interpret: do not ICE on padded non-pow2 SIMD vectors

2 files changed, 9 insertions, 6 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/place.rs b/compiler/rustc_const_eval/src/interpret/place.rs
index 4a86ec3f57a..046ff34e3d0 100644
--- a/compiler/rustc_const_eval/src/interpret/place.rs
+++ b/compiler/rustc_const_eval/src/interpret/place.rs
@@ -499,13 +499,14 @@ where
         &self,
         mplace: &MPlaceTy<'tcx, M::Provenance>,
     ) -> InterpResult<'tcx, (MPlaceTy<'tcx, M::Provenance>, u64)> {
-        // Basically we just transmute this place into an array following simd_size_and_type.
-        // (Transmuting is okay since this is an in-memory place. We also double-check the size
-        // stays the same.)
+        // Basically we want to transmute this place into an array following simd_size_and_type.
         let (len, e_ty) = mplace.layout.ty.simd_size_and_type(*self.tcx);
-        let array = Ty::new_array(self.tcx.tcx, e_ty, len);
-        let layout = self.layout_of(array)?;
-        let mplace = mplace.transmute(layout, self)?;
+        // Some SIMD types have padding, so `len` many `e_ty` does not cover the entire place.
+        // Therefore we cannot transmute, and instead we project at offset 0, which side-steps
+        // the size check.
+        let array_layout = self.layout_of(Ty::new_array(self.tcx.tcx, e_ty, len))?;
+        assert!(array_layout.size <= mplace.layout.size);
+        let mplace = mplace.offset(Size::ZERO, array_layout, self)?;
         Ok((mplace, len))
     }
 
diff --git a/compiler/rustc_const_eval/src/interpret/projection.rs b/compiler/rustc_const_eval/src/interpret/projection.rs
index 0e594914c3a..09e1a59dfa1 100644
--- a/compiler/rustc_const_eval/src/interpret/projection.rs
+++ b/compiler/rustc_const_eval/src/interpret/projection.rs
@@ -81,6 +81,8 @@ pub trait Projectable<'tcx, Prov: Provenance>: Sized + std::fmt::Debug {
         ecx: &InterpCx<'tcx, M>,
     ) -> InterpResult<'tcx, Self> {
         assert!(layout.is_sized());
+        // We sometimes do pointer arithmetic with this function, disregarding the source type.
+        // So we don't check the sizes here.
         self.offset_with_meta(offset, OffsetMode::Inbounds, MemPlaceMeta::None, layout, ecx)
     }