Add initial zeroslice! and zerovec! macros
#3453
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| () => ( | ||
| $crate::ZeroSlice::new_empty() | ||
| ); | ||
| ($aligned:ty; $array_fn:expr; $($x:expr),+ $(,)?) => ( |
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thought: add a case without $array_fn that uses <$aligned as $crate::ule::AsULE>::ULE::from_array.
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Please have a look at #3454
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is the type strictly necessary? We could probably make do with using _ if the type is omitted and rely on context
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We need the unaligned type to declare the const slice, and the cleanest way to get that is to ask for the aligned type.
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To expand on that, the internal const X is necessary for non-const contexts:
let x: &ZeroSlice<char> = zeroslice![<char as AsULE>::ULE::from_array; 'a'];
println!("{:?}", x); // error: creates a temporary value which is freed while still in use ^There was a problem hiding this comment.
Fortunately or unfortunately, I can get the compiler to extend the lifetime of the inner array (avoiding the need for the temporary) by using transmute directly instead of calling the ZeroSlice constructor...
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Would like @Manishearth's thoughts on if there is some other way we can avoid the type. It's totally something the compiler should be able to infer; I just don't know how in this context
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I don't think that code is sound. You're extending the lifetime of the type, but the temporary array still gets dropped at the end of the expression. In fact if you run Miri you get
error: Undefined Behavior: pointer to alloc8209 was dereferenced after this allocation got freed
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Inline const can solve this in the future. However in the follow up we're using the type to look up the conversion function, and I'd rather specify the type than the function.
Part of #1935.
This adds the
zeroslice![]andzerovec![]macros:A follow-up PR will use this macro in ICU4X where applicable.