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//! Visitor for a run-time value with a given layout: Traverse enums, structs and other compound
//! types until we arrive at the leaves, with custom handling for primitive types.
use rustc_index::IndexVec;
use rustc_middle::mir::interpret::InterpResult;
use rustc_middle::ty;
use rustc_target::abi::FieldIdx;
use rustc_target::abi::{FieldsShape, VariantIdx, Variants};
use std::num::NonZeroUsize;
use super::{InterpCx, MPlaceTy, Machine, Projectable};
/// How to traverse a value and what to do when we are at the leaves.
pub trait ValueVisitor<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>>: Sized {
type V: Projectable<'tcx, M::Provenance> + From<MPlaceTy<'tcx, M::Provenance>>;
/// The visitor must have an `InterpCx` in it.
fn ecx(&self) -> &InterpCx<'mir, 'tcx, M>;
/// `read_discriminant` can be hooked for better error messages.
#[inline(always)]
fn read_discriminant(&mut self, v: &Self::V) -> InterpResult<'tcx, VariantIdx> {
Ok(self.ecx().read_discriminant(&v.to_op(self.ecx())?)?)
}
/// This function provides the chance to reorder the order in which fields are visited for
/// `FieldsShape::Aggregate`: The order of fields will be
/// `(0..num_fields).map(aggregate_field_order)`.
///
/// The default means we iterate in source declaration order; alternative this can do an inverse
/// lookup in `memory_index` to use memory field order instead.
#[inline(always)]
fn aggregate_field_order(_memory_index: &IndexVec<FieldIdx, u32>, idx: usize) -> usize {
idx
}
// Recursive actions, ready to be overloaded.
/// Visits the given value, dispatching as appropriate to more specialized visitors.
#[inline(always)]
fn visit_value(&mut self, v: &Self::V) -> InterpResult<'tcx> {
self.walk_value(v)
}
/// Visits the given value as a union. No automatic recursion can happen here.
#[inline(always)]
fn visit_union(&mut self, _v: &Self::V, _fields: NonZeroUsize) -> InterpResult<'tcx> {
Ok(())
}
/// Visits the given value as the pointer of a `Box`. There is nothing to recurse into.
/// The type of `v` will be a raw pointer, but this is a field of `Box<T>` and the
/// pointee type is the actual `T`.
#[inline(always)]
fn visit_box(&mut self, _v: &Self::V) -> InterpResult<'tcx> {
Ok(())
}
/// Called each time we recurse down to a field of a "product-like" aggregate
/// (structs, tuples, arrays and the like, but not enums), passing in old (outer)
/// and new (inner) value.
/// This gives the visitor the chance to track the stack of nested fields that
/// we are descending through.
#[inline(always)]
fn visit_field(
&mut self,
_old_val: &Self::V,
_field: usize,
new_val: &Self::V,
) -> InterpResult<'tcx> {
self.visit_value(new_val)
}
/// Called when recursing into an enum variant.
/// This gives the visitor the chance to track the stack of nested fields that
/// we are descending through.
#[inline(always)]
fn visit_variant(
&mut self,
_old_val: &Self::V,
_variant: VariantIdx,
new_val: &Self::V,
) -> InterpResult<'tcx> {
self.visit_value(new_val)
}
fn walk_value(&mut self, v: &Self::V) -> InterpResult<'tcx> {
let ty = v.layout().ty;
trace!("walk_value: type: {ty}");
// Special treatment for special types, where the (static) layout is not sufficient.
match *ty.kind() {
// If it is a trait object, switch to the real type that was used to create it.
ty::Dynamic(_, _, ty::Dyn) => {
// Dyn types. This is unsized, and the actual dynamic type of the data is given by the
// vtable stored in the place metadata.
// unsized values are never immediate, so we can assert_mem_place
let op = v.to_op(self.ecx())?;
let dest = op.assert_mem_place();
let inner_mplace = self.ecx().unpack_dyn_trait(&dest)?.0;
trace!("walk_value: dyn object layout: {:#?}", inner_mplace.layout);
// recurse with the inner type
return self.visit_field(&v, 0, &inner_mplace.into());
}
ty::Dynamic(_, _, ty::DynStar) => {
// DynStar types. Very different from a dyn type (but strangely part of the
// same variant in `TyKind`): These are pairs where the 2nd component is the
// vtable, and the first component is the data (which must be ptr-sized).
let data = self.ecx().unpack_dyn_star(v)?.0;
return self.visit_field(&v, 0, &data);
}
// Slices do not need special handling here: they have `Array` field
// placement with length 0, so we enter the `Array` case below which
// indirectly uses the metadata to determine the actual length.
// However, `Box`... let's talk about `Box`.
ty::Adt(def, ..) if def.is_box() => {
// `Box` is a hybrid primitive-library-defined type that one the one hand is
// a dereferenceable pointer, on the other hand has *basically arbitrary
// user-defined layout* since the user controls the 'allocator' field. So it
// cannot be treated like a normal pointer, since it does not fit into an
// `Immediate`. Yeah, it is quite terrible. But many visitors want to do
// something with "all boxed pointers", so we handle this mess for them.
//
// When we hit a `Box`, we do not do the usual field recursion; instead,
// we (a) call `visit_box` on the pointer value, and (b) recurse on the
// allocator field. We also assert tons of things to ensure we do not miss
// any other fields.
// `Box` has two fields: the pointer we care about, and the allocator.
assert_eq!(v.layout().fields.count(), 2, "`Box` must have exactly 2 fields");
let (unique_ptr, alloc) =
(self.ecx().project_field(v, 0)?, self.ecx().project_field(v, 1)?);
// Unfortunately there is some type junk in the way here: `unique_ptr` is a `Unique`...
// (which means another 2 fields, the second of which is a `PhantomData`)
assert_eq!(unique_ptr.layout().fields.count(), 2);
let (nonnull_ptr, phantom) = (
self.ecx().project_field(&unique_ptr, 0)?,
self.ecx().project_field(&unique_ptr, 1)?,
);
assert!(
phantom.layout().ty.ty_adt_def().is_some_and(|adt| adt.is_phantom_data()),
"2nd field of `Unique` should be PhantomData but is {:?}",
phantom.layout().ty,
);
// ... that contains a `NonNull`... (gladly, only a single field here)
assert_eq!(nonnull_ptr.layout().fields.count(), 1);
let raw_ptr = self.ecx().project_field(&nonnull_ptr, 0)?; // the actual raw ptr
// ... whose only field finally is a raw ptr we can dereference.
self.visit_box(&raw_ptr)?;
// The second `Box` field is the allocator, which we recursively check for validity
// like in regular structs.
self.visit_field(v, 1, &alloc)?;
// We visited all parts of this one.
return Ok(());
}
_ => {}
};
// Visit the fields of this value.
match &v.layout().fields {
FieldsShape::Primitive => {}
&FieldsShape::Union(fields) => {
self.visit_union(v, fields)?;
}
FieldsShape::Arbitrary { offsets, memory_index } => {
for idx in 0..offsets.len() {
let idx = Self::aggregate_field_order(memory_index, idx);
let field = self.ecx().project_field(v, idx)?;
self.visit_field(v, idx, &field)?;
}
}
FieldsShape::Array { .. } => {
let mut iter = self.ecx().project_array_fields(v)?;
while let Some((idx, field)) = iter.next(self.ecx())? {
self.visit_field(v, idx.try_into().unwrap(), &field)?;
}
}
}
match v.layout().variants {
// If this is a multi-variant layout, find the right variant and proceed
// with *its* fields.
Variants::Multiple { .. } => {
let idx = self.read_discriminant(v)?;
// There are 3 cases where downcasts can turn a Scalar/ScalarPair into a different ABI which
// could be a problem for `ImmTy` (see layout_sanity_check):
// - variant.size == Size::ZERO: works fine because `ImmTy::offset` has a special case for
// zero-sized layouts.
// - variant.fields.count() == 0: works fine because `ImmTy::offset` has a special case for
// zero-field aggregates.
// - variant.abi.is_uninhabited(): triggers UB in `read_discriminant` so we never get here.
let inner = self.ecx().project_downcast(v, idx)?;
trace!("walk_value: variant layout: {:#?}", inner.layout());
// recurse with the inner type
self.visit_variant(v, idx, &inner)?;
}
// For single-variant layouts, we already did anything there is to do.
Variants::Single { .. } => {}
}
Ok(())
}
}