libunwind_panic: adjust miri panic hack

Author: RalfJung

src/libcore/intrinsics.rs

@@ -1348,9 +1348,11 @@ extern "rust-intrinsic" {
     pub fn ptr_offset_from<T>(ptr: *const T, base: *const T) -> isize;
 
     /// Internal hook used by Miri to implement unwinding.
+    /// Compiles to a NOP during non-Miri codegen.
+    ///
     /// Perma-unstable: do not use
     #[cfg(not(bootstrap))]
-    pub fn miri_start_panic(data: *mut (dyn crate::any::Any + crate::marker::Send)) -> !;
+    pub fn miri_start_panic(data: *mut (dyn crate::any::Any + crate::marker::Send)) -> ();
 }
 
 // Some functions are defined here because they accidentally got made

src/libpanic_unwind/lib.rs

@@ -36,10 +36,7 @@ use core::raw;
 use core::panic::BoxMeUp;
 
 cfg_if::cfg_if! {
-    if #[cfg(miri)] {
-        #[path = "miri.rs"]
-        mod imp;
-    } else if #[cfg(target_os = "emscripten")] {
+    if #[cfg(target_os = "emscripten")] {
         #[path = "emcc.rs"]
         mod imp;
     } else if #[cfg(target_arch = "wasm32")] {
@@ -94,5 +91,14 @@ pub unsafe extern "C" fn __rust_maybe_catch_panic(f: fn(*mut u8),
 #[unwind(allowed)]
 pub unsafe extern "C" fn __rust_start_panic(payload: usize) -> u32 {
     let payload = payload as *mut &mut dyn BoxMeUp;
-    imp::panic(Box::from_raw((*payload).take_box()))
+    let payload = (*payload).take_box();
+
+    // Miri panic support: cfg'd out of normal builds just to be sure.
+    // When going through normal codegen, `miri_start_panic` is a NOP, so the
+    // Miri-enabled sysroot still supports normal unwinding. But when executed in
+    // Miri, this line initiates unwinding.
+    #[cfg(miri)]
+    core::intrinsics::miri_start_panic(payload);
+
+    imp::panic(Box::from_raw(payload))
 }

src/libpanic_unwind/miri.rs

@@ -528,18 +528,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             _ => FnAbi::new(&bx, sig, &extra_args)
         };
 
-        // This should never be reachable at runtime:
-        // We should only emit a call to this intrinsic in #[cfg(miri)] mode,
-        // which means that we will never actually use the generate object files
-        // (we will just be interpreting the MIR)
-        //
-        // Note that we still need to be able to codegen *something* for this intrisnic:
-        // Miri currently uses Xargo to build a special libstd. As a side effect,
-        // we generate normal object files for libstd - while these are never used,
-        // we still need to be able to build them.
+        // For normal codegen, this Miri-specific intrinsic is just a NOP.
         if intrinsic == Some("miri_start_panic") {
-            bx.abort();
-            bx.unreachable();
+            let target = destination.as_ref().unwrap().1;
+            helper.maybe_sideeffect(self.mir, &mut bx, &[target]);
+            helper.funclet_br(self, &mut bx, target);
             return;
         }