[ameliaquining]

To give one trivial example, if you want to write an interpreter for a garbage-collected language in Rust, you need some kind of garbage-collection facility. You could in theory do it C-style by just using raw mostly-untyped pointers everywhere, but this defeats most of the benefit of Rust over C, so you might instead prefer to encapsulate the GC into its own library, so that you can use regular Rust idioms in all the parts of the code that aren't specifically about GC.

More generally, there are various situations where garbage collection is effectively a requirement: namely, when you've got a mutable graph where anything can point to anything else, where some nodes are root nodes and others are not, and so it's possible for cycles of unreachable nodes to occur and you don't want this to result in memory leaks.

You could in theory imagine a language like Rust where garbage collection was integrated more deeply on a syntactic level, such that you might sometimes use it for convenience and not just where it's a requirement (and Rust in fact worked like this for a while pre-1.0), but the way things currently work, integrating a garbage-collection library basically always makes your code more unwieldy and you only do it if you have to.