downgrade ptr.is_aligned_to crate-private #121920

Gankra · 2024-03-03T01:36:01Z

Per #96284 (comment) this API is a mess and is regarded as insufficiently motivated. Removing it from the public API is potentially the only blocker on stabilizing its pleasant brother .is_aligned().

I initially just deleted it completely but it's since become Heavily Used for the purposes of assert_unsafe_precondition, and there's clearly Some Stuff going on with that API that I have no interest in poking.

rustbot · 2024-03-03T01:36:09Z

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rustbot · 2024-03-03T01:36:11Z

The Miri subtree was changed

cc @rust-lang/miri

tests/ui/structs-enums/type-sizes.rs

RalfJung · 2024-03-03T08:15:48Z

src/tools/miri/tests/pass-dep/shims/posix_memalign.rs

@@ -37,7 +37,7 @@ fn main() {
        let size = 31;
        assert_eq!(libc::posix_memalign(&mut ptr, align, size), 0);
        assert!(!ptr.is_null());
-        assert!(ptr.is_aligned_to(align));


This does seem like the function can be quite nice to have, doesn't it? ptr.is_aligned_to(align) is a lot easier to read than the modulo check.

And if it's used heavily inside the standard library (and hence can't be removed entirely, only be made private), that is also a data point in favor of it actually being useful outside of the standard library. So I don't quite see why it should get removed.

It seems to only be used for asserts. Maybe we should have a new assert macro?

In general, an application developer probably wants .is_aligned_to::<N>. It is rare that you truly want a dynamic alignment check.

so it's only used heavily in the stdlib at this point because:

people not being aware how powerful ptr.cast::<T>().some_check() is as an idiom

assert_unsafe_precondition was added in a ton of places calling is_aligned_and_not_null which calls this

The former I rewrote, the latter is a bit ??? to me. The functions/macros are clearly written in a very specific style that avoids generics, and I can't tell if this is Important For Something or someone being idiosyncratic for no good reason.

Sorry just to clarify: as used is_aligned_and_not_null could just be changed to be generic and call .is_aligned(). But I'm not sure if that would break some magic.

also Extremely funny saethlin is the one who suggested removing it X3

🤨 I don't and didn't mean that we should never stabilize such a function. But at the current time, I think we have evidence that this doesn't meet the bar for what goes in the standard library.

In particular, I find the API of this function quite vexing. The usize argument raises annoying questions because not every usize is a valid alignment. The design needs to commit to one of

Don't try to detect alignments that aren't a power of 2. I dislike this option because we shouldn't let trivial-to-detect bugs go unnoticed. A debug assertion would get compiled out of the distributed standard library and therefore not really exist for most users so I'm not really in favor of that.

Panic when passed an alignment that's not a power of 2. I dislike this option because generates a substantial amount of IR and I expect that if this function is used by performance nerds like me the minor inefficiency will invite people to roll their own and advocate against using the standard library function. At which point, why are we stabilizing something people will replace in a third-party library.

Make the alignment a const param like Jubilee says above. This would rule out 2 of the 3 users of a function called is_aligned_to that I linked in the tracking issue. So I'm not particularly convinced that there are many users of a const ALIGN version, considering that the 1 user who could use such an API can also use is_aligned::<T>().

Change the argument type to an Alignment. This seems to me like the right way to design a Rust API, because it rules out invalid usage with the type system. This option also increases the difficulty of stabilizing something, but that seems fine when the other options are stabilizing something with a high chance of regret.

To be clear I think it's perfectly fine to stabilize is_aligned (which checks the pointee type, right?) and leave is_aligned_to unstable, if there is consensus that is_aligned is sufficiently useful and will remain useful even if is_aligned_to eventually becomes stable. That would require moving them to different feature gates.

I am just surprised about the proposal of making them private. That's a pretty rare thing to be done with a unstable std API I think. Usually it either gets removed or stabilized.

Ah I guess you're already doing that in #121948. :)

I'll add my 2¢: I think is_aligned_to is useful (even if much less than plain is_aligned), as suggested by the uses in std and other potential uses in the ecosystem. But I also agree that accepting usize is not nice.

IMO we should change it to accepting Alignment and stabilize is_aligned separately.

Per rust-lang#96284 (comment) this API is a mess and is regarded as insufficiently motivated. Removing it from the public API is potentially the only blocker on stabilizing its pleasant brother .is_aligned(). I initially just deleted it completely but it's since become Heavily Used for the purposes of assert_unsafe_precondition, and there's clearly Some Stuff going on with that API that I have no interest in poking.

rust-log-analyzer · 2024-03-03T14:08:06Z

The job x86_64-gnu-llvm-16 failed! Check out the build log: (web) (plain)

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

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#12 writing image sha256:4253a932050ed852ce3d1f0b4c99fd9e4d6d38cd94cdad2f3aefbe18a39ffacf done
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#12 DONE 10.0s
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Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-16]
##[group]Clock drift check
  local time: Sun Mar  3 13:51:26 UTC 2024
  network time: Sun, 03 Mar 2024 13:51:26 GMT
  network time: Sun, 03 Mar 2024 13:51:26 GMT
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sccache: Starting the server...
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configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-16', '--enable-llvm-link-shared', '--set', 'rust.thin-lto-import-instr-limit=10', '--set', 'change-id=99999999', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'build.optimized-compiler-builtins', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-16/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.thin-lto-import-instr-limit := 10
configure: change-id            := 99999999
---
##[endgroup]
##[group]Testing stage2 compiletest suite=assembly mode=assembly (x86_64-unknown-linux-gnu)

running 412 tests
ii.....................iiiiiiiiiiii.....iFi..iiiiiiiiiiiiiiiF...........................  88/412
........................................................................................ 264/412
........................................................................................ 352/412
..............................i..iii........................


failures:

---- [assembly] tests/assembly/is_aligned.rs#opt-speed stdout ----

error in revision `opt-speed`: compilation failed!
status: exit status: 1
command: RUSTC_ICE="0" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/assembly/is_aligned.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "--sysroot" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2" "--target=x86_64-unknown-linux-gnu" "--cfg" "opt_speed" "-O" "-Cdebug-assertions=no" "--emit" "asm" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/assembly/is_aligned.opt-speed/is_aligned.s" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/assembly/is_aligned.opt-speed/auxiliary" "-Copt-level=2" "-Cdebug-assertions=no"
stdout: none
--- stderr -------------------------------
error[E0624]: method `is_aligned_to` is private
##[error]  --> /checkout/tests/assembly/is_aligned.rs:22:9
##[error]  --> /checkout/tests/assembly/is_aligned.rs:22:9
   |
22 |     ptr.is_aligned_to(align)
  --> /rustc/FAKE_PREFIX/library/core/src/ptr/const_ptr.rs:1512:5
   |
   = note: private method defined here

---


---- [assembly] tests/assembly/is_aligned.rs#opt-size stdout ----

error in revision `opt-size`: compilation failed!
status: exit status: 1
command: RUSTC_ICE="0" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/assembly/is_aligned.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "--sysroot" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2" "--target=x86_64-unknown-linux-gnu" "--cfg" "opt_size" "-O" "-Cdebug-assertions=no" "--emit" "asm" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/assembly/is_aligned.opt-size/is_aligned.s" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/assembly/is_aligned.opt-size/auxiliary" "-Copt-level=s" "-Cdebug-assertions=no"
--- stderr -------------------------------
error[E0624]: method `is_aligned_to` is private
##[error]  --> /checkout/tests/assembly/is_aligned.rs:22:9
   |
   |
22 |     ptr.is_aligned_to(align)
  --> /rustc/FAKE_PREFIX/library/core/src/ptr/const_ptr.rs:1512:5
   |
   = note: private method defined here

Gankra · 2024-03-03T14:36:00Z

(PR currently failing because: there's a codegen test for this function, which, is still desirable in a sense. Leaving the PR as-is for the sake of discussing whether "insufficiently motivated" is still true.)

This is an alternative to rust-lang#121920

bors · 2024-03-06T04:48:08Z

☔ The latest upstream changes (presumably #122045) made this pull request unmergeable. Please resolve the merge conflicts.

stabilize ptr.is_aligned, move ptr.is_aligned_to to a new feature gate This is an alternative to rust-lang#121920

Amanieu · 2024-03-19T16:51:18Z

Libs-api discussed this and is happy to remove this unstable API by making is crate-private.

@rfcbot fcp merge

rfcbot · 2024-03-19T16:51:19Z

Team member @Amanieu has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

rfcbot · 2024-03-19T22:19:12Z

🔔 This is now entering its final comment period, as per the review above. 🔔

rfcbot · 2024-03-29T22:20:00Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

Gankra · 2024-03-30T00:07:33Z

uhhh this pr is a strict alternative to #121948, which is also accepted? (i rebased that one, it's... strictly more pleasant, i think?)

This is an alternative to rust-lang#121920

stabilize ptr.is_aligned, move ptr.is_aligned_to to a new feature gate This is an alternative to rust-lang#121920

RalfJung · 2024-03-30T09:06:10Z

Agreed, we should land #121948, not this.

@rust-lang/libs-api you FCP-approved two PRs that contradict each other about how ptr.is_aligned_to should be handled: make it a new nightly feature vs make it crate-private. #121948 made it a new nightly feature and already landed. I think we should close this PR despite FCP having passed here.

m-ou-se · 2024-04-02T15:05:28Z

I believe the point of the FCP on #121948 was to accept stabilization of is_aligned, while the FCP on this PR is to accept the downgrade of is_aligned_to.

RalfJung · 2024-04-02T15:12:25Z

The author of this PR (and other people, like myself) no longer see a need to downgrade is_aligned_to. The original motivation was that it was believed to help stabilizing is_aligned, but that turned out not to be the case. Do you still think that it should be done?

dtolnay · 2024-04-02T16:36:37Z

Taking into account #96284 (comment), those of us in today's libs-api meeting didn't see is_aligned_to being on track for stabilization in the future. We'd like to rip it out and reset the clock on is_aligned_to: the next step would be to bring an API Change Proposal that justifies why this function is worth having once is_aligned already is available.

If, in short order, we can find someone who volunteers that they plan to write an ACP justifying is_aligned_to, we can hold off on ripping it out.

This is an alternative to rust-lang#121920

Dylan-DPC · 2024-08-10T06:26:20Z

@Gankra any updates on the acp required for this? thanks

Gankra · 2024-08-11T22:27:45Z

This PR should have been closed a while ago, per #121948

rustbot assigned m-ou-se Mar 3, 2024

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workingjubilee reviewed Mar 3, 2024

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Gankra force-pushed the is_aligned_to branch from a4f94bb to 481e6b5 Compare March 3, 2024 13:49

Gankra added a commit to Gankra/rust that referenced this pull request Mar 3, 2024

stabilize ptr.is_aligned, move ptr.is_aligned_to to a new feature gate

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This is an alternative to rust-lang#121920

Gankra mentioned this pull request Mar 3, 2024

stabilize ptr.is_aligned, move ptr.is_aligned_to to a new feature gate #121948

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stabilize ptr.is_aligned, move ptr.is_aligned_to to a new feature gate

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This is an alternative to rust-lang#121920

m-ou-se added the I-libs-api-nominated Nominated for discussion during a libs-api team meeting. label Mar 5, 2024

bors added a commit to rust-lang-ci/rust that referenced this pull request Mar 16, 2024

Auto merge of rust-lang#121948 - Gankra:stab-align, r=dtolnay

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stabilize ptr.is_aligned, move ptr.is_aligned_to to a new feature gate This is an alternative to rust-lang#121920

rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Mar 19, 2024

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rfcbot added the final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. label Mar 19, 2024

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bors pushed a commit to rust-lang-ci/rust that referenced this pull request Mar 30, 2024

stabilize ptr.is_aligned, move ptr.is_aligned_to to a new feature gate

ea92fae

This is an alternative to rust-lang#121920

bors added a commit to rust-lang-ci/rust that referenced this pull request Mar 30, 2024

Auto merge of rust-lang#121948 - Gankra:stab-align, r=dtolnay

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stabilize ptr.is_aligned, move ptr.is_aligned_to to a new feature gate This is an alternative to rust-lang#121920

RalfJung added the I-libs-api-nominated Nominated for discussion during a libs-api team meeting. label Mar 30, 2024

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stabilize ptr.is_aligned, move ptr.is_aligned_to to a new feature gate

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This is an alternative to rust-lang#121920

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downgrade ptr.is_aligned_to crate-private #121920

downgrade ptr.is_aligned_to crate-private #121920

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downgrade ptr.is_aligned_to crate-private #121920

downgrade ptr.is_aligned_to crate-private #121920

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