Cromulent Copy closure captures

[Jules-Bertholet]

Rendered

Change closure capture inference rules for Copy captures, allowing more code to compile without changing semantics for existing closures.

Some examples of closures that would start compiling:

fn callable(c: char) -> impl Fn() -> char {
    || c
}

fn main() {
    let cs: u32 = (0..0xCC).flat_map(|c| (0..c).map(|cc| c + cc)).sum();
    assert_eq!(cs, 4203318);
}

@rustbot label T-lang A-closures

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[ds84182]

I think this needs to be aware of lifetimes, otherwise it'll be easy to accidentally bloat the size of closures that previously borrowed instead of copied. If some place outlives the closure, capture by reference.

View changes since the review

[bjorn3]

Borrowck happens after we decided on closure captures. We need MIR to borrowck, but MIR building needs the set of closure captures.

[Jules-Bertholet]

I've added a note about potential breakage around the auto traits a closure is inferred to implement, and how we could resolve that breakage.

[clarfonthey]

Thank you for creating an RFC that taught me what the word cromulent means. By which I mean you did not provide this information, but upon seeing the RFC, I looked it up.

[Jules-Bertholet]

I've updated the RFC to clarify that capturing values behind a reference still happens by-reference:

let x = &([0; 1024],);
let c = || {
    let y = x.0; // x.0 captured by ImmBorrow
};

[Jules-Bertholet]

I've now implemented this RFC: rust-lang/rust#163717

The RFC text has been updated with lessons from the implementation.