ impl Diagnostic {
         expected: DiagnosticStyledString,
         found: DiagnosticStyledString,
     ) -> &mut Self {
-        let mut msg: Vec<_> = vec![("required when trying to coerce from type `", Style::NoStyle)];
+        let mut msg: Vec<_> =
+            vec![(Cow::from("required when trying to coerce from type `"), Style::NoStyle)];
         msg.extend(expected.0.iter().map(|x| match *x {
-            StringPart::Normal(ref s) => (s.as_str(), Style::NoStyle),
-            StringPart::Highlighted(ref s) => (s.as_str(), Style::Highlight),
+            StringPart::Normal(ref s) => (Cow::from(s.clone()), Style::NoStyle),
+            StringPart::Highlighted(ref s) => (Cow::from(s.clone()), Style::Highlight),
         }));
-        msg.push(("` to type '", Style::NoStyle));
+        msg.push((Cow::from("` to type '"), Style::NoStyle));
         msg.extend(found.0.iter().map(|x| match *x {
-            StringPart::Normal(ref s) => (s.as_str(), Style::NoStyle),
-            StringPart::Highlighted(ref s) => (s.as_str(), Style::Highlight),
+            StringPart::Normal(ref s) => (Cow::from(s.clone()), Style::NoStyle),
+            StringPart::Highlighted(ref s) => (Cow::from(s.clone()), Style::Highlight),
         }));
-        msg.push(("`", Style::NoStyle));
+        msg.push((Cow::from("`"), Style::NoStyle));
 
         // For now, just attach these as notes
         self.highlighted_note(msg);
diff --git a/compiler/rustc_errors/src/diagnostic_builder.rs b/compiler/rustc_errors/src/diagnostic_builder.rs
index db97d96fccd..7d9d0c76450 100644
--- a/compiler/rustc_errors/src/diagnostic_builder.rs
+++ b/compiler/rustc_errors/src/diagnostic_builder.rs
@@ -558,7 +558,7 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
         }
 
         // Take the `Diagnostic` by replacing it with a dummy.
-        let dummy = Diagnostic::new(Level::Allow, DiagnosticMessage::Str("".to_string()));
+        let dummy = Diagnostic::new(Level::Allow, DiagnosticMessage::from(""));
         let diagnostic = std::mem::replace(&mut *self.inner.diagnostic, dummy);
 
         // Disable the ICE on `Drop`.
@@ -627,7 +627,7 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
     pub fn span_labels(
         &mut self,
         spans: impl IntoIterator<Item = Span>,
-        label: impl AsRef<str>,
+        label: &str,
     ) -> &mut Self);
 
     forward!(pub fn note_expected_found(
@@ -781,8 +781,8 @@ impl Drop for DiagnosticBuilderInner<'_> {
                 if !panicking() {
                     handler.emit_diagnostic(&mut Diagnostic::new(
                         Level::Bug,
-                        DiagnosticMessage::Str(
-                            "the following error was constructed but not emitted".to_string(),
+                        DiagnosticMessage::from(
+                            "the following error was constructed but not emitted",
                         ),
                     ));
                     handler.emit_diagnostic(&mut self.diagnostic);
diff --git a/compiler/rustc_errors/src/emitter.rs b/compiler/rustc_errors/src/emitter.rs
index e8cd7eaa60f..d8c997b49a1 100644
--- a/compiler/rustc_errors/src/emitter.rs
+++ b/compiler/rustc_errors/src/emitter.rs
@@ -367,7 +367,7 @@ pub trait Emitter: Translate {
 
                 children.push(SubDiagnostic {
                     level: Level::Note,
-                    message: vec![(DiagnosticMessage::Str(msg), Style::NoStyle)],
+                    message: vec![(DiagnosticMessage::from(msg), Style::NoStyle)],
                     span: MultiSpan::new(),
                     render_span: None,
                 });
diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index 3dec0d9299c..6c5f3e62454 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -628,7 +628,7 @@ impl Handler {
         message: DiagnosticMessage,
         args: impl Iterator<Item = DiagnosticArg<'a, 'static>>,
     ) -> SubdiagnosticMessage {
-        SubdiagnosticMessage::Eager(self.eagerly_translate_to_string(message, args))
+        SubdiagnosticMessage::Eager(Cow::from(self.eagerly_translate_to_string(message, args)))
     }
 
     /// Translate `message` eagerly with `args` to `String`.
@@ -1450,14 +1450,14 @@ impl HandlerInner {
         self.emit_stashed_diagnostics();
 
         let warnings = match self.deduplicated_warn_count {
-            0 => String::new(),
-            1 => "1 warning emitted".to_string(),
-            count => format!("{count} warnings emitted"),
+            0 => Cow::from(""),
+            1 => Cow::from("1 warning emitted"),
+            count => Cow::from(format!("{count} warnings emitted")),
         };
         let errors = match self.deduplicated_err_count {
-            0 => String::new(),
-            1 => "aborting due to previous error".to_string(),
-            count => format!("aborting due to {count} previous errors"),
+            0 => Cow::from(""),
+            1 => Cow::from("aborting due to previous error"),
+            count => Cow::from(format!("aborting due to {count} previous errors")),
         };
         if self.treat_err_as_bug() {
             return;