ICE using a trait-associated const in an array size

[quantheory]

This only seems to be triggered if the size has to be evaluated by typeck; for instance you can use size_of on such an array without triggering the issue.

Test case:

// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

#![feature(associated_consts)]

struct Foo;

impl Foo {
    const SIZE: usize = 8;
}

trait Bar {
    const BAR_SIZE: usize;
}

impl Bar for Foo {
    const BAR_SIZE: usize = 12;
}

#[allow(unused_variables)]
fn main() {
    let w: [u8; 12] = [0u8; <Foo as Bar>::BAR_SIZE];
    let x: [u8; 12] = [0u8; <Foo>::BAR_SIZE];
    let y: [u8; 8] = [0u8; <Foo>::SIZE];
    let z: [u8; 8] = [0u8; Foo::SIZE];
}

In fact, it's fine to use the inherent impl above (in the definitions of y and z), but the trait-associated constant causes a problem that resembles #24946 and #24938. I'm not sure if it would have the same solution, though, since this issue is encountered at an earlier stage.

[leoyvens]

Triage: This no longer ICEs.

[apasel422]

This no longer ICEs, but the E0308 errors are somewhat strange:

issue-24947.rs:29:29: 29:51 error: expected constant integer for repeat count, but non-constant path in constant expr [E0307]
issue-24947.rs:29     let w: [u8; 12] = [0u8; <Foo as Bar>::BAR_SIZE];
                                              ^~~~~~~~~~~~~~~~~~~~~~
issue-24947.rs:29:29: 29:51 help: run `rustc --explain E0307` to see a detailed explanation
issue-24947.rs:29:23: 29:52 error: mismatched types:
 expected `[u8; 12]`,
    found `[u8; 0]`
(expected an array with a fixed size of 12 elements,
    found one with 0 elements) [E0308]
issue-24947.rs:29     let w: [u8; 12] = [0u8; <Foo as Bar>::BAR_SIZE];
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
issue-24947.rs:29:23: 29:52 help: run `rustc --explain E0308` to see a detailed explanation
issue-24947.rs:31:29: 31:44 error: expected constant integer for repeat count, but non-constant path in constant expr [E0307]
issue-24947.rs:31     let x: [u8; 12] = [0u8; <Foo>::BAR_SIZE];
                                              ^~~~~~~~~~~~~~~
issue-24947.rs:31:29: 31:44 help: run `rustc --explain E0307` to see a detailed explanation
issue-24947.rs:31:23: 31:45 error: mismatched types:
 expected `[u8; 12]`,
    found `[u8; 0]`
(expected an array with a fixed size of 12 elements,
    found one with 0 elements) [E0308]
issue-24947.rs:31     let x: [u8; 12] = [0u8; <Foo>::BAR_SIZE];
                                        ^~~~~~~~~~~~~~~~~~~~~~
issue-24947.rs:31:23: 31:45 help: run `rustc --explain E0308` to see a detailed explanation
error: aborting due to 4 previous errors

[apasel422]

In particular, the error messages that indicate that the found array type has length 0 are misleading.

[steveklabnik]

This now compiles, just fine, actually. Closing!

[leoyvens]

@steveklabnik Does it still need a test?

[petrochenkov]

Still needs test.