Rollup merge of #92828 - Amanieu:unwind-abort, r=dtolnay

Print a helpful message if unwinding aborts when it reaches a nounwind function

This is implemented by routing `TerminatorKind::Abort` back through the panic handler, but with a special flag in the `PanicInfo` which indicates that the panic handler should *not* attempt to unwind the stack and should instead abort immediately.

This is useful for the planned change in https://github.com/rust-lang/lang-team/issues/97 which would make `Drop` impls `nounwind` by default.

### Code

```rust
#![feature(c_unwind)]

fn panic() {
    panic!()
}

extern "C" fn nounwind() {
    panic();
}

fn main() {
    nounwind();
}
```

### Before

```
$ ./test
thread 'main' panicked at 'explicit panic', test.rs:4:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Illegal instruction (core dumped)
```

### After

```
$ ./test
thread 'main' panicked at 'explicit panic', test.rs:4:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'main' panicked at 'panic in a function that cannot unwind', test.rs:7:1
stack backtrace:
   0:     0x556f8f86ec9b - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hdccefe11a6ac4396
   1:     0x556f8f88ac6c - core::fmt::write::he152b28c41466ebb
   2:     0x556f8f85d6e2 - std::io::Write::write_fmt::h0c261480ab86f3d3
   3:     0x556f8f8654fa - std::panicking::default_hook::{{closure}}::h5d7346f3ff7f6c1b
   4:     0x556f8f86512b - std::panicking::default_hook::hd85803a1376cac7f
   5:     0x556f8f865a91 - std::panicking::rust_panic_with_hook::h4dc1c5a3036257ac
   6:     0x556f8f86f079 - std::panicking::begin_panic_handler::{{closure}}::hdda1d83c7a9d34d2
   7:     0x556f8f86edc4 - std::sys_common::backtrace::__rust_end_short_backtrace::h5b70ed0cce71e95f
   8:     0x556f8f865592 - rust_begin_unwind
   9:     0x556f8f85a764 - core::panicking::panic_no_unwind::h2606ab3d78c87899
  10:     0x556f8f85b910 - test::nounwind::hade6c7ee65050347
  11:     0x556f8f85b936 - test::main::hdc6e02cb36343525
  12:     0x556f8f85b7e3 - core::ops::function::FnOnce::call_once::h4d02663acfc7597f
  13:     0x556f8f85b739 - std::sys_common::backtrace::__rust_begin_short_backtrace::h071d40135adb0101
  14:     0x556f8f85c149 - std::rt::lang_start::{{closure}}::h70dbfbf38b685e93
  15:     0x556f8f85c791 - std::rt::lang_start_internal::h798f1c0268d525aa
  16:     0x556f8f85c131 - std::rt::lang_start::h476a7ee0a0bb663f
  17:     0x556f8f85b963 - main
  18:     0x7f64c0822b25 - __libc_start_main
  19:     0x556f8f85ae8e - _start
  20:                0x0 - <unknown>
thread panicked while panicking. aborting.
Aborted (core dumped)
```

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -477,6 +477,28 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         helper.do_call(self, &mut bx, fn_abi, llfn, &args, None, cleanup);
     }
 
+    fn codegen_abort_terminator(
+        &mut self,
+        helper: TerminatorCodegenHelper<'tcx>,
+        mut bx: Bx,
+        terminator: &mir::Terminator<'tcx>,
+    ) {
+        let span = terminator.source_info.span;
+        self.set_debug_loc(&mut bx, terminator.source_info);
+
+        // Get the location information.
+        let location = self.get_caller_location(&mut bx, terminator.source_info).immediate();
+
+        // Obtain the panic entry point.
+        let def_id = common::langcall(bx.tcx(), Some(span), "", LangItem::PanicNoUnwind);
+        let instance = ty::Instance::mono(bx.tcx(), def_id);
+        let fn_abi = bx.fn_abi_of_instance(instance, ty::List::empty());
+        let llfn = bx.get_fn_addr(instance);
+
+        // Codegen the actual panic invoke/call.
+        helper.do_call(self, &mut bx, fn_abi, llfn, &[location], None, None);
+    }
+
     /// Returns `true` if this is indeed a panic intrinsic and codegen is done.
     fn codegen_panic_intrinsic(
         &mut self,
@@ -1014,10 +1036,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             mir::TerminatorKind::Resume => self.codegen_resume_terminator(helper, bx),
 
             mir::TerminatorKind::Abort => {
-                bx.abort();
-                // `abort` does not terminate the block, so we still need to generate
-                // an `unreachable` terminator after it.
-                bx.unreachable();
+                self.codegen_abort_terminator(helper, bx, terminator);
             }
 
             mir::TerminatorKind::Goto { target } => {

compiler/rustc_hir/src/lang_items.rs

@@ -283,6 +283,7 @@ language_item_table! {
     PanicInfo,               sym::panic_info,          panic_info,                 Target::Struct,         GenericRequirement::None;
     PanicLocation,           sym::panic_location,      panic_location,             Target::Struct,         GenericRequirement::None;
     PanicImpl,               sym::panic_impl,          panic_impl,                 Target::Fn,             GenericRequirement::None;
+    PanicNoUnwind,           sym::panic_no_unwind,     panic_no_unwind,            Target::Fn,             GenericRequirement::Exact(0);
     /// libstd panic entry point. Necessary for const eval to be able to catch it
     BeginPanic,              sym::begin_panic,         begin_panic_fn,             Target::Fn,             GenericRequirement::None;
 

compiler/rustc_monomorphize/src/collector.rs

@@ -807,10 +807,18 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirNeighborCollector<'a, 'tcx> {
                     self.output.push(create_fn_mono_item(tcx, instance, source));
                 }
             }
+            mir::TerminatorKind::Abort { .. } => {
+                let instance = Instance::mono(
+                    tcx,
+                    tcx.require_lang_item(LangItem::PanicNoUnwind, Some(source)),
+                );
+                if should_codegen_locally(tcx, &instance) {
+                    self.output.push(create_fn_mono_item(tcx, instance, source));
+                }
+            }
             mir::TerminatorKind::Goto { .. }
             | mir::TerminatorKind::SwitchInt { .. }
             | mir::TerminatorKind::Resume
-            | mir::TerminatorKind::Abort
             | mir::TerminatorKind::Return
             | mir::TerminatorKind::Unreachable => {}
             mir::TerminatorKind::GeneratorDrop

compiler/rustc_span/src/symbol.rs

@@ -990,6 +990,7 @@ symbols! {
         panic_implementation,
         panic_info,
         panic_location,
+        panic_no_unwind,
         panic_runtime,
         panic_str,
         panic_unwind,

library/core/src/panic/panic_info.rs

@@ -31,6 +31,7 @@ pub struct PanicInfo<'a> {
     payload: &'a (dyn Any + Send),
     message: Option<&'a fmt::Arguments<'a>>,
     location: &'a Location<'a>,
+    can_unwind: bool,
 }
 
 impl<'a> PanicInfo<'a> {
@@ -44,9 +45,10 @@ impl<'a> PanicInfo<'a> {
     pub fn internal_constructor(
         message: Option<&'a fmt::Arguments<'a>>,
         location: &'a Location<'a>,
+        can_unwind: bool,
     ) -> Self {
         struct NoPayload;
-        PanicInfo { location, message, payload: &NoPayload }
+        PanicInfo { location, message, payload: &NoPayload, can_unwind }
     }
 
     #[unstable(
@@ -127,6 +129,18 @@ impl<'a> PanicInfo<'a> {
         // deal with that case in std::panicking::default_hook and core::panicking::panic_fmt.
         Some(&self.location)
     }
+
+    /// Returns whether the panic handler is allowed to unwind the stack from
+    /// the point where the panic occurred.
+    ///
+    /// This is true for most kinds of panics with the exception of panics
+    /// caused by trying to unwind out of a `Drop` implementation or a function
+    /// whose ABI does not support unwinding.
+    #[must_use]
+    #[unstable(feature = "panic_can_unwind", issue = "92988")]
+    pub fn can_unwind(&self) -> bool {
+        self.can_unwind
+    }
 }
 
 #[stable(feature = "panic_hook_display", since = "1.26.0")]

library/core/src/panicking.rs

@@ -77,6 +77,31 @@ fn panic_bounds_check(index: usize, len: usize) -> ! {
     panic!("index out of bounds: the len is {} but the index is {}", len, index)
 }
 
+#[cfg(not(bootstrap))]
+#[cold]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[track_caller]
+#[lang = "panic_no_unwind"] // needed by codegen for panic in nounwind function
+fn panic_no_unwind() -> ! {
+    if cfg!(feature = "panic_immediate_abort") {
+        super::intrinsics::abort()
+    }
+
+    // NOTE This function never crosses the FFI boundary; it's a Rust-to-Rust call
+    // that gets resolved to the `#[panic_handler]` function.
+    extern "Rust" {
+        #[lang = "panic_impl"]
+        fn panic_impl(pi: &PanicInfo<'_>) -> !;
+    }
+
+    // PanicInfo with the `can_unwind` flag set to false forces an abort.
+    let fmt = format_args!("panic in a function that cannot unwind");
+    let pi = PanicInfo::internal_constructor(Some(&fmt), Location::caller(), false);
+
+    // SAFETY: `panic_impl` is defined in safe Rust code and thus is safe to call.
+    unsafe { panic_impl(&pi) }
+}
+
 /// The entry point for panicking with a formatted message.
 ///
 /// This is designed to reduce the amount of code required at the call
@@ -104,7 +129,7 @@ pub const fn panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
         fn panic_impl(pi: &PanicInfo<'_>) -> !;
     }
 
-    let pi = PanicInfo::internal_constructor(Some(&fmt), Location::caller());
+    let pi = PanicInfo::internal_constructor(Some(&fmt), Location::caller(), true);
 
     // SAFETY: `panic_impl` is defined in safe Rust code and thus is safe to call.
     unsafe { panic_impl(&pi) }

library/std/src/lib.rs

@@ -311,6 +311,7 @@
 #![feature(once_cell)]
 #![feature(panic_info_message)]
 #![feature(panic_internals)]
+#![feature(panic_can_unwind)]
 #![feature(panic_unwind)]
 #![feature(pin_static_ref)]
 #![feature(portable_simd)]

library/std/src/panicking.rs

@@ -576,9 +576,14 @@ pub fn begin_panic_handler(info: &PanicInfo<'_>) -> ! {
     let msg = info.message().unwrap(); // The current implementation always returns Some
     crate::sys_common::backtrace::__rust_end_short_backtrace(move || {
         if let Some(msg) = msg.as_str() {
-            rust_panic_with_hook(&mut StrPanicPayload(msg), info.message(), loc);
+            rust_panic_with_hook(&mut StrPanicPayload(msg), info.message(), loc, info.can_unwind());
         } else {
-            rust_panic_with_hook(&mut PanicPayload::new(msg), info.message(), loc);
+            rust_panic_with_hook(
+                &mut PanicPayload::new(msg),
+                info.message(),
+                loc,
+                info.can_unwind(),
+            );
         }
     })
 }
@@ -602,7 +607,7 @@ pub const fn begin_panic<M: Any + Send>(msg: M) -> ! {
 
     let loc = Location::caller();
     return crate::sys_common::backtrace::__rust_end_short_backtrace(move || {
-        rust_panic_with_hook(&mut PanicPayload::new(msg), None, loc)
+        rust_panic_with_hook(&mut PanicPayload::new(msg), None, loc, true)
     });
 
     struct PanicPayload<A> {
@@ -647,6 +652,7 @@ fn rust_panic_with_hook(
     payload: &mut dyn BoxMeUp,
     message: Option<&fmt::Arguments<'_>>,
     location: &Location<'_>,
+    can_unwind: bool,
 ) -> ! {
     let (must_abort, panics) = panic_count::increase();
 
@@ -663,14 +669,14 @@ fn rust_panic_with_hook(
         } else {
             // Unfortunately, this does not print a backtrace, because creating
             // a `Backtrace` will allocate, which we must to avoid here.
-            let panicinfo = PanicInfo::internal_constructor(message, location);
+            let panicinfo = PanicInfo::internal_constructor(message, location, can_unwind);
             rtprintpanic!("{}\npanicked after panic::always_abort(), aborting.\n", panicinfo);
         }
-        intrinsics::abort()
+        crate::sys::abort_internal();
     }
 
     unsafe {
-        let mut info = PanicInfo::internal_constructor(message, location);
+        let mut info = PanicInfo::internal_constructor(message, location, can_unwind);
         let _guard = HOOK_LOCK.read();
         match HOOK {
             // Some platforms (like wasm) know that printing to stderr won't ever actually
@@ -691,13 +697,13 @@ fn rust_panic_with_hook(
         };
     }
 
-    if panics > 1 {
+    if panics > 1 || !can_unwind {
         // If a thread panics while it's already unwinding then we
         // have limited options. Currently our preference is to
         // just abort. In the future we may consider resuming
         // unwinding or otherwise exiting the thread cleanly.
         rtprintpanic!("thread panicked while panicking. aborting.\n");
-        intrinsics::abort()
+        crate::sys::abort_internal();
     }
 
     rust_panic(payload)

library/std/src/sys/unix/process/process_unix/tests.rs

@@ -53,5 +53,10 @@ fn test_command_fork_no_unwind() {
     let status = got.expect("panic unexpectedly propagated");
     dbg!(status);
     let signal = status.signal().expect("expected child process to die of signal");
-    assert!(signal == libc::SIGABRT || signal == libc::SIGILL || signal == libc::SIGTRAP);
+    assert!(
+        signal == libc::SIGABRT
+            || signal == libc::SIGILL
+            || signal == libc::SIGTRAP
+            || signal == libc::SIGSEGV
+    );
 }

src/test/codegen/unwind-and-panic-abort.rs

@@ -9,7 +9,7 @@ extern "C-unwind" {
 
 // CHECK: Function Attrs:{{.*}}nounwind
 // CHECK-NEXT: define{{.*}}void @foo
-// CHECK: call void @llvm.trap()
+// CHECK: call void @_ZN4core9panicking15panic_no_unwind
 #[no_mangle]
 pub unsafe extern "C" fn foo() {
     bar();