How are closures implemented in Rust?

Question

In many languages closures are implemented using a structure in combination with a standard associated function (with a fixed name) which provides a method of making the object "callable".

This is more commonly known as a functor.

For example in C++ typically the compiler will do something along the lines of creating a functor from a lambda expression. There's an example of this in one of the answers here.

Does Rust do something similar? Are closures just syntatic sugar for some kind of functor struct?

@user2722968 Can you give me an approximate indication as to which part of that documentation pages gives an answer? It may do, but right now I can't see it — user2138149
– user2138149, Commented Dec 28, 2024 at 13:06
the main paragraph itself: A closure can be thought of a a struct that contains the closure's state, and which implements at least FnOnce so that it's callable (in Rust, there are specifics around FnOnce, FnMut and Fn which "callable", as in other languages, does not convey). As far as this, the answer to your - broad - question is "Yes, its very similar". — user2722968
– user2722968, Commented Dec 28, 2024 at 13:58
@user2722968 Yes that would answer it but I still don't see that section. eg: I try to search for struct and I cannot find the text you cited there — user2138149
– user2138149, Commented Dec 28, 2024 at 14:13
The first two sentences are "A closure expression produces a closure value with a unique, anonymous type that cannot be written out. A closure type is approximately equivalent to a struct which contains the captured variables." [...] "Closure types all implement FnOnce, indicating that they can be called once by consuming ownership of the closure." — user2722968
– user2722968, Commented Dec 28, 2024 at 14:22

Chayim Friedman · Accepted Answer · 2024-12-28 16:12:16Z

0

Yes; they are compiled to structs implementing the FnX traits. But contrary to C++, you cannot implement these traits yourself (only on nightly).

Also note that it's arguable whether the compiler really "compiles" them to that. For instance, during most of the compilation phases closures have their own type (TyKind::Closure) - they are not represented as structs (TyKind::Adt) that implements some traits. However when lowering to LLVM IR they are represented as such.

answered Dec 28, 2024 at 16:12

Chayim Friedman

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