Unit struct and enum variants are not accepted as the pattern for destructuring assignment

[ChayimFriedman2]

fn main() {
    let mut a;
    struct S;
    (S, a) = (S, ()); // Error
    (S {}, a) = (S, ()); // Works
    enum E { A }
    (E::A, a) = (E::A, ()); // Error
    (E::A {}, a) = (E::A, ()); // Works
}

error[E0070]: invalid left-hand side of assignment
 --> src/main.rs:4:12
  |
4 |     (S, a) = (S, ()); // Error
  |      -     ^
  |      |
  |      cannot assign to this expression

error[E0070]: invalid left-hand side of assignment
 --> src/main.rs:7:15
  |
7 |     (E::A, a) = (E::A, ()); // Error
  |      ----     ^
  |      |
  |      cannot assign to this expression

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=bf807e599b504f489454388eb27acbbb

The RFC states explicitly we should support them:

https://github.com/rust-lang/rfcs/blob/master/text/2909-destructuring-assignment.md#reference-level-explanation

The class of assignee expressions is defined inductively:

• Place: place.
• Underscore: _.
• Tuples: (assignee, assignee, assignee), (assignee, .., assignee), (.., assignee, assignee), (assignee, assignee, ..).
• Slices: [assignee, assignee, assignee], [assignee, .., assignee], [.., assignee, assignee], [assignee, assignee, ..].
• Tuple structs: path(assignee, assignee, assignee), path(assignee, .., assignee), path(.., assignee, assignee), path(assignee, assignee, ..).
• Structs: path { field: assignee, field: assignee }, path { field: assignee, field: assignee, .. }.
• Unit structs: path.

@rustbot label: +F-destructuring_assignment

[compiler-errors]

@rustbot claim