allow refs in our constant folder

[llogiq]

This helps with #3804 (but won't completely fix it).

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changelog: none

[rust-highfive]

r? @matthiaskrgr

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[ebroto]

Even though this is a nice improvement in the constant folder, and catches some false positives in the float_cmp lint, assert_eq is still linted when it shouldn't.

This example:

const ZERO: f32 = 0.0;

fn main() {
    let offset = ZERO;
    assert_eq!(offset, ZERO);
}

(playground)

expands to:

// ...
const ZERO: f32 = 0.0;

fn main() {
    let offset = ZERO;
    {
        match (&offset, &ZERO) {
            (left_val, right_val) => {
                if !(*left_val == *right_val) {
                    // panicking code
                }
            }
        }
    };
}

So to fix this the constant folder should be taught about the intermediate bindings of the match.

Would you be willing to add this enhancement in this PR so we can close that issue? We can also land this as is for the moment, as I said it's already an improvement.

[llogiq]

I'll add deref, but match is a lot more complex; this will require introducing binding scope. I'd like to leave this to a follow-up PR.

[llogiq]

@ebroto r?

[ebroto]

I'll add deref, but match is a lot more complex; this will require introducing binding scope. I'd like to leave this to a follow-up PR.

Seems fair. Thanks for the enhancement!

[ebroto]

@bors r+

[bors]

📌 Commit 26e4de9 has been approved by ebroto

[bors]

⌛ Testing commit 26e4de9 with merge 0b86340...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: ebroto
Pushing 0b86340 to master...