[jerf]

For a long time, people thought closures were only practical in garbage collected languages. I've used closures in C. (Against my will, basically. It was a C library that was best-of-breed and there was no other choice.) Getting the memory management right on them was effectively impossible. You know where the values are created for sure, but creating clean specifications of when the closures were destroyed quickly becomes insane in any real program. Very simple in a 20-line sample, very complicated when you got a long-lived callback interacting with multiple other resources that may or may not have their own "interesting" lifespans.

That Rust can do closures at all, do them in a sufficiently useful way that they are practical, and still maintain its safety guarantees without garbage collection is, in my opinion, in the top five accomplishments of the language. Prior to it actually happening I think it's fair to say most programming language developers would have said it's not possible, that you'll either need GC or the requisite type system will be impractically complex. At least their quirkiness generally fits the rest of the language and isn't much "new" quirkiness.