Remove uses of unstable feature(cfg_target_has_atomic) #2400

taiki-e · 2021-04-20T15:07:26Z

Instead of depending on unstable features of the compiler, use the TARGET environment variables provided by cargo for the build script and a list of targets that do not have atomic CAS.

This completely removes the dependency on unstable features from crates other than futures, futures-util, futures-io.

This way is inspired by the way heapless and defmt do, but this doesn't maintain the target list manually. (It's not really fully automated, but it's very easy to update.)

Closes #2399
Replaces #2294

r? @Nemo157 @cramertj
cc @jamesmunns @Dirbaio

Dirbaio · 2021-04-20T15:19:25Z

This is great! Thanks for working on it :)

Removing cfg-target-has-atomic will break users building for custom targets that have no atomics, though.

taiki-e · 2021-04-20T15:51:14Z

Removing cfg-target-has-atomic will break users building for custom targets that have no atomics, though.

Hmm, you are right.

That said, we want to reduce dependency on unstable features as much as possible... So perhaps the compromise is to make cfg(no_atomic_cas) an unstable public API so that atomics can be disabled via RUSTFLAGS:

RUSTFLAGS='--cfg futures_no_atomic_cas' cargo build

Dirbaio · 2021-04-20T16:00:04Z

Why not simply keep cfg-target-has-atomic as-is? I'm not a fan of having to use RUSTFLAGS. The standard way to customize lib behavior is cargo features.

Aditionally, removing it (or keeping it but making it do nothing) is semver-breaking: it makes code that compiled before stop compiling.

taiki-e · 2021-04-20T16:44:18Z

Why not simply keep cfg-target-has-atomic as-is? I'm not a fan of having to use RUSTFLAGS. The standard way to customize lib behavior is cargo features.

The current design (enabling unstable features by feature flags) is that dependencies can enable unstable features even if the user doesn't enable them directly, and it is problematic.
See also change from unstable cargo feature to RUSTFLAGS rayon-rs/rayon#369, Rayon 0.7.1 is broken rayon-rs/rayon#364, https://github.com/alexcrichton/proc-macro2#unstable-features, Limit futures dependency to Stream via feature flag tokio-rs/tokio#1774 (comment), etc.

Note that this must not only be done for your crate, but for any crate that depends on your crate. This infectious nature is intentional, as it serves as a reminder that you are outside of the normal semver guarantees.
feature(cfg_target_has_atomic) will never stable as far as I understand. Continuing to rely on such "forever unstable" features is a maintenance burden. (Actually, I think it will be removed in favor of cfg(accessible) in the future, not "forever unstable".)

semver-breaking

No, cfg-target-has-atomic feature is unstable and stability is not guaranteed.

futures-rs/futures/Cargo.toml

Lines 46 to 48 in c77cd3c

    
           # Unstable features 
        
           # These features are outside of the normal semver guarantees and require the 
        
           # `unstable` feature as an explicit opt-in to unstable API.

Dirbaio · 2021-04-20T16:48:05Z

I wasn't aware of this issue with unstable features and the shift to use RUSTFLAGS. Looks good to me then! 👍

jamesmunns · 2021-04-23T11:15:56Z

Hey @taiki-e, thank you for implementing this!

I have tested this change in my cassette repo (which prompted this), and I was able to replace my copy/paste exports with just exporting the futures dependency.

I can confirm this change works as expected for my project, at least :)

taiki-e · 2021-04-23T14:56:00Z

@jamesmunns Thanks for testing this!

taiki-e · 2021-04-23T14:58:13Z

futures-channel/build.rs

+// The rustc-cfg listed below are considered public API, but it is *unstable*
+// and outside of the normal semver guarantees:
+//
+// - `futures_no_atomic_cas`
+//      Assume the target does not have atomic CAS (compare-and-swap).
+//      This is usually detected automatically by the build script, but you may
+//      need to enable it manually when building for custom targets or using
+//      non-cargo build systems that don't run the build script.
+//
+// With the exceptions mentioned above, the rustc-cfg strings below are
+// *not* public API. Please let us know by opening a GitHub issue if your build
+// environment requires some way to enable these cfgs other than by executing
+// our build script.


As said in #2400 (comment), exposes cfg(futures_no_atomic_cas) as unstable public API (outside of the normal semver guarantees).

Also, we probably need to set #![cfg_attr(docsrs, doc(cfg_hide(futures_no_atomic_cas)))] when rust-lang/rust#79341 merged...

That might be a while 😅

Some no-std targets do not have atomic CAS and cannot use Arc, etc. This patch detects those targets using the TARGET environment variables provided by cargo for the build script, and a list of targets that do not have atomic CAS. This is a port of rust-lang/futures-rs#2400. See that PR for more.

698: Remove uses of unstable feature(cfg_target_has_atomic) r=taiki-e a=taiki-e Some no-std targets (e.g., ARMv6-M) do not support atomic CAS operations and cannot use Arc, etc. Currently, we are using an unstable feature to detect them, but it has caused breakage in the past (#435). Also, users of stable Rust are not able to compile crossbeam on those targets. Instead of depending on unstable features of the compiler, this patch detects those targets using the TARGET environment variables provided by cargo for the build script, and a list of targets that do not support atomic CAS operations. This way is the same as the way we recently adopted in [futures](rust-lang/futures-rs#2400) and [valuable](tokio-rs/valuable#12), and was originally inspired by the way [heapless](rust-embedded/heapless@44c66a7) and [defmt](https://github.com/knurling-rs/defmt/blob/963152f0fc530fca64ba4ff1492d9c4b7bf76062/build.rs#L42-L51) do, but this doesn't maintain the target list manually. (It's not really fully automated, but [it's very easy to update](https://github.com/crossbeam-rs/crossbeam/blob/a42dbed87a5739228b576f526b1e2fd80260a29b/.github/workflows/ci.yml#L89).) Also, this completely removes the dependency on unstable features from crates other than crossbeam-epoch. refs: rust-lang/rust#51953, rust-lang/futures-rs#2400, tokio-rs/valuable#12 704: Add AtomicCell::fetch_update r=taiki-e a=taiki-e Equivalent of [`std::sync::atomic::AtomicN::fetch_update`](https://doc.rust-lang.org/nightly/core/sync/atomic/struct.AtomicUsize.html#method.fetch_update) that stabilized in Rust 1.45. Co-authored-by: Taiki Endo <te316e89@gmail.com>

taiki-e force-pushed the cfg_target_has_atomic-2 branch from ba8cc00 to 2b5d082 Compare April 20, 2021 15:08

taiki-e mentioned this pull request Apr 20, 2021

Feedback Request: "disable-cas-atomics" feature #2399

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jamesmunns mentioned this pull request Apr 23, 2021

Test with new target detection jamesmunns/cassette#1

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taiki-e force-pushed the cfg_target_has_atomic-2 branch from 0457c51 to 3f73ddd Compare April 23, 2021 14:38

taiki-e commented Apr 23, 2021

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taiki-e added the 0.3-backport: pending The maintainer accepted to backport this to the 0.3 branch, but backport has not been done yet. label Apr 23, 2021

taiki-e force-pushed the cfg_target_has_atomic-2 branch 2 times, most recently from 7a092cb to 75c184b Compare May 5, 2021 09:37

Remove uses of unstable feature(cfg_target_has_atomic)

d0ca511

taiki-e force-pushed the cfg_target_has_atomic-2 branch from 75c184b to d0ca511 Compare May 5, 2021 10:10

taiki-e merged commit c180a3a into rust-lang:master May 6, 2021

taiki-e deleted the cfg_target_has_atomic-2 branch May 6, 2021 11:12

taiki-e mentioned this pull request May 7, 2021

0.3 backports #2419

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taiki-e added 0.3-backport: completed and removed 0.3-backport: pending The maintainer accepted to backport this to the 0.3 branch, but backport has not been done yet. labels May 7, 2021

This was referenced May 7, 2021

Remove unneeded cfg-target-has-atomic feature #2422

Merged

Add feature to support platforms without atomic CAS tokio-rs/bytes#467

Open

Use futuresordered in join_all #2412

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Release 0.3.15 #2436

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This was referenced May 11, 2021

Fix build error with targets that do not support atomic CAS operations tokio-rs/valuable#12

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Remove cfg_target_has_atomic! macro #2439

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Remove uses of unstable feature(cfg_target_has_atomic) crossbeam-rs/crossbeam#698

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This was referenced Aug 7, 2021

Attempting to use Atomics larger than max. atomic width crossbeam-rs/crossbeam#731

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once_cell::race does not compile on ARM matklad/once_cell#155

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taiki-e mentioned this pull request Feb 6, 2022

Automatically creates PR when no_atomic_cas.rs needs to be updated #2562

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Remove uses of unstable feature(cfg_target_has_atomic) #2400

Remove uses of unstable feature(cfg_target_has_atomic) #2400

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Remove uses of unstable feature(cfg_target_has_atomic) #2400

Remove uses of unstable feature(cfg_target_has_atomic) #2400

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