Const flags describing allocator properties

[the8472]

It would be useful if allocators had an associated constant of flags describing their properties, such as

• support for alignment-changing realloc (posix realloc doesn't, jemalloc does)
• does realloc try to avoid memcpy (doing virtual memory schenanigans)
• whether deallocation does anything (vs. an arena that drops all allocations at once)
• if alloc_zeroed is any different from allocate
• do realloc/dealloc care about the layout information being correct
• etc.

Containers can then choose to optimize their behavior based on those properties.

(from https://github.com/rust-lang/wg-allocators/issues/120#issuecomment-1999418118)

[Amanieu]

Can you give examples of how generic code (e.g. Vec) could make use of these properties?

[the8472]

support for alignment-changing realloc

Would allow the in-place collect code in vec to apply in more cases. Currently GlobaAlloc does this: https://doc.rust-lang.org/nightly/src/alloc/alloc.rs.html#302-321 which means shrinking efficiency depends on the layouts, but some allocators can do better than that

does realloc try to avoid memcpy

can help picking between new-allocation-and-grow vs. reuse-allocation-and-shrink strategies. I.e. when shrinking incurs memcpy costs then it's better to use a fresh allocation from the beginning.

It would also let us pick between realloc vs. alloc-manually-copy-dealloc when there's a huge discrepancy between capacity and length.

do realloc/dealloc care about the layout information being correct

lets in-place-collect skip some realloc calls

whether deallocation does anything

for non-drop T we can turn Container's Drop into a noop

And that's just examples. The list should be extensible of course. Flags would need to have conservative defaults.