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//! Check whether a type is representable.
use rustc_data_structures::fx::FxHashMap;
use rustc_hir as hir;
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_span::Span;
use std::cmp;
/// Describes whether a type is representable. For types that are not
/// representable, 'SelfRecursive' and 'ContainsRecursive' are used to
/// distinguish between types that are recursive with themselves and types that
/// contain a different recursive type. These cases can therefore be treated
/// differently when reporting errors.
///
/// The ordering of the cases is significant. They are sorted so that cmp::max
/// will keep the "more erroneous" of two values.
#[derive(Clone, PartialOrd, Ord, Eq, PartialEq, Debug)]
pub enum Representability {
Representable,
ContainsRecursive,
/// Return a list of types that are included in themselves:
/// the spans where they are self-included, and (if found)
/// the HirId of the FieldDef that defines the self-inclusion.
SelfRecursive(Vec<(Span, Option<hir::HirId>)>),
}
/// Check whether a type is representable. This means it cannot contain unboxed
/// structural recursion. This check is needed for structs and enums.
pub fn ty_is_representable<'tcx>(
tcx: TyCtxt<'tcx>,
ty: Ty<'tcx>,
sp: Span,
field_id: Option<hir::HirId>,
) -> Representability {
debug!("is_type_representable: {:?}", ty);
// To avoid a stack overflow when checking an enum variant or struct that
// contains a different, structurally recursive type, maintain a stack of
// seen types and check recursion for each of them (issues #3008, #3779,
// #74224, #84611). `shadow_seen` contains the full stack and `seen` only
// the one for the current type (e.g. if we have structs A and B, B contains
// a field of type A, and we're currently looking at B, then `seen` will be
// cleared when recursing to check A, but `shadow_seen` won't, so that we
// can catch cases of mutual recursion where A also contains B).
let mut seen: Vec<Ty<'_>> = Vec::new();
let mut shadow_seen: Vec<ty::AdtDef<'tcx>> = Vec::new();
let mut representable_cache = FxHashMap::default();
let mut force_result = false;
let r = is_type_structurally_recursive(
tcx,
&mut seen,
&mut shadow_seen,
&mut representable_cache,
ty,
sp,
field_id,
&mut force_result,
);
debug!("is_type_representable: {:?} is {:?}", ty, r);
r
}
// Iterate until something non-representable is found
fn fold_repr<It: Iterator<Item = Representability>>(iter: It) -> Representability {
iter.fold(Representability::Representable, |r1, r2| match (r1, r2) {
(Representability::SelfRecursive(v1), Representability::SelfRecursive(v2)) => {
Representability::SelfRecursive(v1.into_iter().chain(v2).collect())
}
(r1, r2) => cmp::max(r1, r2),
})
}
fn are_inner_types_recursive<'tcx>(
tcx: TyCtxt<'tcx>,
seen: &mut Vec<Ty<'tcx>>,
shadow_seen: &mut Vec<ty::AdtDef<'tcx>>,
representable_cache: &mut FxHashMap<Ty<'tcx>, Representability>,
ty: Ty<'tcx>,
sp: Span,
field_id: Option<hir::HirId>,
force_result: &mut bool,
) -> Representability {
debug!("are_inner_types_recursive({:?}, {:?}, {:?})", ty, seen, shadow_seen);
match ty.kind() {
ty::Tuple(fields) => {
// Find non representable
fold_repr(fields.iter().map(|ty| {
is_type_structurally_recursive(
tcx,
seen,
shadow_seen,
representable_cache,
ty,
sp,
field_id,
force_result,
)
}))
}
// Fixed-length vectors.
// FIXME(#11924) Behavior undecided for zero-length vectors.
ty::Array(ty, _) => is_type_structurally_recursive(
tcx,
seen,
shadow_seen,
representable_cache,
*ty,
sp,
field_id,
force_result,
),
ty::Adt(def, substs) => {
// Find non representable fields with their spans
fold_repr(def.all_fields().map(|field| {
let ty = field.ty(tcx, substs);
let (sp, field_id) = match field
.did
.as_local()
.map(|id| tcx.hir().local_def_id_to_hir_id(id))
.and_then(|id| tcx.hir().find(id))
{
Some(hir::Node::Field(field)) => (field.ty.span, Some(field.hir_id)),
_ => (sp, field_id),
};
let mut result = None;
// First, we check whether the field type per se is representable.
// This catches cases as in #74224 and #84611. There is a special
// case related to mutual recursion, though; consider this example:
//
// struct A<T> {
// z: T,
// x: B<T>,
// }
//
// struct B<T> {
// y: A<T>
// }
//
// Here, without the following special case, both A and B are
// ContainsRecursive, which is a problem because we only report
// errors for SelfRecursive. We fix this by detecting this special
// case (shadow_seen.first() is the type we are originally
// interested in, and if we ever encounter the same AdtDef again,
// we know that it must be SelfRecursive) and "forcibly" returning
// SelfRecursive (by setting force_result, which tells the calling
// invocations of are_inner_types_representable to forward the
// result without adjusting).
if shadow_seen.len() > seen.len() && shadow_seen.first() == Some(def) {
*force_result = true;
result = Some(Representability::SelfRecursive(vec![(sp, field_id)]));
}
if result == None {
result = Some(Representability::Representable);
// Now, we check whether the field types per se are representable, e.g.
// for struct Foo { x: Option<Foo> }, we first check whether Option<_>
// by itself is representable (which it is), and the nesting of Foo
// will be detected later. This is necessary for #74224 and #84611.
// If we have encountered an ADT definition that we have not seen
// before (no need to check them twice), recurse to see whether that
// definition is SelfRecursive. If so, we must be ContainsRecursive.
if shadow_seen.len() > 1
&& !shadow_seen
.iter()
.take(shadow_seen.len() - 1)
.any(|seen_def| seen_def == def)
{
let adt_def_id = def.did();
let raw_adt_ty = tcx.type_of(adt_def_id);
debug!("are_inner_types_recursive: checking nested type: {:?}", raw_adt_ty);
// Check independently whether the ADT is SelfRecursive. If so,
// we must be ContainsRecursive (except for the special case
// mentioned above).
let mut nested_seen: Vec<Ty<'_>> = vec![];
result = Some(
match is_type_structurally_recursive(
tcx,
&mut nested_seen,
shadow_seen,
representable_cache,
raw_adt_ty,
sp,
field_id,
force_result,
) {
Representability::SelfRecursive(_) => {
if *force_result {
Representability::SelfRecursive(vec![(sp, field_id)])
} else {
Representability::ContainsRecursive
}
}
x => x,
},
);
}
// We only enter the following block if the type looks representable
// so far. This is necessary for cases such as this one (#74224):
//
// struct A<T> {
// x: T,
// y: A<A<T>>,
// }
//
// struct B {
// z: A<usize>
// }
//
// When checking B, we recurse into A and check field y of type
// A<A<usize>>. We haven't seen this exact type before, so we recurse
// into A<A<usize>>, which contains, A<A<A<usize>>>, and so forth,
// ad infinitum. We can prevent this from happening by first checking
// A separately (the code above) and only checking for nested Bs if
// A actually looks representable (which it wouldn't in this example).
if result == Some(Representability::Representable) {
// Now, even if the type is representable (e.g. Option<_>),
// it might still contribute to a recursive type, e.g.:
// struct Foo { x: Option<Option<Foo>> }
// These cases are handled by passing the full `seen`
// stack to is_type_structurally_recursive (instead of the
// empty `nested_seen` above):
result = Some(
match is_type_structurally_recursive(
tcx,
seen,
shadow_seen,
representable_cache,
ty,
sp,
field_id,
force_result,
) {
Representability::SelfRecursive(_) => {
Representability::SelfRecursive(vec![(sp, field_id)])
}
x => x,
},
);
}
}
result.unwrap()
}))
}
ty::Closure(..) => {
// this check is run on type definitions, so we don't expect
// to see closure types
bug!("requires check invoked on inapplicable type: {:?}", ty)
}
_ => Representability::Representable,
}
}
fn same_adt<'tcx>(ty: Ty<'tcx>, def: ty::AdtDef<'tcx>) -> bool {
match *ty.kind() {
ty::Adt(ty_def, _) => ty_def == def,
_ => false,
}
}
// Does the type `ty` directly (without indirection through a pointer)
// contain any types on stack `seen`?
fn is_type_structurally_recursive<'tcx>(
tcx: TyCtxt<'tcx>,
seen: &mut Vec<Ty<'tcx>>,
shadow_seen: &mut Vec<ty::AdtDef<'tcx>>,
representable_cache: &mut FxHashMap<Ty<'tcx>, Representability>,
ty: Ty<'tcx>,
sp: Span,
field_id: Option<hir::HirId>,
force_result: &mut bool,
) -> Representability {
debug!("is_type_structurally_recursive: {:?} {:?} {:?}", ty, sp, field_id);
if let Some(representability) = representable_cache.get(&ty) {
debug!(
"is_type_structurally_recursive: {:?} {:?} {:?} - (cached) {:?}",
ty, sp, field_id, representability
);
return representability.clone();
}
let representability = is_type_structurally_recursive_inner(
tcx,
seen,
shadow_seen,
representable_cache,
ty,
sp,
field_id,
force_result,
);
representable_cache.insert(ty, representability.clone());
representability
}
fn is_type_structurally_recursive_inner<'tcx>(
tcx: TyCtxt<'tcx>,
seen: &mut Vec<Ty<'tcx>>,
shadow_seen: &mut Vec<ty::AdtDef<'tcx>>,
representable_cache: &mut FxHashMap<Ty<'tcx>, Representability>,
ty: Ty<'tcx>,
sp: Span,
field_id: Option<hir::HirId>,
force_result: &mut bool,
) -> Representability {
match ty.kind() {
ty::Adt(def, _) => {
{
debug!("is_type_structurally_recursive_inner: adt: {:?}, seen: {:?}", ty, seen);
// Iterate through stack of previously seen types.
let mut iter = seen.iter();
// The first item in `seen` is the type we are actually curious about.
// We want to return SelfRecursive if this type contains itself.
// It is important that we DON'T take generic parameters into account
// for this check, so that Bar<T> in this example counts as SelfRecursive:
//
// struct Foo;
// struct Bar<T> { x: Bar<Foo> }
if let Some(&seen_adt) = iter.next() {
if same_adt(seen_adt, *def) {
debug!("SelfRecursive: {:?} contains {:?}", seen_adt, ty);
return Representability::SelfRecursive(vec![(sp, field_id)]);
}
}
// We also need to know whether the first item contains other types
// that are structurally recursive. If we don't catch this case, we
// will recurse infinitely for some inputs.
//
// It is important that we DO take generic parameters into account
// here, because nesting e.g. Options is allowed (as long as the
// definition of Option doesn't itself include an Option field, which
// would be a case of SelfRecursive above). The following, too, counts
// as SelfRecursive:
//
// struct Foo { Option<Option<Foo>> }
for &seen_adt in iter {
if ty == seen_adt {
debug!("ContainsRecursive: {:?} contains {:?}", seen_adt, ty);
return Representability::ContainsRecursive;
}
}
}
// For structs and enums, track all previously seen types by pushing them
// onto the 'seen' stack.
seen.push(ty);
shadow_seen.push(*def);
let out = are_inner_types_recursive(
tcx,
seen,
shadow_seen,
representable_cache,
ty,
sp,
field_id,
force_result,
);
shadow_seen.pop();
seen.pop();
out
}
_ => {
// No need to push in other cases.
are_inner_types_recursive(
tcx,
seen,
shadow_seen,
representable_cache,
ty,
sp,
field_id,
force_result,
)
}
}
}