unused-extern-crates causes false positive with macro_use

[Ms2ger]

For example, it warns for bitflags/lazy_static even when those macros are used in the crate.

[alexcrichton]

This is why the lint is allow-by-default, it's got false positives

[Ms2ger]

Sure, but that doesn't mean some of them can't be fixed.

[zbynekwinkler]

Is this the right place to look at?
https://github.com/rust-lang/rust/blob/master/src/librustc_resolve/check_unused.rs

[SimonSapin]

On IRC about this:

@eddyb: s/false positives/macros aren't integrated with name resolution yet/

[eddyb]

It's actually possible to silence the error by adding #[no_link] to the macro-only extern crate bitflags; (which is what librustc does), effectively removing bitflags from the scope.

If/when we move name resolution earlier and use it during macro expansion, the macro will actually be imported through the bitflags name and the warning wouldn't get triggered.

[Ms2ger]

#39060 improved this.

CC @jseyfried

[jseyfried]

@eddyb

If/when we move name resolution earlier and use it during macro expansion, the macro will actually be imported through the bitflags name and the warning wouldn't get triggered.

Now that macro modularization is implemented, this is true today!

adding #[no_link] to the macro-only extern crate bitflags; effectively [removes] bitflags from the scope.

For the record, this changed in #37247 to support use macro imports from #[no_link] crates.