shrink_to() may not guarantee capacity >= min_capacity

[yanfenghu]

Hi,

I found a potential API contract violation in the shrink_to method through Kani formal verification.

Location: src/basic.rs (shrink_to implementation)

Issue:

In some edge cases, shrink_to(min_capacity) may result in a capacity less than min_capacity.

Verification:

let mut sm: SlotMap<DefaultKey, u32> = SlotMap::with_capacity(100);
sm.shrink_to(50);
assert!(sm.capacity() >= 50); // May fail in some cases
Impact:

Violates API contract

May cause unexpected behavior

Could you please confirm if this is a valid issue? I can provide a more detailed reproduction case if needed.