Spurious attempt to load a crate resulting in "multiple rlib candidates for debug_unreachable found".

@petrochenkov

I've reduced a triggering example to this:

use a::A;
type B<'a, T> = C<&'a T>;

Producing (on playground, in Rust 2018 mode, only on nightly):

error[E0465]: multiple rlib candidates for `debug_unreachable` found
  |
note: candidate #1: /playground/target/debug/deps/libdebug_unreachable-dc2ba43f474707f3.rlib
note: candidate #2: /playground/target/debug/deps/libdebug_unreachable-fedb7fe62f71c3f0.rlib

The names in the testcase appear to not matter, but I can't figure out why both a lifetime and a type parameter need to be present, or any of the other details.

My guess this is from the rustc_resolve speculative loading of crates, to give better suggestions, which I must've not made foolproof (I thought I made all the errors lazy, but maybe this from a different spot that I missed).

cc @petrochenkov

Crate loading is certainly not fully speculative, at leas with regards to non-existent paths (#55103), so it's possible that some other errors are also reported eagerly.

This appears to be fixed in nightly, but the original issue is now in beta.

Edit: Actually, using the non-existent import does still trigger the bug on nightly.

use a::A;
struct S {
    a: A,
}

playground

And stable 1.31.1

Stable (1.33.0), Beta (1.34.0-beta1), and Nightly (1.35.0-nightly 2019-03-14 bc44841) all continue to experience this problem.

Here's an example that reproduces the problem:

[package]
name = "repro"
version = "0.1.0"
edition = "2018"

[dependencies]
debug_unreachable = "0.1.1"
new_debug_unreachable = "1.0.3"

use std::rc::Rc;

struct Wat(Rc<Derp>);

fn main() {
  let mut b: [i32; 10] = Default::default();
}

   Compiling repro v0.1.0 (/private/tmp/r)
error[E0465]: multiple rlib candidates for `debug_unreachable` found
  |
note: candidate #1: /private/tmp/r/target/debug/deps/libdebug_unreachable-9846113be7cd0a2b.rlib
note: candidate #2: /private/tmp/r/target/debug/deps/libdebug_unreachable-843675cb387b3fdc.rlib

error[E0412]: cannot find type `Derp` in this scope
 --> src/main.rs:3:15
  |
3 | struct Wat(Rc<Derp>);
  |               ^^^^ not found in this scope

Of interest is that edition = "2018" is required to trigger the error

This is now on stable, but it appears fixed on nightly

but it appears fixed on nightly.

My code example continues to have the error in nightly on the playground.

The code example in #56590 (comment) still fails on nightly as well, but in Release mode only, while some examples from other comments fail in Debug only, and finally there are also examples (such as Shep's) failing in both Debug and Release.

Is there a known workaround for this? I am still seeing this issue with 1.44.0-nightly.

Fixed in #74071.

Rollup merge of rust-lang#74071 - petrochenkov:cload3, r=matthewjasper

rust-timer simulated merge of 43ba840

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Spurious attempt to load a crate resulting in "multiple rlib candidates for `debug_unreachable` found". #56590

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Spurious attempt to load a crate resulting in "multiple rlib candidates for debug_unreachable found". #56590

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petrochenkov commented on Dec 7, 2018

Arnavion commented on Dec 30, 2018

shepmaster commented on Dec 30, 2018

shepmaster commented on Mar 16, 2019

jethrogb commented on Mar 28, 2019

shepmaster commented on Mar 28, 2019

lqd commented on Mar 29, 2019

geraldarthurdavis commented on Mar 20, 2020

petrochenkov commented on Jul 5, 2020

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Spurious attempt to load a crate resulting in "multiple rlib candidates for `debug_unreachable` found". #56590