[Jules-Bertholet]

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---

Reference PR:

• Document universal &mut reborrows reference#2373

From the Reference:

Moved and copied types

When a place expression is evaluated in a value expression context, or is bound by value in a pattern, it denotes the value held in that memory location.

If the type of that value implements Copy, then the value will be copied.

In the remaining situations, if that type is Sized, then it may be possible to move the value.

[…]

This PR adds one exception to the above rules: if the type of the value is an &mut _ mutable reference, the value will now always be reborrowed instead of moved. (Reference PR: rust-lang/reference#2373)

This allows the following to compile:

fn generic(_: impl Sized) {}
fn assert_fnmut(_: &mut impl FnMut()) {}

struct Foo<T>(T);

fn main() {
    let mut_ref = &mut ();
    generic(mut_ref);
    {
        mut_ref
    };
    let _local = mut_ref;
    let _ = || mut_ref;
    let mut _tup: (&mut (),) = (&mut (),);
    _tup.0 = mut_ref;
    Foo(mut_ref);
    Foo { 0: mut_ref };
    let mut f = || {
        let _y: &mut _ = mut_ref;
    };
    f();
    f();
    assert_fnmut(&mut f);
    generic(mut_ref);

    let mut_ref_ref = &mut &mut ();
    generic(*mut_ref_ref);
    let _local = *mut_ref_ref;
    let _ = || *mut_ref_ref;
    let mut _tup: (&mut (),) = (&mut (),);
    _tup.0 = *mut_ref_ref;
    Foo(*mut_ref_ref);
    Foo { 0: *mut_ref_ref };
    let mut f = || {
        let _y: &mut _ = *mut_ref_ref;
    };
    f();
    f();
    assert_fnmut(&mut f);
    generic(*mut_ref_ref);
}

Unfortunately, the following continues to not compile:

fn assert_fnmut(_: &mut impl FnMut()) {}
fn main() {
    let x = &mut ();
    let mut f = || {
        //~^ ERROR expected a closure that implements the `FnMut` trait, but this closure only implements `FnOnce` [E0525]
        let _y = x;
    };
    f();
    f();
    assert_fnmut(f);
}

This last case is unfortunately very difficult to fix at present. See #47478 for discussion.

When #![feature(reborrow)] (#145612) is enabled, we also insert reborrows for concrete types implementing Reborrow.

Depends on #164014

@rustbot label A-MIR A-borrow-checker T-lang needs-fcp S-blocked

[lcnr]

should then probably also remove the explicit reborrows in HIR typeck?

View changes since this review

[dianne]

should then probably also remove the explicit reborrows in HIR typeck?

would that work without changing closure upvar inference to match? e.g. I think this closure captures x by mut ref (and accordingly is a FnMut) only because _y's declaration has a type annotation on it that causes coercing to insert a reborrow:

fn main() {
    let x = &mut ();
    let mut f = || {
        let _y: &mut _ = x;
    };
    f();
    f();
}

without the annotation, it captures x by value and fails to compile

edit: oh, of course, this makes the MIR for constructing the closure do the reborrow since it uses consume_by_copy_reborrow_or_move

edit 2: but it'd still be inferred to be a FnOnce, wouldn't it, if it has by-value captures?

[rustbot]

Some changes occurred in match lowering

cc @Nadrieril

[rustbot]

r? @adwinwhite

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[Jules-Bertholet]

#163494 (comment): I don't understand MIR optimizations, don't feel confident touching these tests. Someone who knows more than I do will need to take a look

[Jules-Bertholet]

@rustbot label F-reborrow

[rustbot]

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

[adwinwhite]

r? @dianne if you don't mind, or types

[rustbot]

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.