No warning on assigning to field of const that doesn't succeed

[CraftSpider]

I tried this code:

• Playground Link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=053c3012c504c1cd85a98ff5feeeceab

I expected to see this happen:

• Either successful assignment or a compiler error, I actually would prefer the first but I'm not sure the intended behavior either way.

Instead, this happened:

• Compile succeeds without warning, the value doesn't change.

Meta

Occurs in Stable, Beta, and Nightly builds on the playground.

No backtrace is emitted

[lcnr]

I am not completely sure if this behavior is intended. But even if this is not a bug,
we should still at least warn here.

Inline snippet

struct Foo {
    val: i32,
}

const FOO: Foo = Foo { val: 100 };

const fn bar() -> i32 {
    FOO.val = -100;
    FOO.val
}

fn main() {
    println!("{}", FOO.val);
    println!("{}", bar());
}

[tesuji]

I think this is intentional behavior base on its MIR and how const works: https://rust.godbolt.org/z/G-BPoi.

[petrochenkov]

cc rust-lang/rust-clippy#829
(Also see the many issues linked to it.)

[leonardo-m]

Is this better left in Clippy or better moved to Rustc? (I think the latter)

[CraftSpider]

Ahhh, I get it. Because it's always inlined, the left hand is exactly equivalent to <snip>.foo, which is a valid assignment, but is immediately discarded. I'd definitely prefer the compiler warning, would make it much more clear what's going on.

[LeSeulArtichaut]

Seems related to #16789. I also think we should emit an unused_assignments warn here, because this code behaves correctly, but is useless as the constant gets inlined but isn't assigned to a variable.

[Dylan-DPC-zz]

This doesn't require prioritisation. Removing it based on the discussion