Rollup merge of rust-lang#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: matthiaskrgr

library/core/src/intrinsics.rs

@@ -987,7 +987,7 @@ pub const unsafe fn assume(b: bool) {
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_intrinsic]
 #[rustc_nounwind]
-#[cfg_attr(not(bootstrap), miri::intrinsic_fallback_checks_ub)]
+#[cfg_attr(not(bootstrap), miri::intrinsic_fallback_is_spec)]
 pub const fn likely(b: bool) -> bool {
     b
 }
@@ -1007,7 +1007,7 @@ pub const fn likely(b: bool) -> bool {
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_intrinsic]
 #[rustc_nounwind]
-#[cfg_attr(not(bootstrap), miri::intrinsic_fallback_checks_ub)]
+#[cfg_attr(not(bootstrap), miri::intrinsic_fallback_is_spec)]
 pub const fn unlikely(b: bool) -> bool {
     b
 }
@@ -2483,7 +2483,7 @@ extern "rust-intrinsic" {
 #[rustc_nounwind]
 #[rustc_do_not_const_check]
 #[inline]
-#[cfg_attr(not(bootstrap), miri::intrinsic_fallback_checks_ub)]
+#[cfg_attr(not(bootstrap), miri::intrinsic_fallback_is_spec)]
 pub const fn ptr_guaranteed_cmp<T>(ptr: *const T, other: *const T) -> u8 {
     (ptr == other) as u8
 }
@@ -2748,7 +2748,7 @@ pub const fn ub_checks() -> bool {
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_nounwind]
 #[rustc_intrinsic]
-#[cfg_attr(not(bootstrap), miri::intrinsic_fallback_checks_ub)]
+#[cfg_attr(not(bootstrap), miri::intrinsic_fallback_is_spec)]
 pub const unsafe fn const_allocate(_size: usize, _align: usize) -> *mut u8 {
     // const eval overrides this function, but runtime code for now just returns null pointers.
     // See <https://github.com/rust-lang/rust/issues/93935>.
@@ -2769,7 +2769,7 @@ pub const unsafe fn const_allocate(_size: usize, _align: usize) -> *mut u8 {
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_nounwind]
 #[rustc_intrinsic]
-#[cfg_attr(not(bootstrap), miri::intrinsic_fallback_checks_ub)]
+#[cfg_attr(not(bootstrap), miri::intrinsic_fallback_is_spec)]
 pub const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize) {
     // Runtime NOP
 }

src/tools/miri/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 {

src/tools/miri/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
 }

src/tools/miri/tests/fail/intrinsic_fallback_checks_ub.rs renamed to src/tools/miri/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
+