Rust beta/nightly has a false positive #[warn(unused_imports)] that's not in stable

[pedrocr]

When compiling https://github.com/pedrocr/rawloader with rust beta or nightly I get an unused_import warning that's a false positive (the compile fails without that import). The import in question is the following:

https://github.com/pedrocr/rawloader/blob/rawloader-0.29.0/src/decoders/ljpeg/decompressors.rs#L3

With stable everything works fine. The import is a little too broad (need to break up that code) and changing it to just the two structs that are needed fixes the warning. Still seems like a bug.

[alexcrichton]

cc @jseyfried

[Thinkofname]

Compiles for me on nightly/beta with that line removed. (Edit: fails on stable though)

It seems the use decoders::ljpeg::*; line is also pulling in basics which is a bit surprising to me because that file just use's it not pub use

Example here https://play.rust-lang.org/?gist=c6b422dfbdfa63519ce0e72f597c5a87&version=nightly&backtrace=0

[pedrocr]

Interesting. Removing that line in beta compiles but not in stable where it fails with the BitPump stuff.

[pedrocr]

Could it be something to do with the two modules requiring each other?

[pedrocr]

I've been able to generate a minimal test case for this and it seems you need this specific situation of a submodule and module including each other. Two modules at the same level including each other doesn't work.

test_nightly_unused_imports.tar.gz

[jseyfried]

This was caused by stabilizing RFC 1560 (now in beta), specifically this change and this change.
Due to these changes, use decoders::ljpeg::*; imports everything in basics on beta and nightly (as @Thinkofname pointed out).