Don't suggest changing {ImmRef,MutRef} implicit self to be mutable

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -416,12 +416,28 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                         _,
                     ) = pat.kind
                 {
-                    err.span_suggestion(
-                        upvar_ident.span,
-                        "consider changing this to be mutable",
-                        format!("mut {}", upvar_ident.name),
-                        Applicability::MachineApplicable,
-                    );
+                    if upvar_ident.name == kw::SelfLower {
+                        for (_, node) in self.infcx.tcx.hir().parent_iter(upvar_hir_id) {
+                            if let Some(fn_decl) = node.fn_decl() {
+                                if !matches!(fn_decl.implicit_self, hir::ImplicitSelfKind::ImmRef | hir::ImplicitSelfKind::MutRef) {
+                                    err.span_suggestion(
+                                        upvar_ident.span,
+                                        "consider changing this to be mutable",
+                                        format!("mut {}", upvar_ident.name),
+                                        Applicability::MachineApplicable,
+                                    );
+                                    break;
+                                }
+                            }
+                        }
+                    } else {
+                        err.span_suggestion(
+                            upvar_ident.span,
+                            "consider changing this to be mutable",
+                            format!("mut {}", upvar_ident.name),
+                            Applicability::MachineApplicable,
+                        );
+                    }
                 }
 
                 let tcx = self.infcx.tcx;

tests/ui/borrowck/issue-111554.rs

@@ -0,0 +1,28 @@
+struct Foo {}
+
+impl Foo {
+    pub fn foo(&mut self) {
+        || bar(&mut self);
+        //~^ ERROR cannot borrow `self` as mutable, as it is not declared as mutable
+    }
+
+    pub fn baz(&self) {
+        || bar(&mut self);
+        //~^ ERROR cannot borrow `self` as mutable, as it is not declared as mutable
+        //~| ERROR cannot borrow data in a `&` reference as mutable
+    }
+
+    pub fn qux(mut self) {
+        || bar(&mut self);
+        // OK
+    }
+
+    pub fn quux(self) {
+        || bar(&mut self);
+        //~^ ERROR cannot borrow `self` as mutable, as it is not declared as mutable
+    }
+}
+
+fn bar(_: &mut Foo) {}
+
+fn main() {}

tests/ui/borrowck/issue-111554.stderr

@@ -0,0 +1,29 @@
+error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
+  --> $DIR/issue-111554.rs:5:16
+   |
+LL |         || bar(&mut self);
+   |                ^^^^^^^^^ cannot borrow as mutable
+
+error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
+  --> $DIR/issue-111554.rs:10:16
+   |
+LL |         || bar(&mut self);
+   |                ^^^^^^^^^ cannot borrow as mutable
+
+error[E0596]: cannot borrow data in a `&` reference as mutable
+  --> $DIR/issue-111554.rs:10:16
+   |
+LL |         || bar(&mut self);
+   |                ^^^^^^^^^ cannot borrow as mutable
+
+error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
+  --> $DIR/issue-111554.rs:21:16
+   |
+LL |     pub fn quux(self) {
+   |                 ---- help: consider changing this to be mutable: `mut self`
+LL |         || bar(&mut self);
+   |                ^^^^^^^^^ cannot borrow as mutable
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0596`.