Make _ an expression, to discard values in destructuring assignments

[fanzier]

This is the third and final step towards implementing destructuring assignment (RFC: rust-lang/rfcs#2909, tracking issue: #71126). This PR is the third and final part of #71156, which was split up to allow for easier review.

With this PR, an underscore _ is parsed as an expression but is allowed only on the left-hand side of a destructuring assignment. There it simply discards a value, similarly to the wildcard _ in patterns. For instance,

(a, _) = (1, 2)

will simply assign 1 to a and discard the 2. Note that for consistency,

_ = foo

is also allowed and equivalent to just foo.

Thanks to @varkor who helped with the implementation, particularly around pre-expansion gating.

r? @petrochenkov

[petrochenkov]

r=me after squashing commits.

[fanzier]

Squashed.

[varkor]

@bors r=petrochenkov rollup

[bors]

📌 Commit 419c515 has been approved by petrochenkov

[fanzier]

(just added a forgotten test (a, _) = (8, 9) in src/test/ui/destructuring-assignment/tuple_destructure.rs)

[varkor]

@bors r=petrochenkov

[bors]

📌 Commit 8cf3564 has been approved by petrochenkov

[RalfJung]

Is drop order here consistent with let _? I recall being quite surprised by that one day, I thought let _ = ... was equivalent to let _name = ... and then never using _name, but this is actually not equivalent. _ drops immediately.

[fanzier]

@RalfJung I'm not sure whether this is what you mean but I just tested the following code:

#![feature(destructuring_assignment)]

struct Bomb(&'static str);
impl Drop for Bomb {
    fn drop(&mut self) {
        println!("{}", self.0);
    }
}

fn main() {
    let _val;
    let x;
    _ = Bomb("bomb 1");
    _val = Bomb("bomb 2");
    (x, _) = (Bomb("bomb 3"), Bomb("bomb 4"));
    drop(Bomb("bomb 5"));
}

and it prints

bomb 1
bomb 4
bomb 5
bomb 3
bomb 2

just like the version with let. So I think everything is fine?

[RalfJung]

Yes that sounds great. :) Would be good to have a testcase for this. Can you add one in a follow-up PR?

[fanzier]

@RalfJung Sure! What's the best way of converting this to a test? Is there a way to specify the expected output of a test program? The UI tests I know just compare the compiler output to what's expected.

[RalfJung]

The UI tests I know just compare the compiler output to what's expected.

Ah right, I keep forgetting that.
Here's one way to do this: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=fa4dedeb937020116f2b7123a9409a2d