Auto merge of rust-lang#116088 - nbdd0121:unwind, r=<try>

Stabilise `c_unwind`

Fix rust-lang#74990
Fix rust-lang#115285

Marking as draft PR for now due to `compiler_builtins` issues

r? `@Amanieu`

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

@@ -362,8 +362,6 @@ 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/proc_macro/src/bridge/buffer.rs

@@ -10,8 +10,10 @@ pub struct Buffer {
     data: *mut u8,
     len: usize,
     capacity: usize,
-    reserve: extern "C" fn(Buffer, usize) -> Buffer,
-    drop: extern "C" fn(Buffer),
+    // HACK(nbdd0121): These should be `extern "C"` but LLVM < 17.0.4 will misoptimise if we do so.
+    // Remove this when we drop support for them or when we require patches to be present.
+    reserve: extern "C-unwind" fn(Buffer, usize) -> Buffer,
+    drop: extern "C-unwind" fn(Buffer),
 }
 
 unsafe impl Sync for Buffer {}
@@ -141,13 +143,13 @@ impl From<Vec<u8>> for Buffer {
             }
         }
 
-        extern "C" fn reserve(b: Buffer, additional: usize) -> Buffer {
+        extern "C-unwind" fn reserve(b: Buffer, additional: usize) -> Buffer {
             let mut v = to_vec(b);
             v.reserve(additional);
             Buffer::from(v)
         }
 
-        extern "C" fn drop(b: Buffer) {
+        extern "C-unwind" fn drop(b: Buffer) {
             mem::drop(to_vec(b));
         }
 

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))]

src/tools/miri/tests/fail-dep/concurrency/unwind_top_of_stack.rs

@@ -4,8 +4,6 @@
 
 //! Unwinding past the top frame of a stack is Undefined Behavior.
 
-#![feature(c_unwind)]
-
 use std::{mem, ptr};
 
 extern "C-unwind" fn thread_start(_null: *mut libc::c_void) -> *mut libc::c_void {

src/tools/miri/tests/fail/function_calls/exported_symbol_bad_unwind1.rs

@@ -1,5 +1,4 @@
 //@compile-flags: -Zmiri-disable-abi-check
-#![feature(c_unwind)]
 
 #[no_mangle]
 extern "C-unwind" fn unwind() {

src/tools/miri/tests/fail/function_calls/exported_symbol_bad_unwind2.rs

@@ -4,7 +4,7 @@
 //@normalize-stderr-test: "\n +[0-9]+:[^\n]+" -> ""
 //@normalize-stderr-test: "\n +at [^\n]+" -> ""
 //@[definition,both]error-in-other-file: aborted execution
-#![feature(rustc_attrs, c_unwind)]
+#![feature(rustc_attrs)]
 
 #[cfg_attr(any(definition, both), rustc_nounwind)]
 #[no_mangle]

src/tools/miri/tests/fail/panic/bad_miri_start_panic.rs

@@ -1,6 +1,5 @@
 //@compile-flags: -Zmiri-disable-abi-check
 // This feature is required to trigger the error using the "C" ABI.
-#![feature(c_unwind)]
 
 extern "C" {
     fn miri_start_panic(payload: *mut u8) -> !;

src/tools/miri/tests/fail/panic/bad_unwind.rs

@@ -1,5 +1,3 @@
-#![feature(c_unwind)]
-
 //! Unwinding when the caller ABI is "C" (without "-unwind") is UB.
 
 extern "C-unwind" fn unwind() {

src/tools/miri/tests/fail/terminate-terminator.rs

@@ -7,8 +7,6 @@
 // Enable MIR inlining to ensure that `TerminatorKind::UnwindTerminate` is generated
 // instead of just `UnwindAction::Terminate`.
 
-#![feature(c_unwind)]
-
 struct Foo;
 
 impl Drop for Foo {

src/tools/miri/tests/fail/unwind-action-terminate.rs

@@ -3,8 +3,6 @@
 //@normalize-stderr-test: "\| +\^+" -> "| ^"
 //@normalize-stderr-test: "\n +[0-9]+:[^\n]+" -> ""
 //@normalize-stderr-test: "\n +at [^\n]+" -> ""
-#![feature(c_unwind)]
-
 extern "C" fn panic_abort() {
     panic!()
 }

src/tools/miri/tests/panic/function_calls/exported_symbol_good_unwind.rs

@@ -2,7 +2,7 @@
 // found in this form" errors works without `-C prefer-dynamic` (`panic!` calls foreign function
 // `__rust_start_panic`).
 // no-prefer-dynamic
-#![feature(c_unwind, unboxed_closures)]
+#![feature(unboxed_closures)]
 
 use std::panic;
 

tests/assembly/asm/aarch64-modifiers.rs

@@ -1,5 +1,5 @@
 // assembly-output: emit-asm
-// compile-flags: -O
+// compile-flags: -O -C panic=abort
 // compile-flags: --target aarch64-unknown-linux-gnu
 // needs-llvm-components: aarch64
 

tests/assembly/asm/arm-modifiers.rs

@@ -1,5 +1,5 @@
 // assembly-output: emit-asm
-// compile-flags: -O
+// compile-flags: -O -C panic=abort
 // compile-flags: --target armv7-unknown-linux-gnueabihf
 // compile-flags: -C target-feature=+neon
 // needs-llvm-components: arm

tests/assembly/asm/x86-modifiers.rs

@@ -1,6 +1,6 @@
 // revisions: x86_64 i686
 // assembly-output: emit-asm
-// compile-flags: -O
+// compile-flags: -O -C panic=abort
 //[x86_64] compile-flags: --target x86_64-unknown-linux-gnu
 //[x86_64] needs-llvm-components: x86
 //[i686] compile-flags: --target i686-unknown-linux-gnu

tests/codegen/avr/avr-func-addrspace.rs

@@ -1,4 +1,4 @@
-// compile-flags: -O --target=avr-unknown-gnu-atmega328 --crate-type=rlib
+// compile-flags: -O --target=avr-unknown-gnu-atmega328 --crate-type=rlib -C panic=abort
 // needs-llvm-components: avr
 
 // This test validates that function pointers can be stored in global variables

tests/codegen/catch-unwind.rs

@@ -14,7 +14,6 @@
 // ignore-loongarch64 FIXME
 
 #![crate_type = "lib"]
-#![feature(c_unwind)]
 
 extern "C" {
     fn bar();

tests/codegen/cffi/c-variadic.rs

@@ -4,7 +4,6 @@
 
 #![crate_type = "lib"]
 #![feature(c_variadic)]
-#![feature(c_unwind)]
 #![no_std]
 use core::ffi::VaList;
 

tests/codegen/debuginfo-inline-callsite-location.rs

@@ -20,7 +20,7 @@
 #![crate_type = "lib"]
 
 #[no_mangle]
-extern "C" fn add_numbers(x: &Option<i32>, y: &Option<i32>) -> i32 {
+extern "C-unwind" fn add_numbers(x: &Option<i32>, y: &Option<i32>) -> i32 {
     let x1 = x.unwrap();
     let y1 = y.unwrap();
 

tests/codegen/unwind-abis/aapcs-unwind-abi.rs

@@ -1,7 +1,7 @@
 // needs-llvm-components: arm
 // compile-flags: --target=armv7-unknown-linux-gnueabihf --crate-type=rlib -Cno-prepopulate-passes
 #![no_core]
-#![feature(no_core, lang_items, c_unwind)]
+#![feature(no_core, lang_items)]
 #[lang="sized"]
 trait Sized { }
 

tests/codegen/unwind-abis/c-unwind-abi-panic-abort.rs

@@ -4,7 +4,6 @@
 // when the code is compiled with `panic=abort`.
 
 #![crate_type = "lib"]
-#![feature(c_unwind)]
 
 // CHECK: @rust_item_that_can_unwind() unnamed_addr [[ATTR0:#[0-9]+]]
 #[no_mangle]

tests/codegen/unwind-abis/c-unwind-abi.rs

@@ -6,7 +6,6 @@
 // to prevent LLVM from inferring the attribute.
 
 #![crate_type = "lib"]
-#![feature(c_unwind)]
 
 // CHECK: @rust_item_that_cannot_unwind() unnamed_addr #0 {
 #[no_mangle]

tests/codegen/unwind-abis/cdecl-unwind-abi.rs

@@ -6,7 +6,6 @@
 // disable optimizations above to prevent LLVM from inferring the attribute.
 
 #![crate_type = "lib"]
-#![feature(c_unwind)]
 
 // CHECK: @rust_item_that_cannot_unwind() unnamed_addr #0 {
 #[no_mangle]

tests/codegen/unwind-abis/fastcall-unwind-abi.rs

@@ -1,7 +1,7 @@
 // needs-llvm-components: x86
 // compile-flags: --target=i686-pc-windows-msvc --crate-type=rlib -Cno-prepopulate-passes
 #![no_core]
-#![feature(no_core, lang_items, c_unwind)]
+#![feature(no_core, lang_items)]
 #[lang="sized"]
 trait Sized { }
 

tests/codegen/unwind-abis/nounwind.rs

@@ -2,7 +2,6 @@
 // ignore-wasm32-bare compiled with panic=abort by default
 
 #![crate_type = "lib"]
-#![feature(c_unwind)]
 
 // We disable optimizations to prevent LLVM from inferring the attribute.
 

tests/codegen/unwind-abis/stdcall-unwind-abi.rs

@@ -1,7 +1,7 @@
 // needs-llvm-components: x86
 // compile-flags: --target=i686-pc-windows-msvc --crate-type=rlib -Cno-prepopulate-passes
 #![no_core]
-#![feature(no_core, lang_items, c_unwind)]
+#![feature(no_core, lang_items)]
 #[lang="sized"]
 trait Sized { }
 

tests/codegen/unwind-abis/system-unwind-abi.rs

@@ -6,7 +6,6 @@
 // optimizations above to prevent LLVM from inferring the attribute.
 
 #![crate_type = "lib"]
-#![feature(c_unwind)]
 
 // CHECK: @rust_item_that_cannot_unwind() unnamed_addr #0 {
 #[no_mangle]