Duplicated errors on UB inside a promoted

[estebank]

The following output should only emit the error once:

#![warn(const_err)]

fn main() {
    &{ [1, 2, 3][4] };
}

error: index out of bounds: the len is 3 but the index is 4
  --> $DIR/array-literal-index-oob.rs:2:7
   |
LL |     &{[1, 2, 3][4]};
   |       ^^^^^^^^^^^^
   |
   = note: `#[deny(const_err)]` on by default

error: reaching this expression at runtime will panic or abort
  --> $DIR/array-literal-index-oob.rs:2:7
   |
LL |     &{[1, 2, 3][4]};
   |     --^^^^^^^^^^^^-
   |       |
   |       index out of bounds: the len is 3 but the index is 4

error: aborting due to 2 previous errors

Follow up to #61598

[estebank]

There are only two now.

[RalfJung]

One of the two errors is coming from evaluating the Assert MIR terminator that does the bounds-check and panics if it fails. The other message comes from actually evaluating the place[4] even though the index is out-of-bounds.

What is surprising here is that the evaluation of the place expression even happens even though that should be dead code. @oli-obk @wesleywiser any idea?

(This is not fixed by #66927.)

[RalfJung]

What is surprising here is that the evaluation of the place expression even happens even though that should be dead code.

The reason is that the MIR of the promoted actually looks like this:

promoted[0] in  main: i32 = {
    let mut _0: i32;                     // return place in scope 0 at src/main.rs:3:5: 3:20
    let mut _1: i32;                     // in scope 0 at src/main.rs:3:6: 3:20
    let mut _2: [i32; 3];                // in scope 0 at src/main.rs:3:7: 3:16
    let mut _3: usize;                   // in scope 0 at src/main.rs:3:17: 3:18

    bb0: {
        _2 = [const 1i32, const 2i32, const 3i32]; // bb0[0]: scope 0 at src/main.rs:3:7: 3:16
        _3 = const 4usize;               // bb0[1]: scope 0 at src/main.rs:3:17: 3:18
        _1 = _2[_3];                     // bb0[2]: scope 0 at src/main.rs:3:7: 3:19
        _0 = move _1;                    // bb0[3]: scope 0 at src/main.rs:3:5: 3:20
        return;                          // bb0[4]: scope 0 at src/main.rs:3:5: 3:20
    }
}

Notice the unchecked array access. That's how this is not dead code.

I assume the other error comes from ConstProp (likely of the Assert terminator), but I am not sure.

[RalfJung]

Cc @ecstatic-morse @eddyb because promotion is involved

[eddyb]

I don't think const_err should include promoteds, since any checks for anything they might do are still in the runtime code (doing anything else would require something much more complex than what we have today).

But also, if we constant-fold the assert, we can know that using the promoted is dead code (I guess this doesn't apply to codegen generating those warnings instead of constant folding).

[RalfJung]

@eddyb so basically, const_prop should cease to emit warnings when it is working inside a promoted? Or maybe it shouldn't run there at all?
Cc @wesleywiser