Test missing miri::intrinsic_fallback_checks_ub attribute

Author: oli-obk

src/tools/miri/src/shims/intrinsics/mod.rs

@@ -68,8 +68,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 throw_unsup_format!("unimplemented intrinsic: `{intrinsic_name}`")
             }
             let intrinsic_fallback_checks_ub = Symbol::intern("intrinsic_fallback_checks_ub");
-            if !this.tcx.item_attrs(instance.def_id()).iter().any(|attr| attr.path_matches(&[sym::miri, intrinsic_fallback_checks_ub])) {
-                throw_unsup_format!("miri can only use intrinsics that preserve UB. After verifying that `{intrinsic_name}` does so, add the `#[miri::intrinsic_fallback_checks_ub]` attribute to it");
+            if this.tcx.get_attrs_by_path(instance.def_id(), &[sym::miri, intrinsic_fallback_checks_ub]).next().is_none() {
+                throw_unsup_format!("miri can only use intrinsic fallback bodies that preserve UB. After verifying that `{intrinsic_name}` does so, add the `#[miri::intrinsic_fallback_checks_ub]` attribute to it");
             }
             return Ok(Some(ty::Instance {
                 def: ty::InstanceDef::Item(instance.def_id()),

src/tools/miri/tests/fail/intrinsic_fallback_checks_ub.rs

@@ -0,0 +1,14 @@
+#![feature(rustc_attrs, effects)]
+
+#[rustc_intrinsic]
+#[rustc_nounwind]
+#[rustc_do_not_const_check]
+#[inline]
+pub const fn ptr_guaranteed_cmp<T>(ptr: *const T, other: *const T) -> u8 {
+    (ptr == other) as u8
+}
+
+fn main() {
+    ptr_guaranteed_cmp::<()>(std::ptr::null(), std::ptr::null());
+    //~^ ERROR: can only use intrinsic fallback bodies that preserve UB
+}

src/tools/miri/tests/fail/intrinsic_fallback_checks_ub.stderr

@@ -0,0 +1,14 @@
+error: unsupported operation: miri can only use intrinsic fallback bodies that preserve UB. After verifying that `ptr_guaranteed_cmp` does so, add the `#[miri::intrinsic_fallback_checks_ub]` attribute to it
+  --> $DIR/intrinsic_fallback_checks_ub.rs:LL:CC
+   |
+LL |     ptr_guaranteed_cmp::<()>(std::ptr::null(), std::ptr::null());
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ miri can only use intrinsic fallback bodies that preserve UB. After verifying that `ptr_guaranteed_cmp` does so, add the `#[miri::intrinsic_fallback_checks_ub]` attribute to it
+   |
+   = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/intrinsic_fallback_checks_ub.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+