False negative for trivial_casts with pointer

[Nilstrieb]

After #113262, this will no longer fail to compile:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=8ee721b21617d98d18c5256778b57efe

#[deny(trivial_casts)]
fn main() {
    let a = std::ptr::null::<()>();
    let _b = a as *const ();
}

The cast is trivial, but we miss it because we have to conservatively assume that raw pointer casts are not trivial. A possible fix would be to check for trivial raw pointer casts in MIR (but correctly considering lifetimes is still not quite easy there I think).

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium

[apiraino]

while randomly checking other things I've tried compiling the snippet in the first comment with the current stable 1.76.0: it compiles (since 1.75)

@Nilstrieb wdyt? could this issue been fixed at some point since opening this issue? Don't have time rn to bisect the fix 🙃 but if yes, I'd add the e-need-test to remind us to add a test case

[Nilstrieb]

The issue is that it does compile it shouldn't compile. I regressed this, it used to error with the lint, but now it doesn't anymore. I checked latest nightly and it still passes.