Only suggest split_at_mut on indexing borrowck errors for std types

Author: estebank

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -2018,12 +2018,25 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     ) {
         let tcx = self.infcx.tcx;
         let hir = tcx.hir();
+
+        let has_split_at_mut = |ty: Ty<'tcx>| {
+            let ty = ty.peel_refs();
+            match ty.kind() {
+                ty::Array(..) | ty::Slice(..) => true,
+                ty::Adt(def, _) if tcx.get_diagnostic_item(sym::Vec) == Some(def.did()) => true,
+                _ if ty == tcx.types.str_ => true,
+                _ => false,
+            }
+        };
         if let ([ProjectionElem::Index(index1)], [ProjectionElem::Index(index2)])
         | (
             [ProjectionElem::Deref, ProjectionElem::Index(index1)],
             [ProjectionElem::Deref, ProjectionElem::Index(index2)],
         ) = (&place.projection[..], &borrowed_place.projection[..])
         {
+            let decl1 = &self.body.local_decls[*index1];
+            let decl2 = &self.body.local_decls[*index2];
+
             let mut note_default_suggestion = || {
                 err.help(
                     "consider using `.split_at_mut(position)` or similar method to obtain two \
@@ -2035,14 +2048,12 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 );
             };
 
-            let Some(index1) = self.find_expr(self.body.local_decls[*index1].source_info.span)
-            else {
+            let Some(index1) = self.find_expr(decl1.source_info.span) else {
                 note_default_suggestion();
                 return;
             };
 
-            let Some(index2) = self.find_expr(self.body.local_decls[*index2].source_info.span)
-            else {
+            let Some(index2) = self.find_expr(decl2.source_info.span) else {
                 note_default_suggestion();
                 return;
             };
@@ -2102,16 +2113,19 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             );
             return;
         }
+        let place_ty = PlaceRef::ty(&place.as_ref(), self.body, tcx).ty;
+        let borrowed_place_ty = PlaceRef::ty(&borrowed_place.as_ref(), self.body, tcx).ty;
+        if !has_split_at_mut(place_ty) && !has_split_at_mut(borrowed_place_ty) {
+            // Only mention `split_at_mut` on `Vec`, array and slices.
+            return;
+        }
         let Some(index1) = self.find_expr(span) else { return };
         let hir::Node::Expr(parent) = tcx.parent_hir_node(index1.hir_id) else { return };
         let hir::ExprKind::Index(..) = parent.kind else { return };
         let Some(index2) = self.find_expr(issued_span) else { return };
         let hir::Node::Expr(parent) = tcx.parent_hir_node(index2.hir_id) else { return };
         let hir::ExprKind::Index(..) = parent.kind else { return };
-        err.help(
-            "use `.split_at_mut(position)` or similar method to obtain two mutable non-overlapping \
-             sub-slices",
-        );
+        err.help("use `.split_at_mut(position)` to obtain two mutable non-overlapping sub-slices");
     }
 
     /// Suggest using `while let` for call `next` on an iterator in a for loop.

tests/ui/borrowck/borrowck-overloaded-index-autoderef.stderr

@@ -17,8 +17,6 @@ LL |     let q = &mut f[&s];
    |                  ^ second mutable borrow occurs here
 LL |     p.use_mut();
    |     - first borrow later used here
-   |
-   = help: use `.split_at_mut(position)` or similar method to obtain two mutable non-overlapping sub-slices
 
 error[E0499]: cannot borrow `f.foo` as mutable more than once at a time
   --> $DIR/borrowck-overloaded-index-autoderef.rs:53:18
@@ -29,8 +27,6 @@ LL |     let q = &mut f.foo[&s];
    |                  ^^^^^ second mutable borrow occurs here
 LL |     p.use_mut();
    |     - first borrow later used here
-   |
-   = help: use `.split_at_mut(position)` or similar method to obtain two mutable non-overlapping sub-slices
 
 error[E0502]: cannot borrow `f.foo` as mutable because it is also borrowed as immutable
   --> $DIR/borrowck-overloaded-index-autoderef.rs:65:18

tests/ui/suggestions/suggest-split-at-mut.stderr

@@ -21,7 +21,7 @@ LL |     let b = &mut foo[2..];
 LL |     a[0] = 5;
    |     ---- first borrow later used here
    |
-   = help: use `.split_at_mut(position)` or similar method to obtain two mutable non-overlapping sub-slices
+   = help: use `.split_at_mut(position)` to obtain two mutable non-overlapping sub-slices
 
 error: aborting due to 2 previous errors