I wondered how long it would take for somebody to call me out for that. It seemed like a sketchy claim when I wrote it (it wouldn't be too complicated to make the @-heap lock-free in rust, for starters), but in retrospect, I shouldn't have said it at all. Thanks for pointing that out.
I tried to think of an alternate justification, but couldn't come up with a non-awkward way of saying it, so just redacted that part entirely. I'll say here, though, a few reasons I think avoiding malloc/GC is important:
- pcwalton has mentioned his desire to be able to reason about when the garbage collector runs, so that he can statically guarantee that the graphics rendering task in servo will never take a big pause to do a GC.
- In this-must-never-fail environments, such as writing kernel device drivers, using dynamic allocation has to be avoided (and, if i'm not mistaken, even more likely to fail when done in an "atomic section" using kmalloc(GFP_ATOMIC)). Rust in its current state is not suitable for kernel-level development, but it's not far off; and it's critical to be able to reason about memory allocation for such applications.
Short answer is "all of it"[1]. 'match' statements in rust also exhaustive. I believe ADTs are used and checked exactly the same way as in ocaml (I've used SML and Haskell and they are the same as there).
[1] not to say dynamic failure is totally gone from the language -- but it is if you avoid both writing 'fail' and using library functions that can fail. A common example is option::get(Option<T>) -> T, which fails if the optional value is None -- it leaves a sour taste in my mouth whenever I use it, but I still do sometimes.
My mistake. I meant to write "can SOMETIMES be entirely on the stack" -- fixed that just now.
Functions indeed cannot return closures without heap-allocating them. (It might be possible in specific obscure cases, but definitely not in general.) However, if you wish to curry a function foo(a,b), you can do that in the caller (instead of at the function definition itself) by writing:
let curried_foo = |b| foo(a,b);
That is then stack-allocated and can refer to 'a' in the same frame, and can be passed to other functions you might call from there (but not returned upwards, indeed).
