Suggest deref non-lvalue mutable reference

Author: compiler-errors

compiler/rustc_typeck/src/check/expr.rs

@@ -836,6 +836,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         lhs: &'tcx hir::Expr<'tcx>,
         err_code: &'static str,
         op_span: Span,
+        adjust_err: impl FnOnce(&mut DiagnosticBuilder<'tcx, ErrorGuaranteed>),
     ) {
         if lhs.is_syntactic_place_expr() {
             return;
@@ -858,6 +859,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             );
         });
 
+        adjust_err(&mut err);
+
         err.emit();
     }
 
@@ -1050,9 +1053,24 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return self.tcx.ty_error();
         }
 
-        self.check_lhs_assignable(lhs, "E0070", span);
-
         let lhs_ty = self.check_expr_with_needs(&lhs, Needs::MutPlace);
+
+        self.check_lhs_assignable(lhs, "E0070", span, |err| {
+            let rhs_ty = self.check_expr(&rhs);
+
+            if let ty::Ref(_, lhs_inner_ty, hir::Mutability::Mut) = lhs_ty.kind() {
+                if self.can_coerce(rhs_ty, *lhs_inner_ty) {
+                    err.span_suggestion_verbose(
+                        lhs.span.shrink_to_lo(),
+                        "consider dereferencing here to assign to the mutable \
+                    borrowed piece of memory",
+                        "*".to_string(),
+                        Applicability::MachineApplicable,
+                    );
+                }
+            }
+        });
+
         let rhs_ty = self.check_expr_coercable_to_type(&rhs, lhs_ty, Some(lhs));
 
         self.require_type_is_sized(lhs_ty, lhs.span, traits::AssignmentLhsSized);

compiler/rustc_typeck/src/check/op.rs

@@ -41,7 +41,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 return_ty
             };
 
-        self.check_lhs_assignable(lhs, "E0067", op.span);
+        self.check_lhs_assignable(lhs, "E0067", op.span, |_| {});
 
         ty
     }

src/test/ui/typeck/assign-non-lval-mut-ref.fixed

@@ -0,0 +1,7 @@
+// run-rustfix
+
+fn main() {
+    let mut x = vec![1usize];
+    *x.last_mut().unwrap() = 2usize;
+    //~^ ERROR invalid left-hand side of assignment
+}

src/test/ui/typeck/assign-non-lval-mut-ref.rs

@@ -0,0 +1,7 @@
+// run-rustfix
+
+fn main() {
+    let mut x = vec![1usize];
+    x.last_mut().unwrap() = 2usize;
+    //~^ ERROR invalid left-hand side of assignment
+}

src/test/ui/typeck/assign-non-lval-mut-ref.stderr

@@ -0,0 +1,16 @@
+error[E0070]: invalid left-hand side of assignment
+  --> $DIR/assign-non-lval-mut-ref.rs:5:27
+   |
+LL |     x.last_mut().unwrap() = 2usize;
+   |     --------------------- ^
+   |     |
+   |     cannot assign to this expression
+   |
+help: consider dereferencing here to assign to the mutable borrowed piece of memory
+   |
+LL |     *x.last_mut().unwrap() = 2usize;
+   |     +
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0070`.

src/test/ui/typeck/issue-93486.stderr

@@ -5,6 +5,11 @@ LL |         vec![].last_mut().unwrap() = 3_u8;
    |         -------------------------- ^
    |         |
    |         cannot assign to this expression
+   |
+help: consider dereferencing here to assign to the mutable borrowed piece of memory
+   |
+LL |         *vec![].last_mut().unwrap() = 3_u8;
+   |         +
 
 error: aborting due to previous error