Closure inference fails to infer that move is required when pattern matching

See this example, which fails to compile:

fn foo<F>(f: F)
    where F: FnOnce()
{
}

fn main() {
    let x = Some(vec![1, 2, 3]);
    foo(|| {
        match x {
            Some(y) => { }
            None => { }
        }
    });
}

Error:

<anon>:9:15: 9:16 error: cannot move out of captured outer variable in an `FnOnce` closure
<anon>:9         match x {
                       ^
<anon>:10:18: 10:19 note: attempting to move value to here
<anon>:10             Some(y) => { }
                           ^
<anon>:10:18: 10:19 help: to prevent the move, use `ref y` or `ref mut y` to capture value by reference
error: aborting due to previous error
playpen: application terminated with error code 101

triage: P-medium

A-const-fn?

Do you just mean the help message is wrong, or it should have automatically created a move closure?

@durka

@durka neither. Normally, even for non-move closures, we infer when particular variables are used in the body and we move just those variables. For some reason, that is not triggering in this particular case.

Might be duplicate of #26139?

Related example from #26139:

struct Name(String);

fn apply<F>(f: F) where
    F: FnOnce() {

    f()
}

fn main() {
    let n = Name(String::new());
    let consume = || {
        let Name(i) = n;
    };

    apply(consume);
}

Auto merge of #45864 - nikomatsakis:issue-30046-infer-fn-once-in-clos…

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Closure inference fails to infer that `move` is required when pattern matching #30046

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Closure inference fails to infer that move is required when pattern matching #30046

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nikomatsakis commented on Nov 25, 2015

durka commented on Nov 25, 2015

durka commented on Nov 25, 2015

nikomatsakis commented on Dec 15, 2015

nagisa commented on Jan 29, 2016

nikomatsakis commented on Aug 25, 2016

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Closure inference fails to infer that `move` is required when pattern matching #30046