Stabilize const versions of ptr::slice_from_raw_parts and slice::from_raw_parts. #94946

tobz · 2022-03-15T00:51:02Z

This doesn't close the tracking issue (#67456) as there's still the mutable variants to consider, but the immutable variants should seemingly be fine to stabilize.

Apologies if this stabilization PR is lacking something: it wasn't immediately clear if there was a different process for partial stabilizations.

…_raw_parts.

rust-highfive · 2022-03-15T00:51:04Z

Some changes occured to rustc_codegen_cranelift

cc @bjorn3

rust-highfive · 2022-03-15T00:51:05Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @dtolnay (or someone else) soon.

Please see the contribution instructions for more information.

tobz · 2022-03-15T00:51:24Z

@rustbot modify labels: +T-libs-api

tobz · 2022-03-15T00:55:59Z

library/core/src/slice/raw.rs

-#[unstable(feature = "const_slice_from_raw_parts", issue = "67456")]
-#[rustc_const_unstable(feature = "const_slice_from_raw_parts", issue = "67456")]
+#[rustc_const_stable(since = "1.61.0")]


Admittedly, this is a change I'm not 100% sure on: its callers are stable in a non-const context, and yet it was marked unstable. I have no real clue what unstable even implies for a non-exported function, so this was mostly a no-thinking-involved mechanical change.

tobz · 2022-03-15T01:39:05Z

Looking at the CI failure, it looks like this actually unwinds a bit to depending on ptr::from_raw_parts being const-stable, which makes sense on a surface level but it also seems to power non-const-fns, despite also being marked unstable?

It's not clear to me if it can also be made const stable safely? This is probably one of those "wait for help" scenarios, at this point. :)

dtolnay

Anything used in the implementation of a public stable const fn needs to be marked rustc_const_stable.

error: `ptr::metadata::from_raw_parts` is not yet stable as a const fn
    |
260 |     from_raw_parts(data.cast(), len)
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

rust/library/core/src/ptr/metadata.rs

Lines 108 to 110 in f5975e5

    
           #[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")] 
        
           #[inline] 
        
           pub const fn from_raw_parts<T: ?Sized>(

tobz · 2022-03-19T15:11:50Z

Anything used in the implementation of a public stable const fn needs to be marked rustc_const_stable.

That's reasonable. Is that something we/I can do in this case? Admittedly, it's not clear to me.

bors · 2022-03-26T23:04:50Z

☔ The latest upstream changes (presumably #95274) made this pull request unmergeable. Please resolve the merge conflicts.

…arts

bors · 2022-04-03T18:35:59Z

☔ The latest upstream changes (presumably #92686) made this pull request unmergeable. Please resolve the merge conflicts.

dtolnay

Yes, you will need to add rustc_const_stable attributes for all the private functions getting called by these public functions. Whether it's something we can do depends on what functions end up needing to get rustc_const_stable added. That would be reviewed after the changes are made.

JohnCSimon · 2022-05-07T23:42:50Z

Ping from triage:
@tobz - can you please address the conflicts and build failures?

FYI: when a PR is ready for review, send a message containing
@rustbot ready to switch to S-waiting-on-review so the PR is in the reviewer's backlog.

Signed-off-by: Toby Lawrence <[email protected]>

…arts

Signed-off-by: Toby Lawrence <[email protected]>

rust-log-analyzer · 2022-05-08T01:21:27Z

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling unwind v0.0.0 (/checkout/library/unwind)
error[E0711]: feature `ptr_metadata` is declared stable, but was previously declared unstable
   --> library/core/src/ptr/metadata.rs:108:1
    |
108 | #[rustc_const_stable(feature = "ptr_metadata", since = "1.62.0")]

error: could not compile `core` due to previous error
Build completed unsuccessfully in 0:01:25

tobz · 2022-05-08T02:14:55Z

I'm going to close this PR for now.

Working through adjusting the const stability for the transitive functions landed me at having to do so for the ptr_metadata feature.. which is obviously not ready to be const stable yet, and so spending any more time -- either me making the PR pass or someone reviewing it only to also come to the same conclusion -- wouldn't make much sense.

KamilaBorowska · 2022-05-21T08:25:57Z

Is there a problem with marking std::ptr::from_raw_parts as const stable? Note that marking a function as const stable doesn't mean having to stabilize it, see https://rustc-dev-guide.rust-lang.org/stability.html#rustc_const_stable.

This attribute can make sense even on an unstable function, if that function is called from another rustc_const_stable function.

…r=dtolnay Partially stabilize const_slice_from_raw_parts This doesn't stabilize methods working on mutable pointers. This pull request continues from rust-lang#94946. Pinging `@rust-lang/wg-const-eval` this because I use `rustc_allow_const_fn_unstable`. I believe this is justifiable as it's already possible to use `slice::from_raw_parts` in stable by abusing `transmute`. The stable alternative to this would be to provide a stable const implementation of `std::ptr::from_raw_parts` (as it can already be implemented in stable). ```rust use std::mem; #[repr(C)] struct Slice<T> { data: *const T, len: usize, } fn main() { let data: *const i32 = [1, 2, 3, 4].as_ptr(); let len = 4; println!("{:?}", unsafe { mem::transmute::<Slice<i32>, &[i32]>(Slice { data, len }) }); } ``` `@rustbot` modify labels: +T-libs-api

Stabilize const versions of ptr::slice_from_raw_parts and slice::from…

35446b6

…_raw_parts.

rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Mar 15, 2022

rust-highfive assigned dtolnay Mar 15, 2022

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Mar 15, 2022

rustbot added the T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. label Mar 15, 2022

tobz commented Mar 15, 2022

View reviewed changes

Fix incorrect usages of rustc_const_stable attribute.

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dtolnay removed the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Mar 18, 2022

dtolnay reviewed Mar 18, 2022

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tobz added 3 commits March 26, 2022 22:04

mark dependent const fn as stable as well

78a8268

Merge branch 'master' into stabilize-immut-const-ptr-slice-from-raw-p…

01fd51e

…arts

add missing feature field to const stable attribute

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dtolnay added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 16, 2022

dtolnay requested changes Apr 16, 2022

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tobz added 2 commits May 7, 2022 20:47

mark intrinsics::const_eval_select as const stable

19e1e4d

Signed-off-by: Toby Lawrence <[email protected]>

Merge branch 'master' into stabilize-immut-const-ptr-slice-from-raw-p…

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bisect immut/mut const slicing to make CI happy

d4a95ab

Signed-off-by: Toby Lawrence <[email protected]>

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set theright since version

658a36a

Signed-off-by: Toby Lawrence <[email protected]>

tobz closed this May 8, 2022

tobz deleted the stabilize-immut-const-ptr-slice-from-raw-parts branch May 8, 2022 02:14

KamilaBorowska mentioned this pull request May 29, 2022

Partially stabilize const_slice_from_raw_parts #97522

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Stabilize const versions of ptr::slice_from_raw_parts and slice::from_raw_parts. #94946

Stabilize const versions of ptr::slice_from_raw_parts and slice::from_raw_parts. #94946

tobz commented Mar 15, 2022 •

edited by dtolnay

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KamilaBorowska commented May 21, 2022

	#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
	#[inline]
	pub const fn from_raw_parts<T: ?Sized>(

Stabilize const versions of ptr::slice_from_raw_parts and slice::from_raw_parts. #94946

Stabilize const versions of ptr::slice_from_raw_parts and slice::from_raw_parts. #94946

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