implement and test fn ptr ABI compatibility rules

Author: RalfJung

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -298,8 +298,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         };
         // Check if the inner type is one of the NPO-guaranteed ones.
         Ok(match inner_ty.kind() {
-            ty::Ref(..) => {
-                // Option<&T> behaves like &T
+            ty::Ref(..) | ty::FnPtr(..) => {
+                // Option<&T> behaves like &T, and same for fn()
                 inner_ty
             }
             ty::Adt(def, _)
@@ -368,6 +368,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             return Ok(meta_ty(left) == meta_ty(right));
         }
 
+        // Compatible function pointer types.
+        if let (ty::FnPtr(..), ty::FnPtr(..)) = (caller_ty.kind(), callee_ty.kind()) {
+            return Ok(true);
+        }
+
         // Compatible integer types (in particular, usize vs ptr-sized-u32/u64).
         let int_ty = |ty: Ty<'tcx>| {
             Some(match ty.kind() {

src/tools/miri/tests/pass/function_calls/abi_compat.rs

@@ -5,6 +5,8 @@ use std::ptr;
 #[derive(Copy, Clone, Default)]
 struct Zst;
 
+fn id<T>(x: T) -> T { x }
+
 fn test_abi_compat<T: Clone, U: Clone>(t: T, u: U) {
     fn id<T>(x: T) -> T {
         x
@@ -70,8 +72,11 @@ fn main() {
     test_abi_compat(&(), ptr::NonNull::<()>::dangling());
     // Reference/pointer types with different but sized pointees.
     test_abi_compat(&0u32, &([true; 4], [0u32; 0]));
+    // `fn` types
+    test_abi_compat(main as fn(), id::<i32> as fn(i32) -> i32);
     // Guaranteed null-pointer-optimizations.
     test_abi_compat(&0u32 as *const u32, Some(&0u32));
+    test_abi_compat(main as fn(), Some(main as fn()));
     test_abi_compat(42u32, num::NonZeroU32::new(1).unwrap());
     test_abi_compat(0u32, Some(num::NonZeroU32::new(1).unwrap()));