Fix excessive memory usage in bulk_load due to Vec over-capacity

[NathanHowell]

I came across rust-lang/rust-clippy#15753 while investigating excessive heap usage in one of my applications. The application was using approximately 40GB of RSS to process a 3GB OSM extract.

The OMT bulk loading algorithm employs Vec::split_off to partition elements into slabs. However, split_off leaves the original Vec with its full capacity even though it now holds only slab_size elements. Consequently, the first slab at each partitioning level inherits this over-capacity, and it propagates through the recursive partitioning process.

At the leaf level, Rust’s in-place collect optimization (triggered when size_of::<T>() == size_of::<RTreeNode<T>>()) reuses the allocation. This leads to the permanent storage of the over-sized buffer within the final tree node. For large inputs (e.g., 18.6M elements), this results in a geometric pattern of halving-size, doubling-count allocations, which wastes approximately 20GB of memory.

Fixed by calling shrink_to_fit() in two places:

• ClusterGroupIterator: after split_off, before returning the slab

• bulk_load_recursive: at the leaf level, before the into_iter().collect()

• I agree to follow the project's code of conduct.

• I added an entry to rstar/CHANGELOG.md if knowledge of this change could be valuable to users.

[urschrei]

Thank you! This is a good catch and looks like a solid PR – the 2x headroom allocation seems reasonable, and shrink_to_fit doesn't have any perf impact as far as I can see. I'm happy. Looks like you've got a formatting error, but that's a trivial fix.

[NathanHowell]

@urschrei k, should be clean now.

btw the rstar-demo crate fails to build:

$ cargo build -p rstar-demo
   Compiling parry3d v0.25.3
error[E0308]: mismatched types
   --> /Users/nhowell/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/parry3d-0.25.3/src/transformation/mesh_intersection/mesh_intersection.rs:537:38
    |
537 |     match point_set.nearest_neighbor(&point_to_insert) {
    |                     ---------------- ^^^^^^^^^^^^^^^^ expected `TreePoint`, found `&TreePoint`
    |                     |
    |                     arguments to this method are incorrect
    |
note: method defined here
   --> /Users/nhowell/source/georust/rstar/rstar/src/rtree.rs:940:12
    |
940 |     pub fn nearest_neighbor(&self, query_point: <T::Envelope as Envelope>::Point) -> Option<&T> {
    |            ^^^^^^^^^^^^^^^^
help: consider removing the borrow
    |
537 -     match point_set.nearest_neighbor(&point_to_insert) {
537 +     match point_set.nearest_neighbor(point_to_insert) {
    |

For more information about this error, try `rustc --explain E0308`.
error: could not compile `parry3d` (lib) due to 1 previous error