Rust: Recent nightly doesn't support array length from indirectly referenced trait constant

[azriel91]

Hiya, on rustc 1.42.0-nightly (a9dd56ff9 2019-12-30), compilation fails when defining an array whose length is from a constant from a trait from an associated type. The code compiles fine on rustc 1.42.0-nightly (0de96d37f 2019-12-19).

trait TraitA {
    const VALUE: usize;
}

struct A;
impl TraitA for A {
    const VALUE: usize = 1;
}

trait TraitB {
    type MyA: TraitA;
    const VALUE: usize = Self::MyA::VALUE;
}

struct B;
impl TraitB for B {
    type MyA = A;
}

fn main() {
    let _ = [0; A::VALUE]; // ok
    let _ = [0; B::VALUE]; // fails

    // error: array lengths can't depend on generic parameters
    //   --> src/main.rs:22:17
    //    |
    // 22 |     let _ = [0; B::VALUE];
    //    |                 ^^^^^^^^
    //
}

The error message:

error: array lengths can't depend on generic parameters
  --> src/main.rs:22:17
   |
22 |     let _ = [0; B::VALUE];
   |                 ^^^^^^^^

It seems similar to https://github.com/rust-lang/rust/issues/67739, but in this case, size_of isn't being used. It may also be a duplicate of https://github.com/rust-lang/rust/issues/43408.

It also compiles fine on playpen nightly: 2019-12-29 da3629b05.

[skinny121]

Ouput from cargo-bisect-rustc

Regression found in the compiler

searched nightlies: from nightly-2019-12-29 to nightly-2019-12-31
regressed nightly: nightly-2019-12-31
searched commits: from https://github.com/rust-lang/rust/commit/da3629b05f8f1b425a738bfe9fe9aedd47c5417a to https://github.com/rust-lang/rust/commit/a9dd56ff9a08d74c53d5cc22d18f126a12749608
regressed commit: https://github.com/rust-lang/rust/commit/214548b8afd94924c17dea456c8077931009b944

[jonas-schievink]

cc @oli-obk

[pnkfelix]

triage P-high, removing nomination.

[pnkfelix]

A user reports that this is blocking the Rust Playground from updating its nightly. (See https://github.com/integer32llc/rust-playground/issues/560 )

Nominating for discussion at today's meeting.

[oli-obk]

Sorry everyone, I totally forgot about this. A fix PR is up now.