From b14799754e10977ebbe184c80c63b08a49ebc70d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jana=20D=C3=B6nszelmann?= <jana@donsz.nl>
Date: Tue, 27 Jan 2026 12:23:57 +0100
Subject: [PATCH] test backtraces

---
 compiler/rustc_codegen_llvm/src/mono_item.rs  |  2 +-
 .../auxiliary/decl_with_default_panics.rs     | 10 ++++++
 tests/ui/eii/default/call_default_panics.rs   | 35 +++++++++++++++++++
 .../default/call_default_panics.run.stderr    | 10 ++++++
 4 files changed, 56 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/eii/default/auxiliary/decl_with_default_panics.rs
 create mode 100644 tests/ui/eii/default/call_default_panics.rs
 create mode 100644 tests/ui/eii/default/call_default_panics.run.stderr

diff --git a/compiler/rustc_codegen_llvm/src/mono_item.rs b/compiler/rustc_codegen_llvm/src/mono_item.rs
index 8db334935219..f7e1ca73953e 100644
--- a/compiler/rustc_codegen_llvm/src/mono_item.rs
+++ b/compiler/rustc_codegen_llvm/src/mono_item.rs
@@ -117,7 +117,7 @@ fn predefine_without_aliases(
     /// This is not a problem: instead of using LLVM aliases, we can just generate
     /// a new function symbol (with target architecture!) which effectively comes down to:
     ///
-    /// ```rust,ignore
+    /// ```ignore (illustrative example)
     /// fn alias_name(...args) {
     ///     original_name(...args)
     /// }
diff --git a/tests/ui/eii/default/auxiliary/decl_with_default_panics.rs b/tests/ui/eii/default/auxiliary/decl_with_default_panics.rs
new file mode 100644
index 000000000000..14778a40cde4
--- /dev/null
+++ b/tests/ui/eii/default/auxiliary/decl_with_default_panics.rs
@@ -0,0 +1,10 @@
+//@ no-prefer-dynamic
+//@ needs-unwind
+//@ exec-env:RUST_BACKTRACE=1
+#![crate_type = "rlib"]
+#![feature(extern_item_impls)]
+
+#[eii(eii1)]
+pub fn decl1(x: u64) {
+    panic!("{}", x);
+}
diff --git a/tests/ui/eii/default/call_default_panics.rs b/tests/ui/eii/default/call_default_panics.rs
new file mode 100644
index 000000000000..f71fddb71ba2
--- /dev/null
+++ b/tests/ui/eii/default/call_default_panics.rs
@@ -0,0 +1,35 @@
+//@ no-prefer-dynamic
+//@ aux-build: decl_with_default_panics.rs
+//@ edition: 2021
+//@ run-pass
+//@ needs-unwind
+//@ exec-env:RUST_BACKTRACE=1
+//@ ignore-backends: gcc
+// FIXME: linking on windows (speciifcally mingw) not yet supported, see tracking issue #125418
+//@ ignore-windows
+// A small test to make sure that unwinding works properly.
+//
+// Functions can have target-cpu applied. On apple-darwin this is super important,
+// since you can have binaries which mix x86 and aarch64 code that are compatible
+// with both architectures. So we can't just reject target_cpu on EIIs since apple
+// puts them on by default. The problem: we generate aliases. And aliases cannot
+// get target_cpu applied to them. So, instead we should, in the case of functions,
+// generate a shim function. For statics aliases should keep working.
+// However, to make this work properly,
+// on LLVM we generate shim functions instead of function aliases.
+// Little extra functions that look like
+// ```
+// function alias_symbol(*args) {return (tailcall) aliasee(*args);}
+// ```
+// This is a simple test to make sure that we can unwind through these,
+// and that this wrapper function effectively doesn't show up in the trace.
+#![feature(extern_item_impls)]
+
+extern crate decl_with_default_panics;
+
+fn main() {
+    let result = std::panic::catch_unwind(|| {
+        decl_with_default_panics::decl1(10);
+    });
+    assert!(result.is_err());
+}
diff --git a/tests/ui/eii/default/call_default_panics.run.stderr b/tests/ui/eii/default/call_default_panics.run.stderr
new file mode 100644
index 000000000000..3edd72166736
--- /dev/null
+++ b/tests/ui/eii/default/call_default_panics.run.stderr
@@ -0,0 +1,10 @@
+
+thread 'main' ($TID) panicked at $DIR/auxiliary/decl_with_default_panics.rs:14:5:
+10
+stack backtrace:
+   0: __rustc::rust_begin_unwind
+   1: core::panicking::panic_fmt
+   2: core::panicking::panic_display::<u64>
+   3: decl_with_default_panics::_::decl1
+   4: call_default_panics::main
+note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.