Given code like `v[&field].boo();` where `field: String` and
`.boo(&mut self)`, typeck will have decided that `v` is accessed using
`Index`, but when `boo` adds a new `mut` obligation,
`convert_place_op_to_mutable` is called. When this happens, for *some
reason* the arguments' dereference adjustments are completely ignored
causing an error saying that `IndexMut` is not satisfied:
```
error[E0596]: cannot borrow data in an index of `Indexable` as mutable
--> src/main.rs:30:5
|
30 | v[&field].boo();
| ^^^^^^^^^ cannot borrow as mutable
|
= help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `Indexable`
```
This is not true, but by changing `try_overloaded_place_op` to retry
when given `Needs::MutPlace` without passing the argument types, the
example successfully compiles.
I believe there might be more appropriate ways to deal with this.
match expr.kind {
hir::ExprKind::Index(ref base_expr, ref index_expr) => {
- let index_expr_ty = self.node_ty(index_expr.hir_id);
+ // We need to get the final type in case dereferences were needed for the trait
+ // to apply (#72002).
+ let index_expr_ty = self.tables.borrow().expr_ty_adjusted(index_expr);
self.convert_place_op_to_mutable(
PlaceOp::Index,
expr,
--- /dev/null
+// check-pass
+struct Indexable;
+
+impl Indexable {
+ fn boo(&mut self) {}
+}
+
+impl std::ops::Index<&str> for Indexable {
+ type Output = Indexable;
+
+ fn index(&self, field: &str) -> &Indexable {
+ self
+ }
+}
+
+impl std::ops::IndexMut<&str> for Indexable {
+ fn index_mut(&mut self, field: &str) -> &mut Indexable {
+ self
+ }
+}
+
+fn main() {
+ let mut v = Indexable;
+ let field = "hello".to_string();
+
+ v[field.as_str()].boo();
+
+ v[&field].boo(); // < This should work
+}