"constant evaluation error" points to nothing when crossing the crate boundary

[shepmaster]

These error markers point to nothing, which is less than ideal. The note does at least help somewhat.

error[E0080]: constant evaluation error
 --> src/lib.rs:5:30
  |
5 |
  |                              ^^^^^^^^^^^ unimplemented constant expression: enum variants
  |
note: for repeat count here
 --> examples/main.rs:4:22
  |
4 |     let test_x = [0; constenum::CRATE_ONE];
  |                      ^^^^^^^^^^^^^^^^^^^^

---

src/lib.rs

pub enum MyEnum {
    ONE = 1,
}

pub const CRATE_ONE: usize = MyEnum::ONE as usize;

examples/main.rs

extern crate constenum;

fn main() {
    let test_x = [0; constenum::CRATE_ONE];
}

Execute via cargo build --example=main

• rustc 1.14.0 (e8a012324 2016-12-16)
• Originally from Stack Overflow

[kennytm]

Same as #23898?

[shepmaster]

@kennytm that looks like the underlying issue (non-constness) thanks! But this particular issue was meant to focus on the silly error message specifically. I assume that is easier to fix than the constness ;-)

[eddyb]

@nikomatsakis @jonathandturner So how do we get sources cross-crate? I'd love to just have the snippet to print instead of hiding it somehow.

[nikomatsakis]

@eddyb so it does seem plausible to store where the sources were, I guess, and re-load them...but how do we know if they've changed in the interim?

[eddyb]

We could hash each file, perhaps?

[nikomatsakis]

@eddyb yeah I suppose so. seems like a non-trivial PR in any case.

[oli-obk]

Storing the place where the sources are won't help, since with const fn, an error can hypothetically occur in the standard library (like if b = 0 in a/b in const fn div(a: i32, b: i32) { a / b }), and the sources are not available then (especially not in the location given by debuginfo).

[eddyb]

My point is that they should be. Most linux distros have a notion of "debuginfo" packages and "debugsource" ones, with the former using paths installed by the latter. In NixOS you can just install the source of a package which is how the racer support works (in that case there is a hash in the path so it can only ever refer to one exact version of the files, so if we did hashing ourselves it'd always match).

[Mark-Simulacrum]

So, this example compiles successfully today. I'm failing at coming up with an example that doesn't... could someone comment with one?

[shepmaster]

So, this example compiles successfully today

It has a non-blank error message, I assume you mean? With rustc 1.17.0 (56124baa9 2017-04-24), I get:

error[E0080]: constant evaluation error
 --> examples/main.rs:4:22
  |
4 |     let test_x = [0; constexprcrate::CRATE_ONE];
  |                      ^^^^^^^^^^^ unimplemented constant expression: enum variants

It seems like it shouldn't have 11 ^; maybe that's because that's the length of MyEnum::ONE in the library crate's source?

The error is still misleading as-is. It points at the client's use of a const usize, but complains it is an enum variant.

It seems this issue is still valid.

[Mark-Simulacrum]

Now I'm very confused. I had it compiling fine... I thought.