rustc_borrowck: Don't suggest changing closure param type not under user control

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -1198,6 +1198,26 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             );
             return;
         }
+
+        // If `local` is a closure arg, and the type of the arg is not under
+        // local control, do not suggest to change its type.
+        if self.body.local_kind(local) == LocalKind::Arg
+            && let InstanceKind::Item(def_id) = self.body.source.instance
+            && let Some(Node::Expr(hir::Expr { hir_id, kind, .. })) =
+                self.infcx.tcx.hir_get_if_local(def_id)
+            && let ExprKind::Closure(hir::Closure { kind: hir::ClosureKind::Closure, .. }) = kind
+            && let Node::Expr(expr) = self.infcx.tcx.parent_hir_node(*hir_id)
+            && let ExprKind::MethodCall(path_segment, _, _, _) = expr.kind
+            && self
+                .infcx
+                .tcx
+                .typeck(path_segment.hir_id.owner.def_id)
+                .type_dependent_def_id(expr.hir_id)
+                .is_some_and(|def_id| !def_id.is_local())
+        {
+            return;
+        }
+
         let decl_span = local_decl.source_info.span;
 
         let (amp_mut_sugg, local_var_ty_info) = match *local_decl.local_info() {

tests/ui/borrowck/issue-115259-suggest-iter-mut.stderr

@@ -2,9 +2,7 @@ error[E0596]: cannot borrow `**layer` as mutable, as it is behind a `&` referenc
   --> $DIR/issue-115259-suggest-iter-mut.rs:15:65
    |
 LL |         self.layers.iter().fold(0, |result, mut layer| result + layer.process())
-   |                                             ---------           ^^^^^ `layer` is a `&` reference, so it cannot be borrowed as mutable
-   |                                             |
-   |                                             consider changing this binding's type to be: `&mut Box<dyn Layer>`
+   |                                                                 ^^^^^ `layer` is a `&` reference, so it cannot be borrowed as mutable
    |
 help: you may want to use `iter_mut` here
    |

tests/ui/borrowck/issue-62387-suggest-iter-mut-2.stderr

@@ -2,9 +2,7 @@ error[E0596]: cannot borrow `*container` as mutable, as it is behind a `&` refer
   --> $DIR/issue-62387-suggest-iter-mut-2.rs:30:45
    |
 LL |             vec.iter().flat_map(|container| container.things()).cloned().collect::<Vec<PathBuf>>();
-   |                                  ---------  ^^^^^^^^^ `container` is a `&` reference, so it cannot be borrowed as mutable
-   |                                  |
-   |                                  consider changing this binding's type to be: `&mut Container`
+   |                                             ^^^^^^^^^ `container` is a `&` reference, so it cannot be borrowed as mutable
    |
 help: you may want to use `iter_mut` here
    |

tests/ui/borrowck/issue-62387-suggest-iter-mut.stderr

@@ -2,9 +2,7 @@ error[E0596]: cannot borrow `*a` as mutable, as it is behind a `&` reference
   --> $DIR/issue-62387-suggest-iter-mut.rs:18:27
    |
 LL |     v.iter().for_each(|a| a.double());
-   |                        -  ^ `a` is a `&` reference, so it cannot be borrowed as mutable
-   |                        |
-   |                        consider changing this binding's type to be: `&mut A`
+   |                           ^ `a` is a `&` reference, so it cannot be borrowed as mutable
    |
 help: you may want to use `iter_mut` here
    |
@@ -15,9 +13,7 @@ error[E0596]: cannot borrow `*a` as mutable, as it is behind a `&` reference
   --> $DIR/issue-62387-suggest-iter-mut.rs:25:39
    |
 LL |     v.iter().rev().rev().for_each(|a| a.double());
-   |                                    -  ^ `a` is a `&` reference, so it cannot be borrowed as mutable
-   |                                    |
-   |                                    consider changing this binding's type to be: `&mut A`
+   |                                       ^ `a` is a `&` reference, so it cannot be borrowed as mutable
    |
 help: you may want to use `iter_mut` here
    |

tests/ui/borrowck/option-inspect-mutation.rs

@@ -0,0 +1,13 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/128381>.
+
+struct Struct {
+    field: u32,
+}
+
+fn main() {
+    let mut some_struct = Some(Struct { field: 42 });
+    some_struct.as_mut().inspect(|some_struct| {
+        some_struct.field *= 10; //~ ERROR cannot assign to `some_struct.field`, which is behind a `&` reference
+        // Users can't change type of `some_struct` param, so above error must not suggest it.
+    });
+}

tests/ui/borrowck/option-inspect-mutation.stderr

@@ -0,0 +1,9 @@
+error[E0594]: cannot assign to `some_struct.field`, which is behind a `&` reference
+  --> $DIR/option-inspect-mutation.rs:10:9
+   |
+LL |         some_struct.field *= 10;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^ `some_struct` is a `&` reference, so it cannot be written to
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0594`.

tests/ui/borrowck/suggest-as-ref-on-mut-closure.stderr

@@ -18,9 +18,7 @@ error[E0596]: cannot borrow `*cb` as mutable, as it is behind a `&` reference
   --> $DIR/suggest-as-ref-on-mut-closure.rs:12:26
    |
 LL |     cb.as_ref().map(|cb| cb());
-   |                      --  ^^ `cb` is a `&` reference, so it cannot be borrowed as mutable
-   |                      |
-   |                      consider changing this binding's type to be: `&mut &mut dyn FnMut()`
+   |                          ^^ `cb` is a `&` reference, so it cannot be borrowed as mutable
 
 error: aborting due to 2 previous errors