avoid mixing accesses of ptrs derived from a mutable ref and parent ptrs

[RalfJung]

@Vanille-N is working on a successor for Stacked Borrows. It will mostly accept strictly more code than Stacked Borrows did, with one exception: the following pattern no longer works.

let mut root = 6u8; let mref = &mut root; let ptr = mref as *mut u8; *ptr = 0; // Write assert_eq!(root, 0); // Parent Read *ptr = 0; // Attempted Write

This worked in Stacked Borrows kind of by accident: when doing the "parent read", under SB we Disable mref, but the raw ptrs derived from it remain usable. The fact that we can still use the "children" of a reference that is no longer usable is quite nasty and leads to some undesirable effects (in particular it is the major blocker for resolving rust-lang/unsafe-code-guidelines#257). So in Tree Borrows we no longer do that; instead, reading from root makes mref and all its children read-only.

Due to other improvements in Tree Borrows, the entire Miri test suite still passes with this new behavior, and even the entire libcore and liballoc test suite, except for these 2 cases this PR fixes. Both of these involve code where the programmer wrote &mut but then used pointers derived from that reference in ways that alias with the parent pointer, which arguably is violating uniqueness. They are fixed by properly using raw pointers throughout.

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[m-ou-se]

@bors r+ rollup

[bors]

📌 Commit c3a2e7a has been approved by m-ou-se

It is now in the queue for this repository.