Prefer DiagnosticBuilder over Diagnostic in diagnostic modifiers.

There are lots of functions that modify a diagnostic. This can be via a
`&mut Diagnostic` or a `&mut DiagnosticBuilder`, because the latter type
wraps the former and impls `DerefMut`.

This commit converts all the `&mut Diagnostic` occurrences to `&mut
DiagnosticBuilder`. This is a step towards greatly simplifying
`Diagnostic`. Some of the relevant function are made generic, because
they deal with both errors and warnings. No function bodies are changed,
because all the modifier methods are available on both `Diagnostic` and
`DiagnosticBuilder`.

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -2,7 +2,7 @@
 #![allow(rustc::untranslatable_diagnostic)]
 
 use hir::ExprKind;
-use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::Node;
@@ -540,7 +540,12 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         }
     }
 
-    fn suggest_map_index_mut_alternatives(&self, ty: Ty<'tcx>, err: &mut Diagnostic, span: Span) {
+    fn suggest_map_index_mut_alternatives(
+        &self,
+        ty: Ty<'tcx>,
+        err: &mut DiagnosticBuilder<'tcx>,
+        span: Span,
+    ) {
         let Some(adt) = ty.ty_adt_def() else { return };
         let did = adt.did();
         if self.infcx.tcx.is_diagnostic_item(sym::HashMap, did)
@@ -548,7 +553,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         {
             struct V<'a, 'tcx> {
                 assign_span: Span,
-                err: &'a mut Diagnostic,
+                err: &'a mut DiagnosticBuilder<'tcx>,
                 ty: Ty<'tcx>,
                 suggested: bool,
             }
@@ -790,7 +795,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         tcx: TyCtxt<'_>,
         closure_local_def_id: hir::def_id::LocalDefId,
         the_place_err: PlaceRef<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
     ) {
         let tables = tcx.typeck(closure_local_def_id);
         if let Some((span, closure_kind_origin)) = tcx.closure_kind_origin(closure_local_def_id) {
@@ -852,7 +857,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 
     // Attempt to search similar mutable associated items for suggestion.
     // In the future, attempt in all path but initially for RHS of for_loop
-    fn suggest_similar_mut_method_for_for_loop(&self, err: &mut Diagnostic, span: Span) {
+    fn suggest_similar_mut_method_for_for_loop(&self, err: &mut DiagnosticBuilder<'_>, span: Span) {
         use hir::{
             BorrowKind, Expr,
             ExprKind::{AddrOf, Block, Call, MethodCall},
@@ -936,7 +941,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
     }
 
     /// Targeted error when encountering an `FnMut` closure where an `Fn` closure was expected.
-    fn expected_fn_found_fn_mut_call(&self, err: &mut Diagnostic, sp: Span, act: &str) {
+    fn expected_fn_found_fn_mut_call(&self, err: &mut DiagnosticBuilder<'_>, sp: Span, act: &str) {
         err.span_label(sp, format!("cannot {act}"));
 
         let hir = self.infcx.tcx.hir();