Forbid casting to/from a pointer of unknown kind

[tirr-c]

Fixes #45730.

Before, it ICE'd when pointer_kind encountered TyInfer.

[rust-highfive]

r? @pnkfelix

(rust_highfive has picked a reviewer for you, use r? to override)

[tirr-c]

Does this case occur only by lack of type information? I'm not pretty sure.

[arielb1]

@tirr-c

It's not a good idea to assume things when encountering a TyInfer - trait bounds etc. can make the TyInfer equal to every type, so this could accidentally allow thin-to-fat pointer casts if the _ is later made to be equal fmt::Debug, as in:

use std::fmt;
fn main() {
  let x: *const _ = 0 as *const _; // we don't want to allow this if...
  let y: Option<*const fmt::Debug> = Some(x) as _;
  // ^ later, we have x = *const fmt::Debug
}

fn not_ok() {
  let x = 0 as *const i32 as *const _ as *mut _; //~ ERROR ?
}

Instead, you should make fn pointer_kind return an Option, and emit an error of we are casting to an unknown pointer kind..

[tirr-c]

Added a new error code for casting with unknown pointer kind.

r? @arielb1

[arielb1]

This forbids casting an *const _ to an *const u32, please move the check after the following if to avoid breaking user code.

[tirr-c]

Updated, but I couldn't trigger the *const _ as *const u32 case. What would the code look like?

[arielb1]

@tirr-c

Can't you have 0 as *const u32 as *const _ as *mut u32? Or does that go through a separate code-path?

[arielb1]

@bors r+

[bors]

📌 Commit 99ada04 has been approved by arielb1

[tirr-c]

It just compiles. I'm not sure when inference occurs, but maybe *const _ there is inferred as *const u32?

[bors]

⌛ Testing commit 99ada04 with merge 49bee9d...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: arielb1
Pushing 49bee9d to master...