Unsoundness in dynamic_set.rs and `from_mutated_str`

[Ralith]

Reviewing this crate's use of unsafe identified a few issues:

string-cache/src/dynamic_set.rs

Line 63 in 34f914c

return NonNull::from(&mut **entry);

string-cache/src/dynamic_set.rs

Line 98 in 34f914c

let entry_ptr: *mut Entry = &mut **entry_ptr;

These construct a &mut Entry that may exist concurrently with the &Entry references unsafely constructed by many methods on Atom. These should use the new ptr::addr_of_mut helper which avoids the hazard.

string-cache/src/dynamic_set.rs

Line 105 in 34f914c

current = unsafe { &mut (*entry_ptr).next_in_bucket };

This similarly constructs a unique reference to a field, which may actually get written while an aliasing &Entry is live elsewhere. This probably needs an UnsafeCell.

string-cache/src/atom.rs

Line 309 in 34f914c

let buffer = unsafe { &mut *buffer.as_mut_ptr() };

This constructs a reference to uninitialized memory. Raw pointer writes should be used instead.