Rollup merge of rust-lang#65346 - RalfJung:nounwind-tests, r=nagisa

nounwind tests and cleanup

This is a follow-up to @pnkfelix' rust-lang#65020. In particular it adds some tests as @nagisa  asked. It also does a cleanup that the original PR omitted to reduce backporting risks.

I hope I finally managed to write an uncontroversial PR in this area. ;) This should not change any behavior, just test it better.

src/librustc_codegen_llvm/attributes.rs

@@ -270,23 +270,12 @@ pub fn from_fn_attrs(
         // optimize based on this!
         false
     } else if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::UNWIND) {
-        // If a specific #[unwind] attribute is present, use that
+        // If a specific #[unwind] attribute is present, use that.
         true
     } else if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_ALLOCATOR_NOUNWIND) {
-        // Special attribute for allocator functions, which can't unwind
+        // Special attribute for allocator functions, which can't unwind.
         false
-    } else if let Some(_) = id {
-        // rust-lang/rust#64655, rust-lang/rust#63909: to minimize
-        // risk associated with changing cases where nounwind
-        // attribute is attached, this code is deliberately mimicking
-        // old control flow based on whether `id` is `Some` or `None`.
-        //
-        // However, in the long term we should either:
-        // - fold this into final else (i.e. stop inspecting `id`)
-        // - or, adopt Rust PR #63909.
-        //
-        // see also Rust RFC 2753.
-
+    } else {
         let sig = cx.tcx.normalize_erasing_late_bound_regions(ty::ParamEnv::reveal_all(), &sig);
         if sig.abi == Abi::Rust || sig.abi == Abi::RustCall {
             // Any Rust method (or `extern "Rust" fn` or `extern
@@ -312,15 +301,6 @@ pub fn from_fn_attrs(
             // In either case, we mark item as explicitly nounwind.
             false
         }
-    } else {
-        // assume this can possibly unwind, avoiding the application of a
-        // `nounwind` attribute below.
-        //
-        // (But: See comments in previous branch. Specifically, it is
-        // unclear whether there is real value in the assumption this
-        // can unwind. The conservatism here may just be papering over
-        // a real problem by making some UB a bit harder to hit.)
-        true
     });
 
     // Always annotate functions with the target-cpu they are compiled for.

src/test/codegen/unwind-extern-exports.rs

@@ -0,0 +1,19 @@
+// compile-flags: -C opt-level=0
+
+#![crate_type = "lib"]
+#![feature(unwind_attributes)]
+
+// Make sure these all do *not* get the attribute.
+// We disable optimizations to prevent LLVM from infering the attribute.
+// CHECK-NOT: nounwind
+
+// "C" ABI
+// pub extern fn foo() {} // FIXME right now we don't abort-on-panic but add `nounwind` nevertheless
+#[unwind(allowed)]
+pub extern fn foo_allowed() {}
+
+// "Rust"
+// (`extern "Rust"` could be removed as all `fn` get it implicitly; we leave it in for clarity.)
+pub extern "Rust" fn bar() {}
+#[unwind(allowed)]
+pub extern "Rust" fn bar_allowed() {}

src/test/codegen/unwind-extern-imports.rs

@@ -0,0 +1,41 @@
+// compile-flags: -C no-prepopulate-passes
+
+#![crate_type = "lib"]
+#![feature(unwind_attributes)]
+
+extern {
+// CHECK: Function Attrs:{{.*}}nounwind
+// CHECK-NEXT: declare void @extern_fn
+    fn extern_fn();
+// CHECK-NOT: Function Attrs:{{.*}}nounwind
+// CHECK: declare void @unwinding_extern_fn
+    #[unwind(allowed)]
+    fn unwinding_extern_fn();
+// CHECK-NOT: nounwind
+// CHECK: declare void @aborting_extern_fn
+    #[unwind(aborts)]
+    fn aborting_extern_fn(); // FIXME: we want to have the attribute here
+}
+
+extern "Rust" {
+// CHECK-NOT: nounwind
+// CHECK: declare void @rust_extern_fn
+    fn rust_extern_fn();
+// CHECK-NOT: nounwind
+// CHECK: declare void @rust_unwinding_extern_fn
+    #[unwind(allowed)]
+    fn rust_unwinding_extern_fn();
+// CHECK-NOT: nounwind
+// CHECK: declare void @rust_aborting_extern_fn
+    #[unwind(aborts)]
+    fn rust_aborting_extern_fn(); // FIXME: we want to have the attribute here
+}
+
+pub unsafe fn force_declare() {
+    extern_fn();
+    unwinding_extern_fn();
+    aborting_extern_fn();
+    rust_extern_fn();
+    rust_unwinding_extern_fn();
+    rust_aborting_extern_fn();
+}