[borrowck] Fix help on mutating &self in async fns

Previously, when rustc was provided an async function that tried to
mutate through a shared reference to an implicit self (as shown in the
ui test), rustc would suggest modifying the parameter signature
to `&mut` + the fully qualified name of the ty (in the case of the repro
`S`). If a user modified their code to match the suggestion, the
compiler would not accept it.

This commit modifies the suggestion so that when rustc is provided the
ui test that is also attached in this commit, it suggests (correctly)
`&mut self`. We try to be careful about distinguishing between implicit
and explicit self annotations, since the latter seem to be handled
correctly already.

Fixes #93093

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -488,12 +488,32 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                                     // don't create labels for compiler-generated spans
                                     Some(_) => None,
                                     None => {
-                                        let (span, suggestion) = suggest_ampmut(
-                                            self.infcx.tcx,
-                                            local_decl,
-                                            opt_assignment_rhs_span,
-                                            *opt_ty_info,
-                                        );
+                                        let (span, suggestion) = if name != kw::SelfLower {
+                                            suggest_ampmut(
+                                                self.infcx.tcx,
+                                                local_decl,
+                                                opt_assignment_rhs_span,
+                                                *opt_ty_info,
+                                            )
+                                        } else {
+                                            match local_decl.local_info.as_deref() {
+                                                Some(LocalInfo::User(ClearCrossCrate::Set(
+                                                    mir::BindingForm::Var(mir::VarBindingForm {
+                                                        opt_ty_info: None,
+                                                        ..
+                                                    }),
+                                                ))) => {
+                                                    suggest_ampmut_self(self.infcx.tcx, local_decl)
+                                                }
+                                                // explicit self (eg `self: &'a Self`)
+                                                _ => suggest_ampmut(
+                                                    self.infcx.tcx,
+                                                    local_decl,
+                                                    opt_assignment_rhs_span,
+                                                    *opt_ty_info,
+                                                ),
+                                            }
+                                        };
                                         Some((true, span, suggestion))
                                     }
                                 }

src/test/ui/borrowck/issue-93093.rs

@@ -0,0 +1,14 @@
+// edition:2018
+struct S {
+    foo: usize,
+}
+impl S {
+    async fn bar(&self) { //~ HELP consider changing this to be a mutable reference
+        //~| SUGGESTION &mut self
+        self.foo += 1; //~ ERROR cannot assign to `self.foo`, which is behind a `&` reference [E0594]
+    }
+}
+
+fn main() {
+    S { foo: 1 }.bar();
+}

src/test/ui/borrowck/issue-93093.stderr

@@ -0,0 +1,12 @@
+error[E0594]: cannot assign to `self.foo`, which is behind a `&` reference
+  --> $DIR/issue-93093.rs:8:9
+   |
+LL |     async fn bar(&self) {
+   |                  ----- help: consider changing this to be a mutable reference: `&mut self`
+LL |
+LL |         self.foo += 1;
+   |         ^^^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be written
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0594`.