DerefMut auto-deref error for union fields sometimes doesn't trigger #141621
Description
An implicit DerefMut of a MaybeUninit union field is an error, but sometimes that error does not fire. This is not a soundness issue as it's more like a lint
use std::mem::ManuallyDrop; union U { x: (), f: ManuallyDrop<(Vec<u8>,)>, } fn main() { let mut u = U { x: () }; // Errors about the implicit deref of the ManuallyDrop unsafe { u.f.0 = Vec::new() }; // equivalent to (*u.f).0 let r = &mut u; // implicitly derefs the ManuallyDrop but does not error unsafe { r.f.0 = Vec::new() }; // equivalent to (*(*r).f).0 }I expected to see this happen: Both assignments should error
Instead, this happened: Only the first produces an error
error: not automatically applying `DerefMut` on `ManuallyDrop` union field --> src/main.rs:24:9 | 24 | a.tup.0.num = 0; | ^^^^^^^ | = help: writing to this reference calls the destructor for the old value = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor We ran into a similar case in rust-lang/rust-clippy#14387
use std::mem::ManuallyDrop; struct Data { num: u64, } union DataWithPadding { u: (), tup: (ManuallyDrop<Data>, ()), } fn main() { let mut a = DataWithPadding { u: () }; unsafe { // No error, but the ManuallyDrop in the tuple is being implicitly dereferenced (*&mut a.tup).0.num = 0; // equivalent to (*(*&mut a.tup).0).num // similar (&mut a.tup).0.num = 0; // Does error when written plainly a.tup.0.num = 0; } }