Spurious "useless lint attribute" warning

[joshlf]

Given the following code:

#[macro_use]
extern crate lazy_static;

cargo build produces the following warning:

warning: unused `#[macro_use]` import
 --> src/lib.rs:1:1
  |
1 | #[macro_use]
  | ^^^^^^^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

If we then add #[allow(unused_imports)]:

#[allow(unused_imports)]
#[macro_use]
extern crate lazy_static;

then cargo build produces no warnings. However, if we run cargo clippy on this code, we get a "useless lint attribute" warning:

warning: useless lint attribute
 --> src/lib.rs:1:1
  |
1 | #[allow(unused_imports)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^ help: if you just forgot a `!`, use: `#![allow(unused_imports)]`
  |
  = note: #[warn(useless_attribute)] on by default
  = help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#useless_attribute

[oli-obk]

Question: why do you want to ignore an unused #[macro_use]?

This is a known limitation of the lint (see https://rust-lang-nursery.github.io/rust-clippy/master/index.html#useless_attribute ), but as the limitation states, it's easier to remove the unused import than to add an allow.

[joshlf]

Question: why do you want to ignore an unused #[macro_use]?

My crate exports macros that, internally, use lazy_static. Thus, when compiling the crate itself, nobody has called those macros, and thus lazy_static isn't actually used, so an unused import warning is generated.

as the limitation states, it's easier to remove the unused import than to add an allow.

In my case, I actually need the import.

[oli-obk]

That sounds more like a rustc bug to me. Have you reported this upstream?

[joshlf]

I don't think it's a rustc bug. Maybe I'm missing something though. Here's the full file if you're curious.

[oli-obk]

Ah... your macro is unusable within crates that don't have

#[macro_use]
extern crate lazy_static;

So I suggest to move the #[macro_use] extern crate lazy_static; at the beginning of your macro: https://gist.github.com/joshlf/87783078dad1d888a321db50da5d4097#file-lib-rs-L484

The trick mentioned in https://gist.github.com/joshlf/87783078dad1d888a321db50da5d4097#file-lib-rs-L474 kinda works but is hacky imo. The solution of moving the crate import into the macro itself is also used by serde and others, so I consider that solution idiomatic.

[joshlf]

I'm not sure that it would work. My concern is that since #[macro_use] is only allowed on extern crate declarations at the crate root, it would bar the macro from being used anywhere other than a crate's root. I'll have to try it out to see, though.

[joshlf]

Oh, and more problematically, putting #[macro_use] extern crate lazy_static; in the macro requires a lazy_static dependency in the consuming crate's Cargo.toml, which I definitely want to avoid.

[lilith]

This is a major pain with #![feature(alloc_system)] see

rustc gives a spurious unused crate error for extern crate alloc_system.

When we disable that lint, #[allow(unused_extern_crates)], clippy complains (incorrectly) about that attribute.

Which brings us to this mess:

#![feature(alloc_system)]
#[cfg_attr(feature = "cargo-clippy", allow(useless_attribute))]
#[allow(unused_extern_crates)]
extern crate alloc_system;

[llogiq]

The unused_extern_crates lint was made to warn by default recently, but this lead to many false positives. The newest (or next?) nightly should have a PR that disables the lint again until the false positives are sorted out.

[oli-obk]

Oh, and more problematically, putting #[macro_use] extern crate lazy_static; in the macro requires a lazy_static dependency in the consuming crate's Cargo.toml, which I definitely want to avoid.

You have to do that anyway as far as I can see.

When we disable that lint, #[allow(unused_extern_crates)], clippy complains (incorrectly) about that attribute.

That has been fixed in rustc

[joshlf]

Oh, and more problematically, putting #[macro_use] extern crate lazy_static; in the macro requires a lazy_static dependency in the consuming crate's Cargo.toml, which I definitely want to avoid.

You have to do that anyway as far as I can see.

Nah, I'm able to do:

#[doc(hidden)]
pub use lazy_static::*;

When we disable that lint, #[allow(unused_extern_crates)], clippy complains (incorrectly) about that attribute.

That has been fixed in rustc

Good to know!

[oli-obk]

In that case the lint unused_imports won't be triggered, right?

[brandur]

Another example of a false positive case: I use serde_json in my project, but I only need the macros it provides in my test suite. If I annotate it with #[macro_use], Rustc correctly complains that I'm not using those macros (outside of tests).

The solution is to annotate the extern crate with #[allow(unused_imports)], but unfortunately, this leads to Clippy complaining.

Whether you consider this a Rustc problem or not, it's pretty unambiguous that the "why this is bad" description is wrong isn't it?

Lint attributes have no effect on crate imports. Most likely a ! was forgotten

Having an allow there fixes a compiler warning, so it certainly has an effect.

[sinkuu]

BTW you can simply annotate it with #[cfg_attr(test, macro_use)].

[brandur]

@sinkuu Thanks! I'd basically solved it with:

#[cfg_attr(feature = "cargo-clippy", allow(useless_attribute))]
#[allow(unused_imports)]
#[macro_use]
extern create serde_json;

But that seems better.

[dtolnay]

As reported in #1995 and #2607, this false positive is triggered when re-exporting procedural macros.

#[allow(unused_imports)]
#[macro_use]
extern crate repro_derive;

pub use repro_derive::*;

[gnzlbg]

I have a lot of code using cfg_if in which #[allow(unused_import)] simplifies many branches. Otherwise, I would actually have to code all potential target_arch, target_feature, target_os, ... combinations that would actually need the import, which would be a mess compared to just:

cfg_if! {
    if #[cfg(all(target_arch = "arm", target_feature = "v7", target_feature = "neon"))] {
        // Do something differently
        extern crate coresimd;
        #[allow(unused_import)]
        use coresimd::arch;
    } else {
        // Add the import because some other target_arch/target_feature combinations
        // need it, but not all:
        #[allow(unused_import)]
        use core::arch;
    }
}

[flip1995]

@gnzlbg #[allow(unused_imports)] is whitelisted for use items. The unused_import (without s) lint doesn't exist, see rustc book.