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Update vec! to have the same coercion behavior as arrays #58377

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Update vec! to have the same coercion behavior as arrays #58377

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17 changes: 14 additions & 3 deletions src/liballoc/macros.rs
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Expand Up @@ -39,9 +39,20 @@ macro_rules! vec {
($elem:expr; $n:expr) => (
$crate::vec::from_elem($elem, $n)
);
($($x:expr),*) => (
<[_]>::into_vec(box [$($x),*])
);
($($x:expr),*) => ({
// We use a temporary let binding to work around a coercion
// bug/strangness with `box [a, b]` syntax. Example:
//
// fn foo() {}
// fn bar() {}
//
// let _ = [foo, bar]; // works: fn items coerced to fn pointers
// let _ = <[_]>::into_vec(box [foo, bar]); // doesn't work
//
// See the `macro_fn_pointer_coercion` test.
let tmp = [$($x),*];
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<[_]>::into_vec(box tmp)
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Possibly-silly question: now that there's a temporary, is box syntax gaining us anything over using Box::new? Is there a chance that some code that was making too-big-for-the-stack Vecs with this will no longer work?

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Alternatively, is this something that should be fixed in the compiler about box syntax? Or is that dead enough that we should just stop ever using the keyword in the library?

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Good point. I guess people could use vec! with data too big for the stack, so we shouldn't change this as it might break code. I don't think there is a way to easily fix that, so I will close this PR.

The correct way would be to fix box, I guess. But I heard it's not gonna land in this form anyway, so we might also just ignore this obscure function-pointer problem for now.

});
($($x:expr,)*) => (vec![$($x),*])
}

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8 changes: 8 additions & 0 deletions src/liballoc/tests/vec.rs
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Expand Up @@ -1151,3 +1151,11 @@ fn test_try_reserve_exact() {
}

}

#[test]
fn macro_fn_pointer_coercion() {
fn foo() {}
fn bar() {}

let _v = vec![foo, bar];
}