Regression in -Z dump-mir=all

[oli-obk]

running rustc -Z dump-mir=all src/test/run-pass/issue-33387.rs

yields

error[E0391]: unsupported cyclic reference between types/traits detected
  --> ../rust/src/test/run-pass/issue-33387.rs:19:1
   |
19 | / impl Foo for [u8; 2] {
20 | |     fn get(&self) -> [u8; 2] {
21 | |         *self
22 | |     }
23 | | }
   | |_^ cyclic reference
   |
note: the cycle begins when processing `<impl at ../rust/src/test/run-pass/issue-33387.rs:19:1: 23:2>`...
  --> ../rust/src/test/run-pass/issue-33387.rs:19:1
   |
19 | / impl Foo for [u8; 2] {
20 | |     fn get(&self) -> [u8; 2] {
21 | |         *self
22 | |     }
23 | | }
   | |_^
note: ...which then requires const-evaluating `<impl at ../rust/src/test/run-pass/issue-33387.rs:19:1: 23:2>::{{initializer}}`...
  --> ../rust/src/test/run-pass/issue-33387.rs:19:19
   |
19 | impl Foo for [u8; 2] {
   |                   ^
note: ...which then requires processing `<impl at ../rust/src/test/run-pass/issue-33387.rs:19:1: 23:2>::{{initializer}}`...
  --> ../rust/src/test/run-pass/issue-33387.rs:19:19
   |
19 | impl Foo for [u8; 2] {
   |                   ^
note: ...which then requires processing `<impl at ../rust/src/test/run-pass/issue-33387.rs:19:1: 23:2>::{{initializer}}`...
  --> ../rust/src/test/run-pass/issue-33387.rs:19:19
   |
19 | impl Foo for [u8; 2] {
   |                   ^
note: ...which then requires processing `<impl at ../rust/src/test/run-pass/issue-33387.rs:19:1: 23:2>`...
  --> ../rust/src/test/run-pass/issue-33387.rs:19:1
   |
19 | / impl Foo for [u8; 2] {
20 | |     fn get(&self) -> [u8; 2] {
21 | |         *self
22 | |     }
23 | | }
   | |_^
   = note: ...which then again requires processing `<impl at ../rust/src/test/run-pass/issue-33387.rs:19:1: 23:2>`, completing the cycle.

Happening on rustc 1.19.0-nightly (777ee2079 2017-05-01). Did not happen on the latest nightly available through rustup on 27.04.2017.

[est31]

Last working nightly: rustc 1.18.0-nightly (94e884b63 2017-04-27)
First broken nightly: rustc 1.18.0-nightly (128aa262e 2017-04-28)
list of commits in between.

[est31]

maybe it was #41368 ? cc @nikomatsakis

[nikomatsakis]

could be. let me try it on my big MIR refactoring branch =)

[dwrensha]

I confirm that the error occurs at 128aa26 but not at ad1461e. So it looks to me like #41368 is indeed the culprit.

[nikomatsakis]

Yes. I can reproduce. Will investigate.

[nikomatsakis]

I see the problem. PR coming soon.

[nikomatsakis]

Fix at #41777

[dwrensha]

I'm still seeing this issue, on the latest nightly and on master.

[nikomatsakis]

@dwrensha can you give more details of how you are reproducing the problem? (I added a test for the precise scenario reported, so I'm reasonably sure it's not that case anymore.)

[dwrensha]

$ rustc --version
rustc 1.19.0-nightly (978d2cfee 2017-05-10)
$ rustc -Z dump-mir=all src/test/run-pass/issue-33387.rs
error[E0391]: unsupported cyclic reference between types/traits detected
[...]

[nikomatsakis]

Hmm, perhaps my regression test wasn't sufficient. I see I used -Zdump-mir=NEVER_MATCHED; I remember that the test did fail, but I guess that it passing doesn't prove all issues are resolved. =)

[dwrensha]

The earliest commit where I can reproduce this problem is 2cca256.