Auto merge of #88759 - Amanieu:panic_in_drop, r=nagisa,eddyb

Add -Z panic-in-drop={unwind,abort} command-line option

This PR changes `Drop` to abort if an unwinding panic attempts to escape it, making the process abort instead. This has several benefits:
- The current behavior when unwinding out of `Drop` is very unintuitive and easy to miss: unwinding continues, but the remaining drops in scope are simply leaked.
- A lot of unsafe code doesn't expect drops to unwind, which can lead to unsoundness:
  - https://github.com/servo/rust-smallvec/issues/14
  - https://github.com/bluss/arrayvec/issues/3
- There is a code size and compilation time cost to this: LLVM needs to generate extra landing pads out of all calls in a drop implementation. This can compound when functions are inlined since unwinding will then continue on to process drops in the callee, which can itself unwind, etc.
  - Initial measurements show a 3% size reduction and up to 10% compilation time reduction on some crates (`syn`).

One thing to note about `-Z panic-in-drop=abort` is that *all* crates must be built with this option for it to be sound since it makes the compiler assume that dropping `Box<dyn Any>` will never unwind.

cc https://github.com/rust-lang/lang-team/issues/97

1 files changed, 54 insertions, 0 deletions

diff --git a/src/test/codegen/panic-in-drop-abort.rs b/src/test/codegen/panic-in-drop-abort.rs
new file mode 100644
index 00000000000..62d093507dd
--- /dev/null
+++ b/src/test/codegen/panic-in-drop-abort.rs
@@ -0,0 +1,54 @@
+// compile-flags: -Z panic-in-drop=abort -O
+
+// Ensure that unwinding code paths are eliminated from the output after
+// optimization.
+
+#![crate_type = "lib"]
+use std::any::Any;
+use std::mem::forget;
+
+pub struct ExternDrop;
+impl Drop for ExternDrop {
+    #[inline(always)]
+    fn drop(&mut self) {
+        // This call may potentially unwind.
+        extern "Rust" {
+            fn extern_drop();
+        }
+        unsafe {
+            extern_drop();
+        }
+    }
+}
+
+struct AssertNeverDrop;
+impl Drop for AssertNeverDrop {
+    #[inline(always)]
+    fn drop(&mut self) {
+        // This call should be optimized away as unreachable.
+        extern "C" {
+            fn should_not_appear_in_output();
+        }
+        unsafe {
+            should_not_appear_in_output();
+        }
+    }
+}
+
+// CHECK-LABEL: normal_drop
+// CHECK-NOT: should_not_appear_in_output
+#[no_mangle]
+pub fn normal_drop(x: ExternDrop) {
+    let guard = AssertNeverDrop;
+    drop(x);
+    forget(guard);
+}
+
+// CHECK-LABEL: indirect_drop
+// CHECK-NOT: should_not_appear_in_output
+#[no_mangle]
+pub fn indirect_drop(x: Box<dyn Any>) {
+    let guard = AssertNeverDrop;
+    drop(x);
+    forget(guard);
+}