Ensure miri only uses fallback bodies that have manually been vetted …

…to preserve all UB that the native intrinsic would have

compiler/rustc_resolve/src/macros.rs

@@ -141,7 +141,7 @@ pub(crate) fn registered_tools(tcx: TyCtxt<'_>, (): ()) -> RegisteredTools {
     }
     // We implicitly add `rustfmt`, `clippy`, `diagnostic` to known tools,
     // but it's not an error to register them explicitly.
-    let predefined_tools = [sym::clippy, sym::rustfmt, sym::diagnostic];
+    let predefined_tools = [sym::clippy, sym::rustfmt, sym::diagnostic, sym::miri];
     registered_tools.extend(predefined_tools.iter().cloned().map(Ident::with_dummy_span));
     registered_tools
 }

library/core/src/intrinsics.rs

@@ -987,6 +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)]
 pub const fn likely(b: bool) -> bool {
     b
 }
@@ -1006,6 +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)]
 pub const fn unlikely(b: bool) -> bool {
     b
 }
@@ -2479,6 +2481,7 @@ extern "rust-intrinsic" {
 #[rustc_nounwind]
 #[rustc_do_not_const_check]
 #[inline]
+#[cfg_attr(not(bootstrap), miri::intrinsic_fallback_checks_ub)]
 pub const fn ptr_guaranteed_cmp<T>(ptr: *const T, other: *const T) -> u8 {
     (ptr == other) as u8
 }

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

@@ -11,6 +11,7 @@ use rustc_middle::{
     ty::{self, FloatTy},
 };
 use rustc_target::abi::Size;
+use rustc_span::{sym, Symbol};
 
 use crate::*;
 use atomic::EvalContextExt as _;
@@ -66,6 +67,10 @@ 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");
+            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");
+            }
             return Ok(Some(ty::Instance {
                 def: ty::InstanceDef::Item(instance.def_id()),
                 args: instance.args,