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unused-extern-crates causes false positive with macro_use #30849

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Ms2ger opened this issue Jan 12, 2016 · 7 comments
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unused-extern-crates causes false positive with macro_use #30849

Ms2ger opened this issue Jan 12, 2016 · 7 comments
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A-lint Area: Lints (warnings about flaws in source code) such as unused_mut.

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@Ms2ger
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Ms2ger commented Jan 12, 2016

For example, it warns for bitflags/lazy_static even when those macros are used in the crate.
@alexcrichton
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This is why the lint is allow-by-default, it's got false positives

@Ms2ger
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Ms2ger commented Jan 12, 2016

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

@Ms2ger Ms2ger mentioned this issue Jan 13, 2016
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@steveklabnik steveklabnik added the A-lint Area: Lints (warnings about flaws in source code) such as unused_mut. label Jan 26, 2016
@nox nox mentioned this issue Apr 19, 2016
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@zbynekwinkler
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Is this the right place to look at?
https://github.com/rust-lang/rust/blob/master/src/librustc_resolve/check_unused.rs

@SimonSapin
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On IRC about this:

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

@eddyb
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eddyb commented Apr 29, 2016

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.

@sanxiyn sanxiyn mentioned this issue Jul 28, 2016
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@Ms2ger
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Ms2ger commented Jan 23, 2017

#39060 improved this.

CC @jseyfried

@jseyfried
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@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.

@sanmai-NL sanmai-NL mentioned this issue May 23, 2017
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@ishitatsuyuki ishitatsuyuki mentioned this issue Jun 11, 2017
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bors added a commit that referenced this issue Aug 27, 2017
@bors
Auto merge of #42588 - ishitatsuyuki:patch-1, r=petrochenkov
78e95bb 
Make unused-extern-crate warn-by-default

Apart from enabling the lint, this pull request also removes existing unused crates in the codebase, and fix some amount of false positives on crates with special purposes.

Now that all false positive issues are closed, it should be possible to make it available to wider users.

Quote:
> Now that macro modularization is implemented, this is true today! *#30849 (comment)

Concerns: can break some `#[deny(warnings)]`.

Close #42591
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@steveklabnik @alexcrichton @eddyb @Ms2ger @SimonSapin @zbynekwinkler @jseyfried