Add a function to turn Box<T> into Box<[T]> #71421
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r? @sfackler (rust_highfive has picked a reviewer for you, use r? to override) |
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I think it is better to have an issue about this, and also maybe add a test |
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Added tests, is this what you were looking for? (I'm not sure why every error appears twice) |
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Those are all about the alignment of pointer-like values themselves, regardless of |
That makes sense. the usage of |
cuviper
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Apr 23, 2020
src/liballoc/boxed.rs
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| @@ -816,6 +827,18 @@ impl From<Box<str>> for Box<[u8]> { | |||
| } | |||
| } | |||
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| #[unstable(feature = "box_into_boxed_slice", issue = "none")] | |||
| impl<T> From<Box<T>> for Box<[T]> { | |||
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AFAIK, trait impls are effectively insta-stable -- they can't really be unstable unless the trait and/or types involved are unstable.
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ha, so I'll remove this from, and it can be added back if/when this is stable
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What's the use case for this? |
I have a function that returns https://docs.rs/syn/1.0.18/syn/struct.TypeParen.html EDIT, basically something along these lines: fn extract_tupled_types(attr: Attribute) -> Result<Vec<Type>> {
let ty: Type = syn::parse2(attr.tts)?;
Ok(match ty {
Type::Paren(paren) => vec![*paren.elem],
Type::Tuple(tuple) => tuple.elems.into_iter().collect(),
_ => vec![ty],
}) |
sfackler
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Apr 26, 2020
src/liballoc/boxed.rs
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| /// | ||
| /// This conversion does not allocate on the heap and happens in place. | ||
| /// | ||
| /// This is also available via [`From`]. |
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Ops, thanks. I just removed the From impl because it's insta-stable
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Seems plausible to me. Can you make a tracking issue and update the stability annotation? |
👍 Thanks! |
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@bors r+ rollup |
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📌 Commit 0228ca0 has been approved by |
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Rollup of 5 pull requests Successful merges: - rust-lang#71421 (Add a function to turn Box<T> into Box<[T]>) - rust-lang#71537 (Remove support for self-opening) - rust-lang#71551 (Minor refactoring around IndexVec usage in generator transformation) - rust-lang#71569 ([miri] Throw UB if target size and data size don't match) - rust-lang#71576 (check that `AsRef` and `AsMut` are inlined) Failed merges: - rust-lang#71558 (Cleanup and document `-Z tls-model` ) r? @ghost
shepmaster
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Jun 17, 2020
| /// | ||
| #[unstable(feature = "box_into_boxed_slice", issue = "71582")] | ||
| pub fn into_boxed_slice(boxed: Box<T>) -> Box<[T]> { | ||
| // *mut T and *mut [T; 1] have the same size and alignment |
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This comment doesn't match the code. *mut T ≠ *mut [T]
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That comment is justifying the cast to an array of length 1. The slice comes after.
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What justifies the subsequent cast?
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That's just unsizing, but we could leave that out and let the Box<[T; 1]> implicitly coerce unsized in the return.
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Hi,
I think this is very useful, as currently it's not possible in safe rust to do this without re-allocating.
an alternative implementation of the same function can be:
The only thing that makes me a little uncomfortable is this line :
from https://rust-lang.github.io/unsafe-code-guidelines/layout/arrays-and-slices.html
But then I see:
from https://rust-lang.github.io/unsafe-code-guidelines/layout/pointers.html#representation
So I do believe this is valid(FWIW it also passes in miri https://play.rust-lang.org/?gist=c002b99364ee6b29862aeb3565a91c19)