Stabilise c_unwind

compiler/rustc_feature/src/accepted.rs

@@ -79,6 +79,8 @@ declare_features! (
     (accepted, braced_empty_structs, "1.8.0", Some(29720), None),
     /// Allows `c"foo"` literals.
     (accepted, c_str_literals, "CURRENT_RUSTC_VERSION", Some(105723), None),
+    /// Allows `extern "C-unwind" fn` to enable unwinding across ABI boundaries and treat `extern "C" fn` as nounwind.
+    (accepted, c_unwind, "CURRENT_RUSTC_VERSION", Some(74990), None),
     /// Allows `#[cfg_attr(predicate, multiple, attributes, here)]`.
     (accepted, cfg_attr_multi, "1.33.0", Some(54881), None),
     /// Allows the use of `#[cfg(doctest)]`, set when rustdoc is collecting doctests.

compiler/rustc_feature/src/unstable.rs

@@ -354,8 +354,6 @@ declare_features! (
     (unstable, async_fn_track_caller, "1.73.0", Some(110011), None),
     /// Allows builtin # foo() syntax
     (unstable, builtin_syntax, "1.71.0", Some(110680), None),
-    /// Treat `extern "C"` function as nounwind.
-    (unstable, c_unwind, "1.52.0", Some(74990), None),
     /// Allows using C-variadics.
     (unstable, c_variadic, "1.34.0", Some(44930), None),
     /// Allows the use of `#[cfg(overflow_checks)` to check if integer overflow behaviour.

compiler/rustc_middle/src/ty/layout.rs

@@ -1197,37 +1197,6 @@ pub fn fn_can_unwind(tcx: TyCtxt<'_>, fn_def_id: Option<DefId>, abi: SpecAbi) ->
     // ABIs have such an option. Otherwise the only other thing here is Rust
     // itself, and those ABIs are determined by the panic strategy configured
     // for this compilation.
-    //
-    // Unfortunately at this time there's also another caveat. Rust [RFC
-    // 2945][rfc] has been accepted and is in the process of being implemented
-    // and stabilized. In this interim state we need to deal with historical
-    // rustc behavior as well as plan for future rustc behavior.
-    //
-    // Historically functions declared with `extern "C"` were marked at the
-    // codegen layer as `nounwind`. This happened regardless of `panic=unwind`
-    // or not. This is UB for functions in `panic=unwind` mode that then
-    // actually panic and unwind. Note that this behavior is true for both
-    // externally declared functions as well as Rust-defined function.
-    //
-    // To fix this UB rustc would like to change in the future to catch unwinds
-    // from function calls that may unwind within a Rust-defined `extern "C"`
-    // function and forcibly abort the process, thereby respecting the
-    // `nounwind` attribute emitted for `extern "C"`. This behavior change isn't
-    // ready to roll out, so determining whether or not the `C` family of ABIs
-    // unwinds is conditional not only on their definition but also whether the
-    // `#![feature(c_unwind)]` feature gate is active.
-    //
-    // Note that this means that unlike historical compilers rustc now, by
-    // default, unconditionally thinks that the `C` ABI may unwind. This will
-    // prevent some optimization opportunities, however, so we try to scope this
-    // change and only assume that `C` unwinds with `panic=unwind` (as opposed
-    // to `panic=abort`).
-    //
-    // Eventually the check against `c_unwind` here will ideally get removed and
-    // this'll be a little cleaner as it'll be a straightforward check of the
-    // ABI.
-    //
-    // [rfc]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md
     use SpecAbi::*;
     match abi {
         C { unwind }
@@ -1239,10 +1208,7 @@ pub fn fn_can_unwind(tcx: TyCtxt<'_>, fn_def_id: Option<DefId>, abi: SpecAbi) ->
         | Thiscall { unwind }
         | Aapcs { unwind }
         | Win64 { unwind }
-        | SysV64 { unwind } => {
-            unwind
-                || (!tcx.features().c_unwind && tcx.sess.panic_strategy() == PanicStrategy::Unwind)
-        }
+        | SysV64 { unwind } => unwind,
         PtxKernel
         | Msp430Interrupt
         | X86Interrupt

library/alloc/src/lib.rs

@@ -167,13 +167,13 @@
 //
 // Language features:
 // tidy-alphabetical-start
+#![cfg_attr(bootstrap, feature(c_unwind))]
 #![cfg_attr(not(test), feature(coroutine_trait))]
 #![cfg_attr(test, feature(panic_update_hook))]
 #![cfg_attr(test, feature(test))]
 #![feature(allocator_internals)]
 #![feature(allow_internal_unstable)]
 #![feature(associated_type_bounds)]
-#![feature(c_unwind)]
 #![feature(cfg_sanitize)]
 #![feature(const_mut_refs)]
 #![feature(const_precise_live_drops)]

library/core/src/lib.rs

@@ -198,14 +198,14 @@
 //
 // Language features:
 // tidy-alphabetical-start
+#![cfg_attr(bootstrap, feature(c_unwind))]
 #![feature(abi_unadjusted)]
 #![feature(adt_const_params)]
 #![feature(allow_internal_unsafe)]
 #![feature(allow_internal_unstable)]
 #![feature(asm_const)]
 #![feature(associated_type_bounds)]
 #![feature(auto_traits)]
-#![feature(c_unwind)]
 #![feature(cfg_sanitize)]
 #![feature(cfg_target_has_atomic)]
 #![feature(cfg_target_has_atomic_equal_alignment)]

library/panic_abort/src/lib.rs

@@ -13,7 +13,7 @@
 #![feature(std_internals)]
 #![feature(staged_api)]
 #![feature(rustc_attrs)]
-#![feature(c_unwind)]
+#![cfg_attr(bootstrap, feature(c_unwind))]
 #![allow(internal_features)]
 
 #[cfg(target_os = "android")]

library/panic_unwind/src/lib.rs

@@ -22,7 +22,7 @@
 #![feature(rustc_attrs)]
 #![panic_runtime]
 #![feature(panic_runtime)]
-#![feature(c_unwind)]
+#![cfg_attr(bootstrap, feature(c_unwind))]
 // `real_imp` is unused with Miri, so silence warnings.
 #![cfg_attr(miri, allow(dead_code))]
 #![allow(internal_features)]

library/std/src/lib.rs

@@ -265,11 +265,11 @@
 //
 // Language features:
 // tidy-alphabetical-start
+#![cfg_attr(bootstrap, feature(c_unwind))]
 #![feature(alloc_error_handler)]
 #![feature(allocator_internals)]
 #![feature(allow_internal_unsafe)]
 #![feature(allow_internal_unstable)]
-#![feature(c_unwind)]
 #![feature(cfg_target_thread_local)]
 #![feature(cfi_encoding)]
 #![feature(concat_idents)]

library/unwind/src/lib.rs

@@ -2,7 +2,7 @@
 #![unstable(feature = "panic_unwind", issue = "32837")]
 #![feature(link_cfg)]
 #![feature(staged_api)]
-#![feature(c_unwind)]
+#![cfg_attr(bootstrap, feature(c_unwind))]
 #![feature(cfg_target_abi)]
 #![feature(strict_provenance)]
 #![cfg_attr(not(target_env = "msvc"), feature(libc))]