Auto merge of #96959 - nbdd0121:unwind, r=Amanieu

Prevent unwinding when `-C panic=abort` is used regardless declared ABI

Ensures that Rust code will abort with `-C panic=abort` regardless ABI used.
```rust
extern "C-unwind" {
    fn may_unwind();
}

// Will be nounwind with `-C panic=abort`, despite `C-unwind` ABI.
pub unsafe extern "C-unwind" fn rust_item_that_can_unwind() {
    may_unwind();
}
```

Current behaviour is that unwind will propagate through. While the current behaviour won't cause unsoundness it is inconsistent with the text reading of [RFC2945](https://rust-lang.github.io/rfcs/2945-c-unwind-abi.html).

I tweaked `fn_can_unwind` instead of tweaking `AbortUnwindingCalls` because this approach would allow Rust (non-direct) callers to also see that this function is nounwind, so it can prevent excessive landing pads generation.

For more discussions: https://rust-lang.zulipchat.com/#narrow/stream/210922-project-ffi-unwind/topic/soundness.20in.20mixed.20panic.20mode.

cc `@alexcrichton,` `@BatmanAoD`
r? `@Amanieu`

`@rustbot` label: T-compiler T-lang F-c_unwind

Author: bors

compiler/rustc_middle/src/ty/layout.rs

@@ -2888,6 +2888,14 @@ pub fn fn_can_unwind<'tcx>(tcx: TyCtxt<'tcx>, fn_def_id: Option<DefId>, abi: Spe
             return false;
         }
 
+        // With `-C panic=abort`, all non-FFI functions are required to not unwind.
+        //
+        // Note that this is true regardless ABI specified on the function -- a `extern "C-unwind"`
+        // function defined in Rust is also required to abort.
+        if tcx.sess.panic_strategy() == PanicStrategy::Abort && !tcx.is_foreign_item(did) {
+            return false;
+        }
+
         // With -Z panic-in-drop=abort, drop_in_place never unwinds.
         //
         // This is not part of `codegen_fn_attrs` as it can differ between crates

src/test/codegen/unwind-abis/c-unwind-abi-panic-abort.rs

@@ -1,22 +1,27 @@
 // compile-flags: -C panic=abort
 
-// Test that `nounwind` atributes are not applied to `C-unwind` extern functions
-// even when the code is compiled with `panic=abort`.
+// Test that `nounwind` atributes are also applied to extern `C-unwind` Rust functions
+// when the code is compiled with `panic=abort`.
 
 #![crate_type = "lib"]
 #![feature(c_unwind)]
 
-extern "C-unwind" {
-    fn may_unwind();
-}
-
-// CHECK: @rust_item_that_can_unwind() unnamed_addr #0
+// CHECK: @rust_item_that_can_unwind() unnamed_addr [[ATTR0:#[0-9]+]]
 #[no_mangle]
 pub unsafe extern "C-unwind" fn rust_item_that_can_unwind() {
+    // CHECK: call void @_ZN4core9panicking15panic_no_unwind
     may_unwind();
 }
 
+extern "C-unwind" {
+    // CHECK: @may_unwind() unnamed_addr [[ATTR1:#[0-9]+]]
+    fn may_unwind();
+}
+
 // Now, make sure that the LLVM attributes for this functions are correct.  First, make
 // sure that the first item is correctly marked with the `nounwind` attribute:
 //
-// CHECK-NOT: attributes #0 = { {{.*}}nounwind{{.*}} }
+// CHECK: attributes [[ATTR0]] = { {{.*}}nounwind{{.*}} }
+//
+// Now, check that foreign item is correctly marked without the `nounwind` attribute.
+// CHECK-NOT: attributes [[ATTR1]] = { {{.*}}nounwind{{.*}} }