SlotMap::with_capacity_and_key() integer overflow when capacity == usize::MAX #157

Hi,

I found a potential integer overflow issue in the with_capacity_and_key constructor through Kani formal verification.

Location: src/basic.rs:204

Current Code:

pub fn with_capacity_and_key(capacity: usize) -> Self {
let mut slots = Vec::with_capacity(capacity + 1); // Overflow when capacity == usize::MAX
slots.push(Slot { … });
// …
}
Problem:

When capacity = usize::MAX:

capacity + 1 wraps to 0 (integer overflow)

Vec::with_capacity(0) succeeds but allocates no space

Violates the “with_capacity” contract

First insertion will trigger reallocation

Impact:

Unexpected behavior when user passes extreme capacity values

Violates API contract

Suggested Fix:

pub fn with_capacity_and_key(capacity: usize) -> Self {
let actual_capacity = capacity.checked_add(1)
.expect(“capacity overflow: requested capacity too large”);
let mut slots = Vec::with_capacity(actual_capacity);
// …
}
Could you please confirm if this is a valid concern?