Rollup merge of #99259 - RalfJung:visit-a-place, r=oli-obk

interpret/visitor: support visiting with a PlaceTy

Finally we can visit a `PlaceTy` in a way that will only do `force_allocation` when needed ti visit a field. :)

r? `@oli-obk`

compiler/rustc_const_eval/src/const_eval/valtrees.rs

@@ -436,7 +436,7 @@ fn valtree_into_mplace<'tcx>(
 
                         let offset = place_adjusted.layout.fields.offset(i);
                         place
-                            .offset(
+                            .offset_with_meta(
                                 offset,
                                 MemPlaceMeta::Meta(Scalar::from_machine_usize(
                                     num_elems as u64,

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -297,15 +297,15 @@ impl<'tcx, Tag: Provenance> OpTy<'tcx, Tag> {
         }
     }
 
-    pub fn offset(
+    pub fn offset_with_meta(
         &self,
         offset: Size,
         meta: MemPlaceMeta<Tag>,
         layout: TyAndLayout<'tcx>,
         cx: &impl HasDataLayout,
     ) -> InterpResult<'tcx, Self> {
         match self.try_as_mplace() {
-            Ok(mplace) => Ok(mplace.offset(offset, meta, layout, cx)?.into()),
+            Ok(mplace) => Ok(mplace.offset_with_meta(offset, meta, layout, cx)?.into()),
             Err(imm) => {
                 assert!(
                     matches!(*imm, Immediate::Uninit),
@@ -317,6 +317,16 @@ impl<'tcx, Tag: Provenance> OpTy<'tcx, Tag> {
             }
         }
     }
+
+    pub fn offset(
+        &self,
+        offset: Size,
+        layout: TyAndLayout<'tcx>,
+        cx: &impl HasDataLayout,
+    ) -> InterpResult<'tcx, Self> {
+        assert!(!layout.is_unsized());
+        self.offset_with_meta(offset, MemPlaceMeta::None, layout, cx)
+    }
 }
 
 impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {

compiler/rustc_const_eval/src/interpret/place.rs

@@ -163,7 +163,7 @@ impl<Tag: Provenance> MemPlace<Tag> {
     }
 
     #[inline]
-    pub fn offset<'tcx>(
+    pub fn offset_with_meta<'tcx>(
         self,
         offset: Size,
         meta: MemPlaceMeta<Tag>,
@@ -199,20 +199,30 @@ impl<'tcx, Tag: Provenance> MPlaceTy<'tcx, Tag> {
     }
 
     #[inline]
-    pub fn offset(
+    pub fn offset_with_meta(
         &self,
         offset: Size,
         meta: MemPlaceMeta<Tag>,
         layout: TyAndLayout<'tcx>,
         cx: &impl HasDataLayout,
     ) -> InterpResult<'tcx, Self> {
         Ok(MPlaceTy {
-            mplace: self.mplace.offset(offset, meta, cx)?,
+            mplace: self.mplace.offset_with_meta(offset, meta, cx)?,
             align: self.align.restrict_for_offset(offset),
             layout,
         })
     }
 
+    pub fn offset(
+        &self,
+        offset: Size,
+        layout: TyAndLayout<'tcx>,
+        cx: &impl HasDataLayout,
+    ) -> InterpResult<'tcx, Self> {
+        assert!(!layout.is_unsized());
+        self.offset_with_meta(offset, MemPlaceMeta::None, layout, cx)
+    }
+
     #[inline]
     pub fn from_aligned_ptr(ptr: Pointer<Option<Tag>>, layout: TyAndLayout<'tcx>) -> Self {
         MPlaceTy { mplace: MemPlace::from_ptr(ptr), layout, align: layout.align.abi }

compiler/rustc_const_eval/src/interpret/projection.rs

@@ -63,7 +63,7 @@ where
 
         // We do not look at `base.layout.align` nor `field_layout.align`, unlike
         // codegen -- mostly to see if we can get away with that
-        base.offset(offset, meta, field_layout, self)
+        base.offset_with_meta(offset, meta, field_layout, self)
     }
 
     /// Gets the place of a field inside the place, and also the field's type.
@@ -193,9 +193,7 @@ where
                 let offset = stride * index; // `Size` multiplication
                 // All fields have the same layout.
                 let field_layout = base.layout.field(self, 0);
-                assert!(!field_layout.is_unsized());
-
-                base.offset(offset, MemPlaceMeta::None, field_layout, self)
+                base.offset(offset, field_layout, self)
             }
             _ => span_bug!(
                 self.cur_span(),
@@ -215,10 +213,10 @@ where
         let abi::FieldsShape::Array { stride, .. } = base.layout.fields else {
             span_bug!(self.cur_span(), "operand_array_fields: expected an array layout");
         };
-        let layout = base.layout.field(self, 0);
+        let field_layout = base.layout.field(self, 0);
         let dl = &self.tcx.data_layout;
         // `Size` multiplication
-        Ok((0..len).map(move |i| base.offset(stride * i, MemPlaceMeta::None, layout, dl)))
+        Ok((0..len).map(move |i| base.offset(stride * i, field_layout, dl)))
     }
 
     /// Index into an array.
@@ -326,7 +324,7 @@ where
             }
         };
         let layout = self.layout_of(ty)?;
-        base.offset(from_offset, meta, layout, self)
+        base.offset_with_meta(from_offset, meta, layout, self)
     }
 
     pub fn place_subslice(

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -853,7 +853,8 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
                 self.visit_scalar(scalar, scalar_layout)?;
             }
             Abi::ScalarPair(a_layout, b_layout) => {
-                // We would validate these things as we descend into the fields,
+                // There is no `rustc_layout_scalar_valid_range_start` for pairs, so
+                // we would validate these things as we descend into the fields,
                 // but that can miss bugs in layout computation. Layout computation
                 // is subtle due to enums having ScalarPair layout, where one field
                 // is the discriminant.
@@ -867,7 +868,8 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
             }
             Abi::Vector { .. } => {
                 // No checks here, we assume layout computation gets this right.
-                // (This is harder to check since Miri does not represent these as `Immediate`.)
+                // (This is harder to check since Miri does not represent these as `Immediate`. We
+                // also cannot use field projections since this might be a newtype around a vector.)
             }
             Abi::Aggregate { .. } => {
                 // Nothing to do.