Add SubdiagnosticMessageOp as a trait alias.

It avoids a lot of repetition.

compiler/rustc_errors/src/diagnostic.rs

@@ -77,11 +77,12 @@ where
 
     /// Add a subdiagnostic to an existing diagnostic where `f` is invoked on every message used
     /// (to optionally perform eager translation).
-    fn add_to_diagnostic_with<F>(self, diag: &mut Diagnostic, f: F)
-    where
-        F: Fn(&mut Diagnostic, SubdiagnosticMessage) -> SubdiagnosticMessage;
+    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, f: F);
 }
 
+pub trait SubdiagnosticMessageOp =
+    Fn(&mut Diagnostic, SubdiagnosticMessage) -> SubdiagnosticMessage;
+
 /// Trait implemented by lint types. This should not be implemented manually. Instead, use
 /// `#[derive(LintDiagnostic)]` -- see [rustc_macros::LintDiagnostic].
 #[rustc_diagnostic_item = "DecorateLint"]

compiler/rustc_errors/src/diagnostic_impls.rs

@@ -2,7 +2,7 @@ use crate::diagnostic::DiagnosticLocation;
 use crate::{fluent_generated as fluent, AddToDiagnostic};
 use crate::{
     DiagCtxt, DiagnosticArgValue, DiagnosticBuilder, EmissionGuarantee, ErrCode, IntoDiagnostic,
-    IntoDiagnosticArg, Level,
+    IntoDiagnosticArg, Level, SubdiagnosticMessageOp,
 };
 use rustc_ast as ast;
 use rustc_ast_pretty::pprust;
@@ -299,7 +299,7 @@ pub struct SingleLabelManySpans {
     pub label: &'static str,
 }
 impl AddToDiagnostic for SingleLabelManySpans {
-    fn add_to_diagnostic_with<F>(self, diag: &mut crate::Diagnostic, _: F) {
+    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut crate::Diagnostic, _: F) {
         diag.span_labels(self.spans, self.label);
     }
 }

compiler/rustc_errors/src/lib.rs

@@ -19,6 +19,7 @@
 #![feature(never_type)]
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
+#![feature(trait_alias)]
 #![feature(try_blocks)]
 #![feature(yeet_expr)]
 // tidy-alphabetical-end
@@ -35,6 +36,7 @@ pub use codes::*;
 pub use diagnostic::{
     AddToDiagnostic, DecorateLint, Diagnostic, DiagnosticArg, DiagnosticArgName,
     DiagnosticArgValue, DiagnosticStyledString, IntoDiagnosticArg, StringPart, SubDiagnostic,
+    SubdiagnosticMessageOp,
 };
 pub use diagnostic_builder::{
     BugAbort, DiagnosticBuilder, EmissionGuarantee, FatalAbort, IntoDiagnostic,