CTFE interns allocations multiple times

[oli-obk]

This is refactoring fallout during the miri merger.

First it is interned in

rust/src/librustc_mir/interpret/memory.rs

Line 604 in 1558ae7

let alloc = self.tcx.intern_const_alloc(alloc);

and then again in

rust/src/librustc_mir/interpret/const_eval.rs

Line 98 in 1558ae7

let alloc = ecx.tcx.intern_const_alloc(alloc);

The second place is wholly unnecessary barring bugs in the first place

[RalfJung]

The interning in memory.rs is also fishy. It determines once, using is_freeze(), whether there is interior mutability, and then recursively uses that information when interning stuff behind references. However, is_freeze() does not say anything about what is behind references: It is guided by the Freeze marker trait, which has

unsafe impl<T: ?Sized> Freeze for *const T {}
unsafe impl<T: ?Sized> Freeze for *mut T {}
unsafe impl<'a, T: ?Sized> Freeze for &'a T {}
unsafe impl<'a, T: ?Sized> Freeze for &'a mut T {}

So really it has to check again after following any pointer indirection.

Of course, that doe snot work either because at that point we do not have type information...

I think right now this is not a problem because this does not compile

static FOO2 : &'static AtomicUsize = &AtomicUsize::new(3);

so the "inner" allocations we arrive when following pointers will never have an UnsafeCell.

But I have a hard time following any of this code, so that analysis may be totally wrong.

[RalfJung]

Ah and because I don't know there else to dump this: const_to_allocation is a strange beast; it seems to be called for statics only so the logic that copies is unnecessary (and it is non-recursive anyway which seems incomplete).

Likely we want a query for statics that just returns an allocation, or maybe an AllocId instead, done?

[RalfJung]

@oli-obk I think this got fixed by your big interning refactoring (#58351)?

[oli-obk]

Yes, the call in memory.rs is gone now