`redundant_closure` false positive, extern fn

[SamRodri]

Lint name: redundant_closure

I tried this code:

pub extern "C" fn foo(func: extern fn()) {
    bar(|| func())
}

pub fn bar(func: impl FnOnce()) {
    func()
}

I expected no lints here, because FnOnce is not implemented for extern fn.

The suggestion causes a compile time error by passing func directly:

pub extern "C" fn foo(func: extern fn()) {
    bar(func)
}

Meta

Rust version (rustc -Vv):

rustc 1.56.0 (09c42c458 2021-10-18)
binary: rustc
commit-hash: 09c42c45858d5f3aedfa670698275303a3d19afa
commit-date: 2021-10-18
host: x86_64-pc-windows-msvc
release: 1.56.0
LLVM version: 13.0.0

@rustbot label +I-suggestion-causes-error

[josephlr]

I also ran into this issue when using a FnOnce-based API. Is the solution here to fix the Clippy lint, or to have extern fn() implement FnOnce()?

See also: rust-lang/rust#69250 and rust-lang/rust#40710

[dswij]

I'm pretty sure #8431 fix this FP, but I haven't tested this yet

[dswij]

Actually, just tried this with #8431, the changes introduced in #8431 doesn't seem to cover extern fn. This FP still shows up

[dswij]

Actually, just tried this with #8431, the changes introduced in #8431 doesn't seem to cover extern fn