Add new rustc_errors::DiagDecorator type

compiler/rustc_errors/src/diagnostic.rs

@@ -128,6 +128,17 @@ fn into_diag(self, dcx: DiagCtxtHandle<'a>, level: Level) -> Diag<'a, ()> {
     }
 }
 
+/// Type used to emit diagnostic through a closure instead of implementing the `Diagnostic` trait.
+pub struct DiagDecorator<F: FnOnce(&mut Diag<'_, ()>)>(pub F);
+
+impl<'a, F: FnOnce(&mut Diag<'_, ()>)> Diagnostic<'a, ()> for DiagDecorator<F> {
+    fn into_diag(self, dcx: DiagCtxtHandle<'a>, level: Level) -> Diag<'a, ()> {
+        let mut diag = Diag::new(dcx, level, "");
+        (self.0)(&mut diag);
+        diag
+    }
+}
+
 /// Trait implemented by error types. This should not be implemented manually. Instead, use
 /// `#[derive(Subdiagnostic)]` -- see [rustc_macros::Subdiagnostic].
 #[rustc_diagnostic_item = "Subdiagnostic"]

compiler/rustc_errors/src/lib.rs

@@ -36,8 +36,8 @@
 pub use codes::*;
 pub use decorate_diag::{BufferedEarlyLint, DecorateDiagCompat, LintBuffer};
 pub use diagnostic::{
-    BugAbort, Diag, DiagInner, DiagLocation, DiagStyledString, Diagnostic, EmissionGuarantee,
-    FatalAbort, StringPart, Subdiag, Subdiagnostic,
+    BugAbort, Diag, DiagDecorator, DiagInner, DiagLocation, DiagStyledString, Diagnostic,
+    EmissionGuarantee, FatalAbort, StringPart, Subdiag, Subdiagnostic,
 };
 pub use diagnostic_impls::{
     DiagSymbolList, ElidedLifetimeInPathSubdiag, ExpectedLifetimeParameter,