Borrow checker unsoundness with unions #45157
Description
#![allow(unused)] #[derive(Clone, Copy, Default)] struct S { a: u8, b: u8, } #[derive(Clone, Copy, Default)] struct Z { c: u8, d: u8, } union U { s: S, z: Z, } fn main() { unsafe { let mut u = U { s: Default::default() }; let mref = &mut u.s.a; let err = &u.z.c; // This line compiles, but it certainly shouldn't ... drop(mref); // ... (at least if `mref` is used after `err`) }}"Cousins" of borrowed union sub-fields (and their further children) are not marked as borrowed.
The same bug should happen with move checking as well, but I haven't made an example yet.