Destructuring assignment doesn't work for bare unit structs

[jsgf]

I tried this code:

struct Unit;

Unit = Unit;

I expected to see this being treated as a (trivial) destructuring assignment.

Instead, it fails with:

error[E0070]: invalid left-hand side of assignment
 --> src/main.rs:3:10
  |
3 |     Unit = Unit;
  |     ---- ^
  |     |
  |     cannot assign to this expression

For more information about this error, try `rustc --explain E0070`.
error: could not compile `playground` (bin "playground") due to previous error

Likewise:

enum Enum { Unit }

Enum::Unit = Enum::Unit;

fails similarly.

On the other hand the very similar:

struct Tup0();

Tup0() = Tup0();

works as expected.

The RFC for this says:

We support the following classes of expressions:

• Tuples.
• Slices.
• Structs (including unit and tuple structs).
• Unique variants of enums.

It looks like enums are also not implemented. The PR implementing this for structs seems to overlook unit structs, and I couldn't find any place claiming to implement it for enums.

Meta

rustc --version --verbose:

rustc 1.71.1 (eb26296b5 2023-08-03)
binary: rustc
commit-hash: eb26296b556cef10fb713a38f3d16b9886080f26
commit-date: 2023-08-03
host: x86_64-unknown-linux-gnu
release: 1.71.1
LLVM version: 16.0.5

And is also present in nightly.

[jsgf]

Oh, this was supposed to have been fixed in 1.62.0 with #95380. But that implements:

struct Unit;

(Unit, _) = (Unit, 123);

which isn't how I read "We support the following classes of expressions ... structs (including unit)", as I'd expect all those classes of expression to be valid at the top-level of the assignment.

[jsgf]

Unit {} = Unit;

does work, using the "{} can be used on any unit struct" loophole.