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core/src/intrinsics/fallback.rs

@@ -114,13 +114,8 @@ impl const CarryingMulAdd for i128 {
 #[const_trait]
 #[rustc_const_unstable(feature = "core_intrinsics_fallbacks", issue = "none")]
 pub trait DisjointBitOr: Copy + 'static {
-    /// This is always just `assume((self & other) == 0); self | other`.
-    ///
-    /// It's essential that the assume is there so that this is sufficient to
-    /// specify the UB for MIRI, rather than it needing to re-implement it.
-    ///
-    /// # Safety
-    /// See [`super::disjoint_bitor`].
+    /// See [`super::disjoint_bitor`]; we just need the trait indirection to handle
+    /// different types since calling intrinsics with generics doesn't work.
     unsafe fn disjoint_bitor(self, other: Self) -> Self;
 }
 macro_rules! zero {
@@ -135,8 +130,11 @@ macro_rules! impl_disjoint_bitor {
     ($($t:ident,)+) => {$(
         #[rustc_const_unstable(feature = "core_intrinsics_fallbacks", issue = "none")]
         impl const DisjointBitOr for $t {
+            #[cfg_attr(miri, track_caller)]
             #[inline]
             unsafe fn disjoint_bitor(self, other: Self) -> Self {
+                // Note that the assume here is required for UB detection in Miri!
+
                 // SAFETY: our precondition is that there are no bits in common,
                 // so this is just telling that to the backend.
                 unsafe { super::assume((self & other) == zero!($t)) };

core/src/intrinsics/mod.rs

@@ -3261,7 +3261,8 @@ pub const fn three_way_compare<T: Copy>(_lhs: T, _rhss: T) -> crate::cmp::Orderi
 #[rustc_const_unstable(feature = "disjoint_bitor", issue = "135758")]
 #[rustc_nounwind]
 #[cfg_attr(not(bootstrap), rustc_intrinsic)]
-#[miri::intrinsic_fallback_is_spec] // the fallbacks all `assume` to tell MIRI
+#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
+#[miri::intrinsic_fallback_is_spec] // the fallbacks all `assume` to tell Miri
 pub const unsafe fn disjoint_bitor<T: ~const fallback::DisjointBitOr>(a: T, b: T) -> T {
     // SAFETY: same preconditions as this function.
     unsafe { fallback::DisjointBitOr::disjoint_bitor(a, b) }

core/src/num/uint_macros.rs

@@ -1213,7 +1213,7 @@ macro_rules! uint_impl {
         ///
         /// # Safety
         ///
-        /// Requires that `(self | other) == 0`, otherwise it's immediate UB.
+        /// Requires that `(self & other) == 0`, otherwise it's immediate UB.
         ///
         /// Equivalently, requires that `(self | other) == (self + other)`.
         #[unstable(feature = "disjoint_bitor", issue = "135758")]