Missing index out of bounds error

[varkor]

The following example no longer throws a warning about indices being out of bounds, on master:

fn main() {
    let a: [i32; 1] = [0; 1];
    a[1];
}

I expect a index out of bounds: the len is 1 but the index is 1 error. I suspect this is due to #46882.

cc @oli-obk

[oli-obk]

Yea, there's a FIXME comment in librustc_mir::transform:: const_prop for this.

[varkor]

I imagine it was specific to the old implementation, but just a note that we want to make sure #48900 doesn't reappear when this is reimplemented.

[steveklabnik]

This code still errors, but this case doesn't:

fn main() {
    let a = [1, 2, 3];
    let index = 10;
    let element = a[index];
    println!("element is {}", element);
}

is this expected?

[oli-obk]

yes. Constant propagation does not cross variables (for now). To do this correctly we'll need to detect loops and conditions. Without such a detection mechanism our current implemementation will do a few bogus things.

[paolieri]

Since rustc 1.61, the following code is not giving a compile-time error:

fn main() {
    let xs = [10, 20, 30];
    println!("{}", xs[10]);

    let _n = xs.len();
}

If I uncomment the last line, the compile-time error shows up.
Is this right? I would expect the compile-time error to show up in both cases (this is what happens for rustc < 1.61).

cc @oli-obk