fix: Do not consider mutable usage of deref to `*mut T` as deref_mut
Fixes#15799
We are doing some heuristics for deciding whether the given deref is deref or deref_mut here;
5982d9c420/crates/hir-ty/src/infer/mutability.rs (L182-L200)
But this heuristic is erroneous if we are dereferencing to a mut ptr and normally those cases are filtered out here as builtin;
5982d9c420/crates/hir-ty/src/mir/lower/as_place.rs (L165-L177)
Howerver, this works not so well if the given dereferencing is double dereferencings like the case in the #15799.
```rust
struct WrapPtr(*mut u32);
impl core::ops::Deref for WrapPtr {
type Target = *mut u32;
fn deref(&self) -> &Self::Target {
&self.0
}
}
fn main() {
let mut x = 0u32;
let wrap = WrapPtr(&mut x);
unsafe {
**wrap = 6;
}
}
```
Here are two - outer and inner - dereferences here, and the outer dereference is marked as deref_mut because there is an assignment operation.
And this deref_mut marking is propagated into the inner dereferencing.
In the later MIR lowering, the outer dereference is filtered out as it's expr type is `*mut u32`, but the expr type in the inner dereference is an ADT, so this false-mutablility is not filtered out.
This PR cuts propagation of this false mutablilty chain if the expr type is mut ptr.
Since this happens before the resolve_all, it may have some limitations when the expr type is determined as mut ptr at the very end of inferencing, but I couldn't find simple fix for it 🤔
9f1f5cd8f6