PR feedback

- Add `:Sized` assertion in interpreter impl
- Use `Scalar::from_bool` instead of `ScalarInt: From<bool>`
- Remove unneeded comparison in intrinsic typeck
- Make this UB to call with undef, not just return undef in that case

Author: scottmcm

compiler/rustc_mir/src/interpret/intrinsics.rs

@@ -570,11 +570,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         rhs: &OpTy<'tcx, <M as Machine<'mir, 'tcx>>::PointerTag>,
     ) -> InterpResult<'tcx, Scalar<M::PointerTag>> {
         let layout = self.layout_of(lhs.layout.ty.builtin_deref(true).unwrap().ty)?;
+        assert!(!layout.is_unsized());
 
         let lhs = self.read_scalar(lhs)?.check_init()?;
         let rhs = self.read_scalar(rhs)?.check_init()?;
         let lhs_bytes = self.memory.read_bytes(lhs, layout.size)?;
         let rhs_bytes = self.memory.read_bytes(rhs, layout.size)?;
-        Ok(Scalar::Int((lhs_bytes == rhs_bytes).into()))
+        Ok(Scalar::from_bool(lhs_bytes == rhs_bytes))
     }
 }

compiler/rustc_typeck/src/check/intrinsic.rs

@@ -381,11 +381,10 @@ pub fn check_intrinsic_type(tcx: TyCtxt<'_>, it: &hir::ForeignItem<'_>) {
             sym::nontemporal_store => (1, vec![tcx.mk_mut_ptr(param(0)), param(0)], tcx.mk_unit()),
 
             sym::raw_eq => {
-                let param_count = if intrinsic_name == sym::raw_eq { 2 } else { 1 };
                 let br = ty::BoundRegion { var: ty::BoundVar::from_u32(0), kind: ty::BrAnon(0) };
                 let param_ty =
                     tcx.mk_imm_ref(tcx.mk_region(ty::ReLateBound(ty::INNERMOST, br)), param(0));
-                (1, vec![param_ty; param_count], tcx.types.bool)
+                (1, vec![param_ty; 2], tcx.types.bool)
             }
 
             other => {

library/core/src/intrinsics.rs

@@ -1924,8 +1924,12 @@ extern "rust-intrinsic" {
     ///
     /// # Safety
     ///
-    /// This doesn't take into account padding, so if `T` has padding
-    /// the result will be `undef`, which cannot be exposed to safe code.
+    /// It's UB to call this if any of the *bytes* in `*a` or `*b` are uninitialized.
+    /// Note that this is a stricter criterion than just the *values* being
+    /// fully-initialized: if `T` has padding, it's UB to call this intrinsic.
+    ///
+    /// (The implementation is allowed to branch on the results of comparisons,
+    /// which is UB if any of their inputs are `undef`.)
     #[cfg(not(bootstrap))]
     #[rustc_const_unstable(feature = "const_intrinsic_raw_eq", issue = "none")]
     pub fn raw_eq<T>(a: &T, b: &T) -> bool;