Closed
Description
Finally getting around to filing this issue at @estebank's encouragement in this (rather embarrassing) Reddit thread. CC @WalterSmuts and @pali6.
Code
I've tried to capture things in this Playground link; to summarize:
struct Foo;
impl std::ops::Mul for &Foo {
type Output = Foo;
fn mul(self, _rhs: Self) -> Self::Output {
unimplemented!()
}
}
#[test]
fn it_works() {
let ref_mut_foo: &mut Foo = &mut Foo;
let ref_foo: &Foo = &Foo;
let owned_foo: Foo = Foo;
// The following assignments cause compiler errors that can be resolved by
// getting immutable borrows of `ref_mut_foo`'s pointee (i.e.,
// `&*ref_mut_foo`):
let _ = ref_mut_foo * ref_foo;
let _ = ref_mut_foo * ref_mut_foo;
let _ = ref_mut_foo * &owned_foo;
// …but we currently miss the opportunity to inform a user of the
// possibility (😢).
}Current output
error[E0369]: cannot multiply `&mut Foo` by `&Foo`
--> src/lib.rs:20:25
|
20 | let _ = ref_mut_foo * ref_foo;
| ----------- ^ ------- &Foo
| |
| &mut Foo
error[E0369]: cannot multiply `&mut Foo` by `&mut Foo`
--> src/lib.rs:21:25
|
21 | let _ = ref_mut_foo * ref_mut_foo;
| ----------- ^ ----------- &mut Foo
| |
| &mut Foo
error[E0369]: cannot multiply `&mut Foo` by `&Foo`
--> src/lib.rs:22:25
|
22 | let _ = ref_mut_foo * &owned_foo;
| ----------- ^ ---------- &Foo
| |
| &mut FooDesired output
error[E0369]: cannot multiply `&mut Foo` by `&Foo`
--> src/lib.rs:20:25
|
20 | let _ = ref_mut_foo * ref_foo;
| ----------- ^ ------- &Foo
| |
| &mut Foo
|
= help: consider re-borrowing here: `&*ref_mut_foo * ref_foo`
error[E0369]: cannot multiply `&mut Foo` by `&mut Foo`
--> src/lib.rs:21:25
|
21 | let _ = ref_mut_foo * ref_mut_foo;
| ----------- ^ ----------- &mut Foo
| |
| &mut Foo
|
= help: consider re-borrowing here: `&*ref_mut_foo * &*ref_mut_foo`
error[E0369]: cannot multiply `&mut Foo` by `&Foo`
--> src/lib.rs:22:25
|
22 | let _ = ref_mut_foo * &owned_foo;
| ----------- ^ ---------- &Foo
| |
| &mut Foo
|
= help: consider re-borrowing here: `&*ref_mut_foo * &owned_foo`Rationale and extra context
It seems like this should generally be a valid suggestion, and we have to create a diagnostic for this; current error output doesn't help the user to discover the "right" solution at all.
Other cases
No response
Anything else?
No response