add simulated non-regression test for issue 105637

Author: lqd

src/test/ui/lto/auxiliary/thinlto-dylib.rs

@@ -0,0 +1,23 @@
+// Auxiliary crate for test issue-105637: the LTOed dylib which had duplicate symbols from libstd,
+// breaking the panic hook feature.
+//
+// This simulates the `rustc_driver` crate, and the main crate simulates rustc's main binary hooking
+// into this driver.
+
+// compile-flags: -Zdylib-lto -C lto=thin
+
+use std::panic;
+
+pub fn main() {
+    // Install the hook we want to see executed
+    panic::set_hook(Box::new(|_| {
+        eprintln!("LTOed auxiliary crate panic hook");
+    }));
+
+    // Trigger the panic hook with an ICE
+    run_compiler();
+}
+
+fn run_compiler() {
+    panic!("ICEing");
+}

src/test/ui/lto/issue-105637.rs

@@ -0,0 +1,28 @@
+// Regression test for issue #105637: `-Zdylib-lto` with LTO duplicated symbols from other dylibs,
+// in this case from libstd.
+//
+// That manifested as both `rustc_driver` and rustc's "main" (`compiler/rustc`) having their own
+// `std::panicking::HOOK` static, and the hook in rustc's main (the default stdlib's) being executed
+// when rustc ICEs, instead of the overriden hook from `rustc_driver` (which also displays the query
+// stack and information on how to open a GH issue for the encountered ICE).
+//
+// In this test, we reproduce this setup by installing a panic hook in both the main and an LTOed
+// dylib: the last hook set should be the one being executed, the dylib's.
+
+// aux-build: thinlto-dylib.rs
+// run-fail
+// check-run-results
+
+extern crate thinlto_dylib;
+
+use std::panic;
+
+fn main() {
+    // We don't want to see this panic hook executed
+    std::panic::set_hook(Box::new(|_| {
+        eprintln!("main crate panic hook");
+    }));
+
+    // Have the LTOed dylib install its own hook and panic, we want to see its hook executed.
+    thinlto_dylib::main();
+}

src/test/ui/lto/issue-105637.run.stderr

@@ -0,0 +1 @@
+LTOed auxiliary crate panic hook