Rollup merge of rust-lang#124293 - oli-obk:miri_intrinsic_fallback_body, r=RalfJung

Let miri and const eval execute intrinsics' fallback bodies

fixes rust-lang/miri#3397

r? ``@RalfJung``

Author: matthiaskrgr

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
 }
@@ -2469,6 +2471,7 @@ 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
 }
@@ -2733,8 +2736,10 @@ 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)]
 pub const unsafe fn const_allocate(_size: usize, _align: usize) -> *mut u8 {
-    // const eval overrides this function, but runtime code should always just return null pointers.
+    // const eval overrides this function, but runtime code for now just returns null pointers.
+    // See <https://github.com/rust-lang/rust/issues/93935>.
     crate::ptr::null_mut()
 }
 
@@ -2752,7 +2757,10 @@ pub const unsafe fn const_allocate(_size: usize, _align: usize) -> *mut u8 {
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_nounwind]
 #[rustc_intrinsic]
-pub const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize) {}
+#[cfg_attr(not(bootstrap), miri::intrinsic_fallback_checks_ub)]
+pub const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize) {
+    // Runtime NOP
+}
 
 /// `ptr` must point to a vtable.
 /// The intrinsic will return the size stored in that vtable.