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as *mut _ does not cause by-mut place evaluation, causing an error stating DerefMut is not implemented, but it is #86262
Description
Given the code:
#![allow(unused_mut)]
use core::mem::ManuallyDrop;
pub struct RefWrapper<'a, T: ?Sized> {
slot: &'a mut T,
}
impl<'a, T: ?Sized> RefWrapper<'a, T> {
pub fn into_raw(this: Self) -> *mut T {
let mut this = ManuallyDrop::new(this);
// let this = &mut *this;
this.slot as *mut T
}
}The current output is:
error[E0596]: cannot borrow data in a dereference of `ManuallyDrop<RefWrapper<'_, T>>` as mutable
--> src/lib.rs:13:9
|
13 | this.slot as *mut T
| ^^^^^^^^^ cannot borrow as mutable
|
= help: trait `DerefMut` is required to modify through a dereference, but it is not implemented for `ManuallyDrop<RefWrapper<'_, T>>`
Ideally, this should just work1. Uncommenting the commented line to add an explicit mutable reborrow allows the code to work as intended. But even if it can't work, the error is clearly very unhelpful, as the issue isn't the lack of DerefMut, but that the compiler selected a shared Deref evaluation of the place.
@rustbot modify labels +C-bug +D-incorrect
Footnotes
-
Importantly, just work and produce a pointer with valid
mutprovenance. It would be technically correct but misleading to allow&mut T as *mut Tbehind a shared reference to behave like&mut T as *const T as *mut T. The actually useful behavior is for the fact thatas *mutis a by-mutuse of thethis.slotplace to flow back into the selection of whether to evaluate thethis.slotplace by-ref (viaDeref) or by-mut (viaDerefMut) the same way other by-mutplace usage does. ↩
Activity
estebank commentedon Jan 9, 2023
Triage: no change
CAD97 commentedon Jan 9, 2023
I believe the change implemented in #86647 is the correct fix; namely, considering an
as *mut _use of a place a by-mutusage for the purpose of selecting whetherDereforDerefMutis used in autoborrow evaluation of the place.For comparison, writing
ptr::from_mut(this.slot)(docs instead, compiles as expected.That PR stalled waiting for lang/compiler confirmation of the change's impact.
Since this does indeed have lang implications,
@rustbot modify labels +T-lang
If we can get team confirmation that #86647 did indeed have the correct approach, I can rebase and resubmit.
[-]Bad error: DerefMut is not implemented, but it is[/-][+]`as *mut _` does not cause by-`mut` place evaluation, causing an error stating `DerefMut` is not implemented, but it is[/+]