TAIT: `ref mut` destructuring does not require marking variable as mutable

[matthiaskrgr]

#![feature(type_alias_impl_trait)]
#[derive(Copy, Clone)]
struct Foo((u32, u32));
fn main() {
    type T = impl Copy;
    let foo: T = Foo((2, 2));
    || {
        let Foo(ref mut cdr) = foo;
    };
}

this code fails to build with --edition=2015

error[E0596]: cannot borrow `foo.0` as mutable, as `foo` is not declared as mutable
 --> code.rs:8:17
  |
8 |         let Foo(ref mut cdr) = foo;
  |                 ^^^^^^^^^^^ cannot borrow as mutable
  |
help: consider changing this to be mutable
  |
6 |     let mut foo: T = Foo((2, 2));
  |         +++

warning: unused variable: `cdr`
 --> code.rs:8:17
  |
8 |         let Foo(ref mut cdr) = foo;
  |                 ^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_cdr`
  |
  = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

but it compiles with --edition=2021 or 2024:

warning: unused closure that must be used
 --> code.rs:7:5
  |
7 | /     || {
8 | |         let Foo(ref mut cdr) = foo;
9 | |     };
  | |_____^
  |
  = note: closures are lazy and do nothing unless called
  = note: `#[warn(unused_must_use)]` (part of `#[warn(unused)]`) on by default

UNLESS we turn on the next solver, then it fails as above on all editions again 🤔

maybe there's some edition-specific code missing from the next solver?

[lcnr]

that's just a bug with the old solver afaict? https://rust.godbolt.org/z/f8GxdETqM

This should error

[lcnr]

The new solver handles this correctly by normalizing opaque types in the defining scope. I think we should keep this as a "won't fix" as properly fixing this requires properly normalizing opaque types which is too large of a change given the impending next-solver stabilization