Here's the code performing the allocation reuse: https://github.com/rust-lang/rust/blob/25f8d01fd8bda339612d0c0a8844173a09205f7c/library/alloc/src/vec/spec_fro

Here's the code performing the allocation reuse: https://github.com/rust-lang/rust/blob/25f8d01fd8bda339612d0c0a8844173a09205f7c/library/alloc/src/vec/spec_from_iter.rs#L43 https://github.com/rust-lang/rust/blob/25f8d01fd8bda339612d0... It's not supposed to waste more space than a regular `collect()` or `push()` would when growing the capacity exponentially.