> Rust-the-language doesn't know anything about allocations

AIUI, Rust knows more about allocations than it should, since it's still limited to a single global allocator, and Box<> (which relies on that global allocator) is implemented via compiler magic and cannot be supplemented by equivalent library code. If Rust supported "pluggable" allocators in some fashion, fancy features (including heap compaction, although I find that a bit dubious TBH) might become easier to implement.