Avoding the clippy lint with type placeholder `_`

Lint name:
zero_ptr

I tried this code:

fn dummy_raw_waker() -> RawWaker {
    fn no_op(_: *const ()) {}
    fn clone(_: *const ()) -> RawWaker {
        dummy_raw_waker()
    }
    let vtable = &RawWakerVTable::new(clone, no_op, no_op, no_op);
    RawWaker::new(0 as _, vtable)
}

Clippy is not able to deduce a style lint in cases of implicit cast, but would look at the signature of function RawWaker::new():

pub const fn new(data: *const (), vtable: &'static RawWakerVTable) -> RawWaker { ... }

Manually placing the type makes clippy recognize the style lint

 RawWaker::new(0 as *const (), vtable)

Meta

• cargo clippy -V: clippy 0.1.54 (a85f584 2021-06-16)
• rustc -Vv:

rustc 1.55.0-nightly (a85f584ae 2021-06-16)
binary: rustc
commit-hash: a85f584aebd9b08314bf30b9adc17b4a752143e5
commit-date: 2021-06-16
host: x86_64-unknown-linux-gnu
release: 1.55.0-nightly
LLVM version: 12.0.1

Start by reproducing the reported case with cargo clippy and the zero_ptr lint, comparing RawWaker::new(0 as _, vtable) with the explicit *const () cast. Trace the lint's handling of implicit casts and function signatures. Done means the implicit-cast form is recognized consistently and regression coverage verifies the behavior.