ICE on loop {|_: [_; continue]| {}}

[est31]

The following code ICEs on nightly, stable and beta:

fn main() {
    loop {|_: [_; continue]| {}}
}

The ICE message is:

error: internal compiler error: librustc_mir/hair/cx/expr.rs:551: invalid loop id for continue: not inside loop scope

As of filing this bug, PR #51731 does not fix it. If I apply it, I'm getting:

error: internal compiler error: librustc_mir/build/scope.rs:552: no enclosing breakable scope found
  --> $DIR/issue-XXXX.rs:12:19
   |
LL |     loop {|_: [_; continue]| {}}
   |                   ^^^^^^^^

[varkor]

Counterintuitively, this should probably compile successfully. It's not surprising that it's not working, though: it's a really weird edge case. I'll look into it.

[hanna-kruppe]

I don't think it should compile successfully. The continue is in an array length constant expression inside the closure's signature. Therefore, it's not part of the loop body and should cause an "continue outside of loop" error.

[varkor]

I feel perhaps control-flow should just not be allowed inside type-level constants like this. That seems like the most logical decision. In which case, we should just add a new error for this.

Edit: Just saw @rkruppe's comment. I agree, though I'm not sure there's precedent for that or not? Is there an existing error we can make us of?

[hanna-kruppe]

As I said, I believe this is a plain old "continue outside of loop" errors. The continue token is lexically within the loop { ... }, but it's not part of the loop body, so it's invalid without even considering that it's supposed to be a constant. It's morally the same as loop { fn foo() { continue } }.

[DutchGhost]

What about the break version?
Not sure if the fix is applied in the latest nightly, but this still ICE's:
#51707

Rewritting it using a loop {} instead of while:

fn main() {
    loop {
        |_: [_; break] | {}
    }
}

[est31]

@DutchGhost that's fixed by @varkor 's PR #51731 . The fix is not in nightly yet, not even merged. I've applied the PR locally in order to test whether the loop {loop {|_: [_; continue]| {}} } bug still exists.

It would probably make sense if @varkor would also fix the break version right away.