miri: accept fieldless repr-int enums as ABI-compatible with the integer

Author: RalfJung

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -404,6 +404,20 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 ty::Int(ity) => (Integer::from_int_ty(&self.tcx, *ity), /* signed */ true),
                 ty::Uint(uty) => (Integer::from_uint_ty(&self.tcx, *uty), /* signed */ false),
                 ty::Char => (Integer::I32, /* signed */ false),
+                ty::Adt(def, _) => {
+                    // Ensure it is an enum, with a suitable repr, and fieldless.
+                    if !def.is_enum() {
+                        return None;
+                    }
+                    let Some(int_ty) = def.repr().int else {
+                        return None;
+                    };
+                    if !def.variants().iter().all(|variant| variant.fields.is_empty()) {
+                        return None;
+                    }
+                    let int = Integer::from_attr(&self.tcx, int_ty);
+                    (int, int_ty.is_signed())
+                }
                 _ => return None,
             })
         };

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

@@ -71,6 +71,23 @@ fn main() {
         test_abi_compat(0isize, 0i64);
     }
     test_abi_compat(42u32, num::NonZero::new(1u32).unwrap());
+    // - `repr(int)` enums and the corresponding integer.
+    #[repr(i16)]
+    #[derive(Copy, Clone)]
+    enum I16 {
+        Var1 = 0,
+        Var2 = -5,
+    }
+    test_abi_compat(I16::Var1, 0i16);
+    test_abi_compat(I16::Var2, -5i16);
+    #[repr(u64)]
+    #[derive(Copy, Clone)]
+    enum U64 {
+        Var1 = 0,
+        Var2 = u64::MAX,
+    }
+    test_abi_compat(U64::Var1, 0u64);
+    test_abi_compat(U64::Var2, u64::MAX);
     // - `char` and `u32`.
     test_abi_compat(42u32, 'x');
     // - Reference/pointer types with the same pointee.