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.

Author: nnethercote

compiler/rustc_builtin_macros/src/compile_error.rs

@@ -18,7 +18,7 @@ pub fn expand_compile_error<'cx>(
         reason = "diagnostic message is specified by user"
     )]
     #[expect(rustc::untranslatable_diagnostic, reason = "diagnostic message is specified by user")]
-    cx.span_err(sp, var.as_str());
+    cx.span_err(sp, var.to_string());
 
     DummyResult::any(sp)
 }

compiler/rustc_builtin_macros/src/errors.rs

@@ -377,7 +377,7 @@ impl<'a, G: EmissionGuarantee> IntoDiagnostic<'a, G> for EnvNotDefined {
                 rustc::untranslatable_diagnostic,
                 reason = "cannot translate user-provided messages"
             )]
-            handler.struct_diagnostic(msg.as_str())
+            handler.struct_diagnostic(msg.to_string())
         } else {
             handler.struct_diagnostic(crate::fluent_generated::builtin_macros_env_not_defined)
         };

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -893,7 +893,7 @@ fn link_natively<'a>(
                     linker_path: &linker_path,
                     exit_status: prog.status,
                     command: &cmd,
-                    escaped_output: &escaped_output,
+                    escaped_output,
                 };
                 sess.diagnostic().emit_err(err);
                 // If MSVC's `link.exe` was expected but the return code

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -1800,7 +1800,7 @@ impl SharedEmitterMain {
                     handler.emit_diagnostic(&mut d);
                 }
                 Ok(SharedEmitterMessage::InlineAsmError(cookie, msg, level, source)) => {
-                    let msg = msg.strip_prefix("error: ").unwrap_or(&msg);
+                    let msg = msg.strip_prefix("error: ").unwrap_or(&msg).to_string();
 
                     let mut err = match level {
                         Level::Error { lint: false } => sess.struct_err(msg).forget_guarantee(),

compiler/rustc_codegen_ssa/src/errors.rs

@@ -336,7 +336,7 @@ pub struct LinkingFailed<'a> {
     pub linker_path: &'a PathBuf,
     pub exit_status: ExitStatus,
     pub command: &'a Command,
-    pub escaped_output: &'a str,
+    pub escaped_output: String,
 }
 
 impl IntoDiagnostic<'_> for LinkingFailed<'_> {
@@ -345,11 +345,13 @@ impl IntoDiagnostic<'_> for LinkingFailed<'_> {
         diag.set_arg("linker_path", format!("{}", self.linker_path.display()));
         diag.set_arg("exit_status", format!("{}", self.exit_status));
 
-        diag.note(format!("{:?}", self.command)).note(self.escaped_output);
+        let contains_undefined_ref = self.escaped_output.contains("undefined reference to");
+
+        diag.note(format!("{:?}", self.command)).note(self.escaped_output.to_string());
 
         // Trying to match an error from OS linkers
         // which by now we have no way to translate.
-        if self.escaped_output.contains("undefined reference to") {
+        if contains_undefined_ref {
             diag.note(fluent::codegen_ssa_extern_funcs_not_found)
                 .note(fluent::codegen_ssa_specify_libraries_to_link)
                 .note(fluent::codegen_ssa_use_cargo_directive);

compiler/rustc_driver_impl/src/lib.rs

@@ -1258,7 +1258,7 @@ pub fn install_ice_hook(bug_report_url: &'static str, extra_info: fn(&Handler))
         if let Some(msg) = info.payload().downcast_ref::<String>() {
             if msg.starts_with("failed printing to stdout: ") && msg.ends_with("(os error 232)") {
                 // the error code is already going to be reported when the panic unwinds up the stack
-                let _ = early_error_no_abort(ErrorOutputType::default(), msg.as_str());
+                let _ = early_error_no_abort(ErrorOutputType::default(), msg.clone());
                 return;
             }
         };

compiler/rustc_error_messages/src/lib.rs

@@ -263,8 +263,7 @@ type FluentId = Cow<'static, str>;
 #[rustc_diagnostic_item = "SubdiagnosticMessage"]
 pub enum SubdiagnosticMessage {
     /// Non-translatable diagnostic message.
-    // FIXME(davidtwco): can a `Cow<'static, str>` be used here?
-    Str(String),
+    Str(Cow<'static, str>),
     /// Translatable message which has already been translated eagerly.
     ///
     /// Some diagnostics have repeated subdiagnostics where the same interpolated variables would
@@ -275,8 +274,7 @@ pub enum SubdiagnosticMessage {
     /// incorrect diagnostics. Eager translation results in translation for a subdiagnostic
     /// happening immediately after the subdiagnostic derive's logic has been run. This variant
     /// stores messages which have been translated eagerly.
-    // FIXME(#100717): can a `Cow<'static, str>` be used here?
-    Eager(String),
+    Eager(Cow<'static, str>),
     /// Identifier of a Fluent message. Instances of this variant are generated by the
     /// `Subdiagnostic` derive.
     FluentIdentifier(FluentId),
@@ -290,17 +288,17 @@ pub enum SubdiagnosticMessage {
 
 impl From<String> for SubdiagnosticMessage {
     fn from(s: String) -> Self {
-        SubdiagnosticMessage::Str(s)
+        SubdiagnosticMessage::Str(Cow::Owned(s))
     }
 }
-impl<'a> From<&'a str> for SubdiagnosticMessage {
-    fn from(s: &'a str) -> Self {
-        SubdiagnosticMessage::Str(s.to_string())
+impl From<&'static str> for SubdiagnosticMessage {
+    fn from(s: &'static str) -> Self {
+        SubdiagnosticMessage::Str(Cow::Borrowed(s))
     }
 }
 impl From<Cow<'static, str>> for SubdiagnosticMessage {
     fn from(s: Cow<'static, str>) -> Self {
-        SubdiagnosticMessage::Str(s.to_string())
+        SubdiagnosticMessage::Str(s)
     }
 }
 
@@ -312,8 +310,7 @@ impl From<Cow<'static, str>> for SubdiagnosticMessage {
 #[rustc_diagnostic_item = "DiagnosticMessage"]
 pub enum DiagnosticMessage {
     /// Non-translatable diagnostic message.
-    // FIXME(#100717): can a `Cow<'static, str>` be used here?
-    Str(String),
+    Str(Cow<'static, str>),
     /// Translatable message which has already been translated eagerly.
     ///
     /// Some diagnostics have repeated subdiagnostics where the same interpolated variables would
@@ -324,8 +321,7 @@ pub enum DiagnosticMessage {
     /// incorrect diagnostics. Eager translation results in translation for a subdiagnostic
     /// happening immediately after the subdiagnostic derive's logic has been run. This variant
     /// stores messages which have been translated eagerly.
-    // FIXME(#100717): can a `Cow<'static, str>` be used here?
-    Eager(String),
+    Eager(Cow<'static, str>),
     /// Identifier for a Fluent message (with optional attribute) corresponding to the diagnostic
     /// message.
     ///
@@ -363,17 +359,17 @@ impl DiagnosticMessage {
 
 impl From<String> for DiagnosticMessage {
     fn from(s: String) -> Self {
-        DiagnosticMessage::Str(s)
+        DiagnosticMessage::Str(Cow::Owned(s))
     }
 }
-impl<'a> From<&'a str> for DiagnosticMessage {
-    fn from(s: &'a str) -> Self {
-        DiagnosticMessage::Str(s.to_string())
+impl From<&'static str> for DiagnosticMessage {
+    fn from(s: &'static str) -> Self {
+        DiagnosticMessage::Str(Cow::Borrowed(s))
     }
 }
 impl From<Cow<'static, str>> for DiagnosticMessage {
     fn from(s: Cow<'static, str>) -> Self {
-        DiagnosticMessage::Str(s.to_string())
+        DiagnosticMessage::Str(s)
     }
 }
 

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);

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);

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,
                 });