In a sense Rust could have tried to become a "tiered" language with 2 standard libraries: one low level that allows precise control on heap allocations (including custom allocators) and another higher level that relied on a GC. They could both be used at the same time.

There would also be a compiler attribute #![no_gc] that make it so that you have to provide your own implementation of the GC runtime to use GC types. (similar to how executors work for futures)