Usage of pattern on lhs of assignment results in unsafety check for raw pointer deferences being skipped

[Aiden2207]

I tried this code:

fn main() {
    let _ = *(0 as *const ());
}

playground

I expected to see this happen: A compiler error, dereferencing pointers is unsafe, and no unsafe block is provided.
Instead, this happened:

   Compiling playground v0.0.1 (/playground)
warning: dereferencing a null pointer
 --> src/main.rs:2:13
  |
2 |     let _ = *(0 as *const ());
  |             ^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed
  |
  = note: `#[warn(deref_nullptr)]` on by default

warning: `playground` (bin "playground") generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.97s
     Running `target/debug/playground`

As far as I can tell, this code erroneously compiles for any Copy type, so long as the pattern doesn't bind anything to an identifier. Calls to unsafe functions cause a compiler error as normal.

Meta

Works on the playground when I tested with stable-1.66.1, beta-1.67.0-beta.10, and nightly-1.69.0 (2023-01-23)

[steffahn]

Looks like a duplicate of #79735

Edit: Maybe it isn't an exact duplicate, but the behavior is one that's sufficiently tracked in that other issue already.

Edit2: Actually, it isn't sufficiently tracked in the other issue. The code here gives a warning claiming "undefined behavior", so in case the code is considered safe, we would still have a diagnostic issue.

[steffahn]

Ah, there's also #80059. But that discussion does not encounter the "this code causes undefined behavior" warning either.

[CAD97]

That warning feels like it's coming from const evaluation (rvalue promotion). I don't know if it actually is, but I would not be surprised if promotion/promotability is involved here somehow.

[lukas-code]

This will be fixed by #102256.

[WaffleLapkin]

This is in fact fixed by #102256 on the latest stable:

error[E0133]: dereference of raw pointer is unsafe and requires unsafe function or block
 --> src/main.rs:2:13
  |
2 |     let _ = *(0 as *const ());
  |             ^^^^^^^^^^^^^^^^^ dereference of raw pointer
  |
  = note: raw pointers may be null, dangling or unaligned; they can violate aliasing rules and cause data races: all of these are undefined behavior