Add SubdiagnosticMessageOp as a trait alias.

It avoids a lot of repetition.

compiler/rustc_trait_selection/src/errors.rs

@@ -1,7 +1,7 @@
 use crate::fluent_generated as fluent;
 use rustc_errors::{
     codes::*, AddToDiagnostic, Applicability, DiagCtxt, Diagnostic, DiagnosticBuilder,
-    EmissionGuarantee, IntoDiagnostic, Level, SubdiagnosticMessage,
+    EmissionGuarantee, IntoDiagnostic, Level, SubdiagnosticMessageOp,
 };
 use rustc_macros::Diagnostic;
 use rustc_middle::ty::{self, ClosureKind, PolyTraitRef, Ty};
@@ -102,10 +102,7 @@ pub enum AdjustSignatureBorrow {
 }
 
 impl AddToDiagnostic for AdjustSignatureBorrow {
-    fn add_to_diagnostic_with<F>(self, diag: &mut Diagnostic, _: F)
-    where
-        F: Fn(&mut Diagnostic, SubdiagnosticMessage) -> SubdiagnosticMessage,
-    {
+    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
         match self {
             AdjustSignatureBorrow::Borrow { to_borrow } => {
                 diag.arg("len", to_borrow.len());