Rust-analyzer shows an error for code that compiles

[dirkvdb]

Rust analyzer version 0.3.2593-standalone (6b2e677795 2025-08-25)
rustc version 1.89.0 (29483883e 2025-08-04)
editor extension: both vscode and zed show the same errors

For a few months I'm getting a few lines of code that are flagged as an error by rust-analyzer but the code compiles just fine with rustc. I tried to simplify the code to a minimal snippet that still triggers the error. The error reported by rust-analyzer is: cannot mutate immutable variable 'b' [E0384]

code snippet to reproduce:

pub trait A: std::ops::IndexMut<usize> {
    type T: A;
}

fn func(a: &mut impl A, b: &mut [i32]) {
    b[0] += 1;
}

Removing the unused argument a causes the error to go away.

[lnicola]

Saw this on my previous version, but I can't reproduce it on master:

[ChayimFriedman2]

So it's fixed? #20654 could very well fix this. What's weird is that OP isn't using an unstable version of r-a.

[lnicola]

I think it predates the next solver, I just tested on the latest stable version and it's there.

[ShoyuVanilla]

I think it predates the next solver, I just tested on the latest stable version and it's there.

Yes, I can reproduce this with stable releases.

[jsonDoge]

Same, showed old issues, reverting back to 0.3.2577 fixed it for me

[ChayimFriedman2]

Can't reproduce on master, so closing.

[lnicola]

Might be worth adding a test.

[ChayimFriedman2]

Done: #20802