Allow using C-unwind in stable Rust

Improves the situation in #539.

Author: madsmtm

.github/workflows/ci.yml

@@ -182,7 +182,7 @@ jobs:
         key: cargo-${{ github.job }}-${{ matrix.name }}-${{ hashFiles('**/Cargo.lock') }}
 
     - name: cargo check
-      run: cargo check $PUBLIC_CRATES $FRAMEWORKS_MACOS_14 $INTERESTING_FEATURES
+      run: cargo check $PUBLIC_CRATES $FRAMEWORKS_MACOS_14 --features=all
       env:
         RUSTFLAGS: "--codegen=debuginfo=0" # Removed --deny=warnings
 

crates/objc-sys/Cargo.toml

@@ -54,7 +54,12 @@ unstable-winobjc = ["gnustep-1-8"]
 # Link to ObjFW
 unstable-objfw = []
 
-# Use nightly c_unwind feature
+# Uses `extern "C-unwind"` on relevant function declarations.
+#
+# This raises MSRV to `1.71`.
+#
+# Warning: Enabling this is a breaking change for consumer crates, as it
+# changes the signature of functions.
 unstable-c-unwind = []
 
 # Private
@@ -65,6 +70,7 @@ cc = { version = "1.0.80", optional = true }
 
 [package.metadata.docs.rs]
 default-target = "aarch64-apple-darwin"
+features = ["unstable-c-unwind"]
 targets = [
     "aarch64-apple-darwin",
     "x86_64-apple-darwin",

crates/objc-sys/src/lib.rs

@@ -170,9 +170,8 @@
 #![allow(non_snake_case)]
 #![allow(missing_debug_implementations)]
 #![doc(html_root_url = "https://docs.rs/objc-sys/0.3.5")]
-#![cfg_attr(feature = "unstable-c-unwind", feature(c_unwind))]
 #![cfg_attr(docsrs, feature(doc_auto_cfg, doc_cfg_hide))]
-#![cfg_attr(docsrs, doc(cfg_hide(doc)))]
+#![cfg_attr(docsrs, doc(cfg_hide(doc, feature = "unstable-c-unwind")))]
 
 // TODO: Remove this and add "no-std" category to Cargo.toml
 // Requires a better solution for C-types in `no_std` crates.
@@ -213,10 +212,6 @@ macro_rules! generate_linking_tests {
                     // Get function pointer to make the linker require the
                     // symbol to be available.
                     let f: unsafe extern $abi fn($($(#[$a_m])* $t),*) $(-> $r)? = crate::$name;
-                    // Workaround for https://github.com/rust-lang/rust/pull/92964
-                    #[cfg(feature = "unstable-c-unwind")]
-                    #[allow(clippy::useless_transmute)]
-                    let f: unsafe extern "C" fn() = unsafe { core::mem::transmute(f) };
                     // Execute side-effect to ensure it is not optimized away.
                     std::println!("{:p}", f);
                 }

crates/objc2/Cargo.toml

@@ -76,7 +76,9 @@ unstable-static-class-inlined = ["unstable-static-class"]
 # Uses nightly features to make autorelease pools fully sound
 unstable-autoreleasesafe = []
 
-# Uses the nightly c_unwind feature to make throwing safe
+# Uses `extern "C-unwind"` to make method calls that throw safe.
+#
+# This raises MSRV to `1.71`.
 #
 # You must manually enable `objc-sys/unstable-c-unwind` to use this.
 unstable-c-unwind = []
@@ -137,7 +139,7 @@ harness = false
 
 [package.metadata.docs.rs]
 default-target = "aarch64-apple-darwin"
-features = ["exception"]
+features = ["unstable-c-unwind", "exception"]
 targets = [
     "aarch64-apple-darwin",
     "x86_64-apple-darwin",

crates/objc2/src/__macro_helpers/common_selectors.rs

@@ -56,7 +56,7 @@ fn cxx_construct_sel() -> Sel {
 /// take a selector, unlike every other Objective-C method, see:
 /// <https://github.com/apple-oss-distributions/objc4/blob/objc4-906/runtime/objc-class.mm#L457>
 ///
-/// So the signature is `extern "C" fn(*mut AnyObject)`.
+/// So the signature is `extern "C-unwind" fn(*mut AnyObject)`.
 ///
 /// This is likely because it's not a real Objective-C method that can be
 /// called from userspace / objc_msgSend, and it's more efficient to not pass
@@ -67,8 +67,6 @@ fn cxx_construct_sel() -> Sel {
 /// convention, where such an ignored parameter would be allowed on all
 /// relevant architectures.
 ///
-/// TODO: Unsure whether "C-unwind" is allowed?
-///
 /// [gcc-docs]: https://gcc.gnu.org/onlinedocs/gcc/Objective-C-and-Objective-C_002b_002b-Dialect-Options.html#index-fobjc-call-cxx-cdtors
 #[inline]
 #[allow(dead_code)] // May be useful in the future

crates/objc2/src/exception.rs

@@ -172,12 +172,11 @@ impl RefUnwindSafe for Exception {}
 /// Objective-C exception handler like [`catch`] (and specifically not
 /// [`catch_unwind`]).
 ///
-/// This also invokes undefined behaviour until `C-unwind` is stabilized, see
-/// [RFC-2945] - you can try this out on nightly using the `unstable-c-unwind`
-/// feature flag.
+/// This also invokes undefined behaviour unless `C-unwind` is used, which it
+/// only is if the `unstable-c-unwind` feature flag is enabled (raises MSRV to
+/// 1.71).
 ///
 /// [`catch_unwind`]: std::panic::catch_unwind
-/// [RFC-2945]: https://rust-lang.github.io/rfcs/2945-c-unwind-abi.html
 #[inline]
 #[cfg(feature = "exception")] // For consistency, not strictly required
 pub unsafe fn throw(exception: Retained<Exception>) -> ! {
@@ -274,11 +273,9 @@ unsafe fn try_no_ret<F: FnOnce()>(closure: F) -> Result<(), Option<Retained<Exce
 /// The given closure must not panic (e.g. normal Rust unwinding into this
 /// causes undefined behaviour).
 ///
-/// Additionally, this unwinds through the closure from Objective-C, which is
-/// undefined behaviour until `C-unwind` is stabilized, see [RFC-2945] - you
-/// can try this out on nightly using the `unstable-c-unwind` feature flag.
-///
-/// [RFC-2945]: https://rust-lang.github.io/rfcs/2945-c-unwind-abi.html
+/// This also invokes undefined behaviour unless `C-unwind` is used, which it
+/// only is if the `unstable-c-unwind` feature flag is enabled (raises MSRV to
+/// 1.71).
 #[cfg(feature = "exception")]
 pub unsafe fn catch<R>(
     closure: impl FnOnce() -> R + UnwindSafe,

crates/objc2/src/lib.rs

@@ -151,7 +151,6 @@
     feature = "unstable-autoreleasesafe",
     feature(negative_impls, auto_traits)
 )]
-#![cfg_attr(feature = "unstable-c-unwind", feature(c_unwind))]
 #![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
 #![warn(missing_docs)]
 #![warn(clippy::missing_errors_doc)]

crates/objc2/src/macros/mod.rs

@@ -831,17 +831,15 @@ macro_rules! __class_inner {
 /// # Panics
 ///
 /// Panics if the `"catch-all"` feature is enabled and the Objective-C method
-/// throws an exception. Exceptions may still cause UB until
-/// `extern "C-unwind"` is stable, see [RFC-2945].
+/// throws an exception. Exceptions may still cause UB unless you enable the
+/// `"unstable-c-unwind"` feature (raises MSRV to 1.71).
 ///
 /// Panics if `debug_assertions` are enabled and the Objective-C method's
 /// encoding does not match the encoding of the given arguments and return.
 ///
 /// And panics if the `NSError**` handling functionality described above is
 /// used, and the error object was unexpectedly `NULL`.
 ///
-/// [RFC-2945]: https://rust-lang.github.io/rfcs/2945-c-unwind-abi.html
-///
 ///
 /// # Safety
 ///