Rollup merge of #124896 - RalfJung:miri-intrinsic-fallback, r=oli-obk

miri: rename intrinsic_fallback_checks_ub to intrinsic_fallback_is_spec

Checking UB is not the only concern, we also have to make sure we are not losing out on non-determinism.

r? ``@oli-obk`` (not urgent, take your time)

Author: fmease

src/intrinsics/mod.rs

@@ -43,18 +43,18 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 if this.tcx.intrinsic(instance.def_id()).unwrap().must_be_overridden {
                     throw_unsup_format!("unimplemented intrinsic: `{intrinsic_name}`")
                 }
-                let intrinsic_fallback_checks_ub = Symbol::intern("intrinsic_fallback_checks_ub");
+                let intrinsic_fallback_is_spec = Symbol::intern("intrinsic_fallback_is_spec");
                 if this
                     .tcx
                     .get_attrs_by_path(
                         instance.def_id(),
-                        &[sym::miri, intrinsic_fallback_checks_ub],
+                        &[sym::miri, intrinsic_fallback_is_spec],
                     )
                     .next()
                     .is_none()
                 {
                     throw_unsup_format!(
-                        "miri can only use intrinsic fallback bodies that check UB. After verifying that `{intrinsic_name}` does so, add the `#[miri::intrinsic_fallback_checks_ub]` attribute to it; also ping @rust-lang/miri when you do that"
+                        "Miri can only use intrinsic fallback bodies that exactly reflect the specification: they fully check for UB and are as non-deterministic as possible. After verifying that `{intrinsic_name}` does so, add the `#[miri::intrinsic_fallback_is_spec]` attribute to it; also ping @rust-lang/miri when you do that"
                     );
                 }
                 Ok(Some(ty::Instance {

tests/fail/intrinsic_fallback_checks_ub.stderr

@@ -10,5 +10,5 @@ pub const fn ptr_guaranteed_cmp<T>(ptr: *const T, other: *const T) -> u8 {
 
 fn main() {
     ptr_guaranteed_cmp::<()>(std::ptr::null(), std::ptr::null());
-    //~^ ERROR: can only use intrinsic fallback bodies that check UB.
+    //~^ ERROR: can only use intrinsic fallback bodies that exactly reflect the specification
 }

tests/fail/intrinsic_fallback_checks_ub.rs renamed to tests/fail/intrinsic_fallback_is_spec.rs

@@ -0,0 +1,14 @@
+error: unsupported operation: Miri can only use intrinsic fallback bodies that exactly reflect the specification: they fully check for UB and are as non-deterministic as possible. After verifying that `ptr_guaranteed_cmp` does so, add the `#[miri::intrinsic_fallback_is_spec]` attribute to it; also ping @rust-lang/miri when you do that
+  --> $DIR/intrinsic_fallback_is_spec.rs:LL:CC
+   |
+LL |     ptr_guaranteed_cmp::<()>(std::ptr::null(), std::ptr::null());
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Miri can only use intrinsic fallback bodies that exactly reflect the specification: they fully check for UB and are as non-deterministic as possible. After verifying that `ptr_guaranteed_cmp` does so, add the `#[miri::intrinsic_fallback_is_spec]` attribute to it; also ping @rust-lang/miri when you do that
+   |
+   = help: this is likely not a bug in the program; it indicates that the program performed an operation that Miri does not support
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/intrinsic_fallback_is_spec.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
+