Unrelated Code Fails to Typecheck when Feature Enabled

[cwfitzgerald]

When building the following wgpu-rs commit with default features, everything compiles. When the subscriber feature is enabled with --features subscriber, the these two lines of code fail to compile. They are unrelated to the feature. The error is:

error[E0282]: type annotations needed
  --> src\util\mod.rs:38:9
   |
38 |         debug_assert_eq!(pre, &[]);
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type
   |
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

gfx-rs/wgpu-rs@9a9bf53

I have no idea how to even start making a reduced example.

Meta

Issue happens on both stable and nightly:

rustc --version --verbose:

rustc 1.44.1 (c7087fe00 2020-06-17)
binary: rustc
commit-hash: c7087fe00d2ba919df1d813c040a5d47e43b0fe7
commit-date: 2020-06-17
host: x86_64-pc-windows-msvc
release: 1.44.1
LLVM version: 9.0

rustc +nightly --version --verbose

rustc 1.47.0-nightly (6c8927b0c 2020-07-26)
binary: rustc
commit-hash: 6c8927b0cf80ceee19386026cf9d7fd4fd9d486f
commit-date: 2020-07-26
host: x86_64-pc-windows-msvc
release: 1.47.0-nightly
LLVM version: 10.0

[tesuji]

Cannot reproduce in rustc 6c8927b 2020-07-26 x86_64-unknown-linux-gnu with both check and build build-profile
with cargo build --features subscriber or cargo build --no-default-features --features subscriber.

[cwfitzgerald]

Thanks for trying it! Will check on linux tomorrow, but silly question, did you test the exact commit, as the commit immediately after the referenced one refactors the code so it compiles.

[tesuji]

Yeah, I can reproduce it with gfx-rs/wgpu-rs@9a9bf53 .

[Noratrieb]

This is a known issue/feature where type inference is guided if only a single trait implementation exists. I assume that the feature adds a new trait impl (maybe it brings in serde_json, which often causes this).