Implement ptr_cast_array
#144515
Implement ptr_cast_array
#144515
Conversation
| @@ -330,7 +330,7 @@ impl<T> [T] { | |||
| } else { | |||
| // SAFETY: We explicitly check for the correct number of elements, | |||
| // and do not let the reference outlive the slice. | |||
| Some(unsafe { &*(self.as_ptr().cast::<[T; N]>()) }) | |||
| Some(unsafe { &*(self.as_ptr().cast_array()) }) | |||
There was a problem hiding this comment.
I noticed this in the libs-api thread before it was accepted but didn't want to be off-topic.
I was wondering how many uses of unsafe ptr casts are really just working around lack of const casting traits.
I think all of chunk methods could also be a fixme(const-hack) for TryFrom not being const before the const trait initiative. So the unsafe could also be removed here eventually.
I thought the other cast_array uses were more motivating outside of this module.
There was a problem hiding this comment.
True, there's a bunch of different ways this could be done. Certainly there's also options like self.as_chunks().0.first() that would work for this function.
I don't think these chunk methods would be TryFrom, though, because the &[T; N] as TryFrom<&[T]> intentionally requires an exact length match. (That's aligned with the general guidance that "from" stuff gives the "same" thing, and having them be eq needs the lengths to be the same.)
But yeah, .get(..N)?.as_array() (https://doc.rust-lang.org/nightly/std/primitive.slice.html#method.as_array) would be an option if get worked in const. (That's still two checks in the MIR, though, so dunno if it's better that way or not.)
EDIT: Oh, wait, not, the as_chunks version doesn't work because that panics for N == 0. But yes, lots of options. Even split_at_checked(N)?.0.as_array(), I suppose, which doesn't need to wait for const-trait.
There was a problem hiding this comment.
I was thinking of TryInto (unless that has the same issues and I'm just mistaken/misremembering). But as you said, now we have a lot of other options.
There was a problem hiding this comment.
I was curious and ended up implementing them with the slice_as_array feature in this branch okaneco@a64b928
I haven't checked them all, but it seems to optimize to the current assembly output without extra length checks for the array cast.
split_last_chunk_mut - https://rust.godbolt.org/z/Y4oa9rh5c
|
r? libs |
|
@bors r+ rollup |
Rollup of 13 pull requests Successful merges: - #140434 (rustdoc: Allow multiple references to a single footnote) - #142372 (Improve `--remap-path-prefix` documentation) - #142741 (Fix unsoundness in some tests) - #144515 (Implement `ptr_cast_array`) - #144727 (Add tracing to resolve-related functions) - #144959 (fix(unicode-table-generator): fix duplicated unique indices) - #145179 (Avoid abbreviating "numerator" as "numer", to allow catching typo "numer" elsewhere) - #145250 (Add regression test for a former ICE involving helper attributes containing interpolated tokens) - #145266 (Reduce some queries around associated items) - #145299 (doc test: fix mpsc.rs try_send doc test) - #145323 (Port the `#[linkage]` attribute to the new attribute system) - #145361 (Suppress wrapper suggestion when expected and actual ty are the same adt and the variant is unresolved) - #145372 (resolve: Miscellaneous cleanups) r? `@ghost` `@rustbot` modify labels: rollup
Rollup merge of #144515 - scottmcm:ptr_cast_array, r=Mark-Simulacrum Implement `ptr_cast_array` ACP: rust-lang/libs-team#602 Tracking Issue: #144514
Rollup of 13 pull requests Successful merges: - rust-lang/rust#140434 (rustdoc: Allow multiple references to a single footnote) - rust-lang/rust#142372 (Improve `--remap-path-prefix` documentation) - rust-lang/rust#142741 (Fix unsoundness in some tests) - rust-lang/rust#144515 (Implement `ptr_cast_array`) - rust-lang/rust#144727 (Add tracing to resolve-related functions) - rust-lang/rust#144959 (fix(unicode-table-generator): fix duplicated unique indices) - rust-lang/rust#145179 (Avoid abbreviating "numerator" as "numer", to allow catching typo "numer" elsewhere) - rust-lang/rust#145250 (Add regression test for a former ICE involving helper attributes containing interpolated tokens) - rust-lang/rust#145266 (Reduce some queries around associated items) - rust-lang/rust#145299 (doc test: fix mpsc.rs try_send doc test) - rust-lang/rust#145323 (Port the `#[linkage]` attribute to the new attribute system) - rust-lang/rust#145361 (Suppress wrapper suggestion when expected and actual ty are the same adt and the variant is unresolved) - rust-lang/rust#145372 (resolve: Miscellaneous cleanups) r? `@ghost` `@rustbot` modify labels: rollup
ACP: rust-lang/libs-team#602
Tracking Issue: #144514