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use crate::infer::InferCtxt;
use crate::traits;
use rustc_hir as hir;
use rustc_hir::lang_items::LangItem;
use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
use rustc_middle::ty::{GenericArg, GenericArgKind, GenericArgsRef};
use rustc_span::def_id::{DefId, LocalDefId, CRATE_DEF_ID};
use rustc_span::{Span, DUMMY_SP};
use std::iter;
/// Returns the set of obligations needed to make `arg` well-formed.
/// If `arg` contains unresolved inference variables, this may include
/// further WF obligations. However, if `arg` IS an unresolved
/// inference variable, returns `None`, because we are not able to
/// make any progress at all. This is to prevent "livelock" where we
/// say "$0 is WF if $0 is WF".
pub fn obligations<'tcx>(
infcx: &InferCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
body_id: LocalDefId,
recursion_depth: usize,
arg: GenericArg<'tcx>,
span: Span,
) -> Option<Vec<traits::PredicateObligation<'tcx>>> {
// Handle the "livelock" case (see comment above) by bailing out if necessary.
let arg = match arg.unpack() {
GenericArgKind::Type(ty) => {
match ty.kind() {
ty::Infer(ty::TyVar(_)) => {
let resolved_ty = infcx.shallow_resolve(ty);
if resolved_ty == ty {
// No progress, bail out to prevent "livelock".
return None;
} else {
resolved_ty
}
}
_ => ty,
}
.into()
}
GenericArgKind::Const(ct) => {
match ct.kind() {
ty::ConstKind::Infer(_) => {
let resolved = infcx.shallow_resolve(ct);
if resolved == ct {
// No progress.
return None;
} else {
resolved
}
}
_ => ct,
}
.into()
}
// There is nothing we have to do for lifetimes.
GenericArgKind::Lifetime(..) => return Some(Vec::new()),
};
let mut wf =
WfPredicates { infcx, param_env, body_id, span, out: vec![], recursion_depth, item: None };
wf.compute(arg);
debug!("wf::obligations({:?}, body_id={:?}) = {:?}", arg, body_id, wf.out);
let result = wf.normalize(infcx);
debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", arg, body_id, result);
Some(result)
}
/// Compute the predicates that are required for a type to be well-formed.
///
/// This is only intended to be used in the new solver, since it does not
/// take into account recursion depth or proper error-reporting spans.
pub fn unnormalized_obligations<'tcx>(
infcx: &InferCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
arg: GenericArg<'tcx>,
) -> Option<Vec<traits::PredicateObligation<'tcx>>> {
debug_assert_eq!(arg, infcx.resolve_vars_if_possible(arg));
// However, if `arg` IS an unresolved inference variable, returns `None`,
// because we are not able to make any progress at all. This is to prevent
// "livelock" where we say "$0 is WF if $0 is WF".
if arg.is_non_region_infer() {
return None;
}
if let ty::GenericArgKind::Lifetime(..) = arg.unpack() {
return Some(vec![]);
}
let mut wf = WfPredicates {
infcx,
param_env,
body_id: CRATE_DEF_ID,
span: DUMMY_SP,
out: vec![],
recursion_depth: 0,
item: None,
};
wf.compute(arg);
Some(wf.out)
}
/// Returns the obligations that make this trait reference
/// well-formed. For example, if there is a trait `Set` defined like
/// `trait Set<K: Eq>`, then the trait bound `Foo: Set<Bar>` is WF
/// if `Bar: Eq`.
pub fn trait_obligations<'tcx>(
infcx: &InferCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
body_id: LocalDefId,
trait_pred: ty::TraitPredicate<'tcx>,
span: Span,
item: &'tcx hir::Item<'tcx>,
) -> Vec<traits::PredicateObligation<'tcx>> {
let mut wf = WfPredicates {
infcx,
param_env,
body_id,
span,
out: vec![],
recursion_depth: 0,
item: Some(item),
};
wf.compute_trait_pred(trait_pred, Elaborate::All);
debug!(obligations = ?wf.out);
wf.normalize(infcx)
}
/// Returns the requirements for `clause` to be well-formed.
///
/// For example, if there is a trait `Set` defined like
/// `trait Set<K: Eq>`, then the trait bound `Foo: Set<Bar>` is WF
/// if `Bar: Eq`.
#[instrument(skip(infcx), ret)]
pub fn clause_obligations<'tcx>(
infcx: &InferCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
body_id: LocalDefId,
clause: ty::Clause<'tcx>,
span: Span,
) -> Vec<traits::PredicateObligation<'tcx>> {
let mut wf = WfPredicates {
infcx,
param_env,
body_id,
span,
out: vec![],
recursion_depth: 0,
item: None,
};
// It's ok to skip the binder here because wf code is prepared for it
match clause.kind().skip_binder() {
ty::ClauseKind::Trait(t) => {
wf.compute_trait_pred(t, Elaborate::None);
}
ty::ClauseKind::RegionOutlives(..) => {}
ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(ty, _reg)) => {
wf.compute(ty.into());
}
ty::ClauseKind::Projection(t) => {
wf.compute_projection(t.projection_ty);
wf.compute(match t.term.unpack() {
ty::TermKind::Ty(ty) => ty.into(),
ty::TermKind::Const(c) => c.into(),
})
}
ty::ClauseKind::ConstArgHasType(ct, ty) => {
wf.compute(ct.into());
wf.compute(ty.into());
}
ty::ClauseKind::WellFormed(arg) => {
wf.compute(arg);
}
ty::ClauseKind::ConstEvaluatable(ct) => {
wf.compute(ct.into());
}
}
wf.normalize(infcx)
}
struct WfPredicates<'a, 'tcx> {
infcx: &'a InferCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
body_id: LocalDefId,
span: Span,
out: Vec<traits::PredicateObligation<'tcx>>,
recursion_depth: usize,
item: Option<&'tcx hir::Item<'tcx>>,
}
/// Controls whether we "elaborate" supertraits and so forth on the WF
/// predicates. This is a kind of hack to address #43784. The
/// underlying problem in that issue was a trait structure like:
///
/// ```ignore (illustrative)
/// trait Foo: Copy { }
/// trait Bar: Foo { }
/// impl<T: Bar> Foo for T { }
/// impl<T> Bar for T { }
/// ```
///
/// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
/// we decide that this is true because `T: Bar` is in the
/// where-clauses (and we can elaborate that to include `T:
/// Copy`). This wouldn't be a problem, except that when we check the
/// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
/// impl. And so nowhere did we check that `T: Copy` holds!
///
/// To resolve this, we elaborate the WF requirements that must be
/// proven when checking impls. This means that (e.g.) the `impl Bar
/// for T` will be forced to prove not only that `T: Foo` but also `T:
/// Copy` (which it won't be able to do, because there is no `Copy`
/// impl for `T`).
#[derive(Debug, PartialEq, Eq, Copy, Clone)]
enum Elaborate {
All,
None,
}
fn extend_cause_with_original_assoc_item_obligation<'tcx>(
tcx: TyCtxt<'tcx>,
trait_ref: ty::TraitRef<'tcx>,
item: Option<&hir::Item<'tcx>>,
cause: &mut traits::ObligationCause<'tcx>,
pred: ty::Predicate<'tcx>,
) {
debug!(
"extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
trait_ref, item, cause, pred
);
let (items, impl_def_id) = match item {
Some(hir::Item { kind: hir::ItemKind::Impl(impl_), owner_id, .. }) => {
(impl_.items, *owner_id)
}
_ => return,
};
let fix_span =
|impl_item_ref: &hir::ImplItemRef| match tcx.hir().impl_item(impl_item_ref.id).kind {
hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::Type(ty) => ty.span,
_ => impl_item_ref.span,
};
// It is fine to skip the binder as we don't care about regions here.
match pred.kind().skip_binder() {
ty::PredicateKind::Clause(ty::ClauseKind::Projection(proj)) => {
// The obligation comes not from the current `impl` nor the `trait` being implemented,
// but rather from a "second order" obligation, where an associated type has a
// projection coming from another associated type. See
// `tests/ui/associated-types/point-at-type-on-obligation-failure.rs` and
// `traits-assoc-type-in-supertrait-bad.rs`.
if let Some(ty::Alias(ty::Projection, projection_ty)) = proj.term.ty().map(|ty| ty.kind())
&& let Some(&impl_item_id) =
tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.def_id)
&& let Some(impl_item_span) = items
.iter()
.find(|item| item.id.owner_id.to_def_id() == impl_item_id)
.map(fix_span)
{
cause.span = impl_item_span;
}
}
ty::PredicateKind::Clause(ty::ClauseKind::Trait(pred)) => {
// An associated item obligation born out of the `trait` failed to be met. An example
// can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
if let ty::Alias(ty::Projection, ty::AliasTy { def_id, .. }) = *pred.self_ty().kind()
&& let Some(&impl_item_id) =
tcx.impl_item_implementor_ids(impl_def_id).get(&def_id)
&& let Some(impl_item_span) = items
.iter()
.find(|item| item.id.owner_id.to_def_id() == impl_item_id)
.map(fix_span)
{
cause.span = impl_item_span;
}
}
_ => {}
}
}
impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
fn tcx(&self) -> TyCtxt<'tcx> {
self.infcx.tcx
}
fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
traits::ObligationCause::new(self.span, self.body_id, code)
}
fn normalize(self, infcx: &InferCtxt<'tcx>) -> Vec<traits::PredicateObligation<'tcx>> {
// Do not normalize `wf` obligations with the new solver.
//
// The current deep normalization routine with the new solver does not
// handle ambiguity and the new solver correctly deals with unnnormalized goals.
// If the user relies on normalized types, e.g. for `fn implied_outlives_bounds`,
// it is their responsibility to normalize while avoiding ambiguity.
if infcx.next_trait_solver() {
return self.out;
}
let cause = self.cause(traits::WellFormed(None));
let param_env = self.param_env;
let mut obligations = Vec::with_capacity(self.out.len());
for mut obligation in self.out {
assert!(!obligation.has_escaping_bound_vars());
let mut selcx = traits::SelectionContext::new(infcx);
// Don't normalize the whole obligation, the param env is either
// already normalized, or we're currently normalizing the
// param_env. Either way we should only normalize the predicate.
let normalized_predicate = traits::project::normalize_with_depth_to(
&mut selcx,
param_env,
cause.clone(),
self.recursion_depth,
obligation.predicate,
&mut obligations,
);
obligation.predicate = normalized_predicate;
obligations.push(obligation);
}
obligations
}
/// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
fn compute_trait_pred(&mut self, trait_pred: ty::TraitPredicate<'tcx>, elaborate: Elaborate) {
let tcx = self.tcx();
let trait_ref = trait_pred.trait_ref;
// Negative trait predicates don't require supertraits to hold, just
// that their args are WF.
if trait_pred.polarity == ty::ImplPolarity::Negative {
self.compute_negative_trait_pred(trait_ref);
return;
}
// if the trait predicate is not const, the wf obligations should not be const as well.
let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.args);
debug!("compute_trait_pred obligations {:?}", obligations);
let param_env = self.param_env;
let depth = self.recursion_depth;
let item = self.item;
let extend = |traits::PredicateObligation { predicate, mut cause, .. }| {
if let Some(parent_trait_pred) = predicate.to_opt_poly_trait_pred() {
cause = cause.derived_cause(
parent_trait_pred,
traits::ObligationCauseCode::DerivedObligation,
);
}
extend_cause_with_original_assoc_item_obligation(
tcx, trait_ref, item, &mut cause, predicate,
);
traits::Obligation::with_depth(tcx, cause, depth, param_env, predicate)
};
if let Elaborate::All = elaborate {
let implied_obligations = traits::util::elaborate(tcx, obligations);
let implied_obligations = implied_obligations.map(extend);
self.out.extend(implied_obligations);
} else {
self.out.extend(obligations);
}
self.out.extend(
trait_ref
.args
.iter()
.enumerate()
.filter(|(_, arg)| {
matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
})
.filter(|(_, arg)| !arg.has_escaping_bound_vars())
.map(|(i, arg)| {
let mut cause = traits::ObligationCause::misc(self.span, self.body_id);
// The first subst is the self ty - use the correct span for it.
if i == 0 {
if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
item.map(|i| &i.kind)
{
cause.span = self_ty.span;
}
}
traits::Obligation::with_depth(
tcx,
cause,
depth,
param_env,
ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(
arg,
))),
)
}),
);
}
// Compute the obligations that are required for `trait_ref` to be WF,
// given that it is a *negative* trait predicate.
fn compute_negative_trait_pred(&mut self, trait_ref: ty::TraitRef<'tcx>) {
for arg in trait_ref.args {
self.compute(arg);
}
}
/// Pushes the obligations required for `trait_ref::Item` to be WF
/// into `self.out`.
fn compute_projection(&mut self, data: ty::AliasTy<'tcx>) {
// A projection is well-formed if
//
// (a) its predicates hold (*)
// (b) its args are wf
//
// (*) The predicates of an associated type include the predicates of
// the trait that it's contained in. For example, given
//
// trait A<T>: Clone {
// type X where T: Copy;
// }
//
// The predicates of `<() as A<i32>>::X` are:
// [
// `(): Sized`
// `(): Clone`
// `(): A<i32>`
// `i32: Sized`
// `i32: Clone`
// `i32: Copy`
// ]
let obligations = self.nominal_obligations(data.def_id, data.args);
self.out.extend(obligations);
self.compute_projection_args(data.args);
}
fn compute_inherent_projection(&mut self, data: ty::AliasTy<'tcx>) {
// An inherent projection is well-formed if
//
// (a) its predicates hold (*)
// (b) its args are wf
//
// (*) The predicates of an inherent associated type include the
// predicates of the impl that it's contained in.
if !data.self_ty().has_escaping_bound_vars() {
// FIXME(inherent_associated_types): Should this happen inside of a snapshot?
// FIXME(inherent_associated_types): This is incompatible with the new solver and lazy norm!
let args = traits::project::compute_inherent_assoc_ty_args(
&mut traits::SelectionContext::new(self.infcx),
self.param_env,
data,
self.cause(traits::WellFormed(None)),
self.recursion_depth,
&mut self.out,
);
let obligations = self.nominal_obligations(data.def_id, args);
self.out.extend(obligations);
}
self.compute_projection_args(data.args);
}
fn compute_projection_args(&mut self, args: GenericArgsRef<'tcx>) {
let tcx = self.tcx();
let cause = self.cause(traits::WellFormed(None));
let param_env = self.param_env;
let depth = self.recursion_depth;
self.out.extend(
args.iter()
.filter(|arg| {
matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
})
.filter(|arg| !arg.has_escaping_bound_vars())
.map(|arg| {
traits::Obligation::with_depth(
tcx,
cause.clone(),
depth,
param_env,
ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(
arg,
))),
)
}),
);
}
fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
if !subty.has_escaping_bound_vars() {
let cause = self.cause(cause);
let trait_ref =
ty::TraitRef::from_lang_item(self.tcx(), LangItem::Sized, cause.span, [subty]);
self.out.push(traits::Obligation::with_depth(
self.tcx(),
cause,
self.recursion_depth,
self.param_env,
ty::Binder::dummy(trait_ref),
));
}
}
/// Pushes all the predicates needed to validate that `ty` is WF into `out`.
#[instrument(level = "debug", skip(self))]
fn compute(&mut self, arg: GenericArg<'tcx>) {
let mut walker = arg.walk();
let param_env = self.param_env;
let depth = self.recursion_depth;
while let Some(arg) = walker.next() {
debug!(?arg, ?self.out);
let ty = match arg.unpack() {
GenericArgKind::Type(ty) => ty,
// No WF constraints for lifetimes being present, any outlives
// obligations are handled by the parent (e.g. `ty::Ref`).
GenericArgKind::Lifetime(_) => continue,
GenericArgKind::Const(ct) => {
match ct.kind() {
ty::ConstKind::Unevaluated(uv) => {
if !ct.has_escaping_bound_vars() {
let obligations = self.nominal_obligations(uv.def, uv.args);
self.out.extend(obligations);
let predicate = ty::Binder::dummy(ty::PredicateKind::Clause(
ty::ClauseKind::ConstEvaluatable(ct),
));
let cause = self.cause(traits::WellFormed(None));
self.out.push(traits::Obligation::with_depth(
self.tcx(),
cause,
self.recursion_depth,
self.param_env,
predicate,
));
}
}
ty::ConstKind::Infer(_) => {
let cause = self.cause(traits::WellFormed(None));
self.out.push(traits::Obligation::with_depth(
self.tcx(),
cause,
self.recursion_depth,
self.param_env,
ty::Binder::dummy(ty::PredicateKind::Clause(
ty::ClauseKind::WellFormed(ct.into()),
)),
));
}
ty::ConstKind::Expr(_) => {
// FIXME(generic_const_exprs): this doesn't verify that given `Expr(N + 1)` the
// trait bound `typeof(N): Add<typeof(1)>` holds. This is currently unnecessary
// as `ConstKind::Expr` is only produced via normalization of `ConstKind::Unevaluated`
// which means that the `DefId` would have been typeck'd elsewhere. However in
// the future we may allow directly lowering to `ConstKind::Expr` in which case
// we would not be proving bounds we should.
let predicate = ty::Binder::dummy(ty::PredicateKind::Clause(
ty::ClauseKind::ConstEvaluatable(ct),
));
let cause = self.cause(traits::WellFormed(None));
self.out.push(traits::Obligation::with_depth(
self.tcx(),
cause,
self.recursion_depth,
self.param_env,
predicate,
));
}
ty::ConstKind::Error(_)
| ty::ConstKind::Param(_)
| ty::ConstKind::Bound(..)
| ty::ConstKind::Placeholder(..) => {
// These variants are trivially WF, so nothing to do here.
}
ty::ConstKind::Value(..) => {
// FIXME: Enforce that values are structurally-matchable.
}
}
continue;
}
};
debug!("wf bounds for ty={:?} ty.kind={:#?}", ty, ty.kind());
match *ty.kind() {
ty::Bool
| ty::Char
| ty::Int(..)
| ty::Uint(..)
| ty::Float(..)
| ty::Error(_)
| ty::Str
| ty::GeneratorWitness(..)
| ty::Never
| ty::Param(_)
| ty::Bound(..)
| ty::Placeholder(..)
| ty::Foreign(..) => {
// WfScalar, WfParameter, etc
}
// Can only infer to `ty::Int(_) | ty::Uint(_)`.
ty::Infer(ty::IntVar(_)) => {}
// Can only infer to `ty::Float(_)`.
ty::Infer(ty::FloatVar(_)) => {}
ty::Slice(subty) => {
self.require_sized(subty, traits::SliceOrArrayElem);
}
ty::Array(subty, _) => {
self.require_sized(subty, traits::SliceOrArrayElem);
// Note that we handle the len is implicitly checked while walking `arg`.
}
ty::Tuple(ref tys) => {
if let Some((_last, rest)) = tys.split_last() {
for &elem in rest {
self.require_sized(elem, traits::TupleElem);
}
}
}
ty::RawPtr(_) => {
// Simple cases that are WF if their type args are WF.
}
ty::Alias(ty::Projection, data) => {
walker.skip_current_subtree(); // Subtree handled by compute_projection.
self.compute_projection(data);
}
ty::Alias(ty::Inherent, data) => {
walker.skip_current_subtree(); // Subtree handled by compute_inherent_projection.
self.compute_inherent_projection(data);
}
ty::Adt(def, args) => {
// WfNominalType
let obligations = self.nominal_obligations(def.did(), args);
self.out.extend(obligations);
}
ty::FnDef(did, args) => {
let obligations = self.nominal_obligations(did, args);
self.out.extend(obligations);
}
ty::Ref(r, rty, _) => {
// WfReference
if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
self.out.push(traits::Obligation::with_depth(
self.tcx(),
cause,
depth,
param_env,
ty::Binder::dummy(ty::PredicateKind::Clause(
ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(rty, r)),
)),
));
}
}
ty::Generator(did, args, ..) => {
// Walk ALL the types in the generator: this will
// include the upvar types as well as the yield
// type. Note that this is mildly distinct from
// the closure case, where we have to be careful
// about the signature of the closure. We don't
// have the problem of implied bounds here since
// generators don't take arguments.
let obligations = self.nominal_obligations(did, args);
self.out.extend(obligations);
}
ty::Closure(did, args) => {
// Only check the upvar types for WF, not the rest
// of the types within. This is needed because we
// capture the signature and it may not be WF
// without the implied bounds. Consider a closure
// like `|x: &'a T|` -- it may be that `T: 'a` is
// not known to hold in the creator's context (and
// indeed the closure may not be invoked by its
// creator, but rather turned to someone who *can*
// verify that).
//
// The special treatment of closures here really
// ought not to be necessary either; the problem
// is related to #25860 -- there is no way for us
// to express a fn type complete with the implied
// bounds that it is assuming. I think in reality
// the WF rules around fn are a bit messed up, and
// that is the rot problem: `fn(&'a T)` should
// probably always be WF, because it should be
// shorthand for something like `where(T: 'a) {
// fn(&'a T) }`, as discussed in #25860.
walker.skip_current_subtree(); // subtree handled below
// FIXME(eddyb) add the type to `walker` instead of recursing.
self.compute(args.as_closure().tupled_upvars_ty().into());
// Note that we cannot skip the generic types
// types. Normally, within the fn
// body where they are created, the generics will
// always be WF, and outside of that fn body we
// are not directly inspecting closure types
// anyway, except via auto trait matching (which
// only inspects the upvar types).
// But when a closure is part of a type-alias-impl-trait
// then the function that created the defining site may
// have had more bounds available than the type alias
// specifies. This may cause us to have a closure in the
// hidden type that is not actually well formed and
// can cause compiler crashes when the user abuses unsafe
// code to procure such a closure.
// See tests/ui/type-alias-impl-trait/wf_check_closures.rs
let obligations = self.nominal_obligations(did, args);
self.out.extend(obligations);
}
ty::FnPtr(_) => {
// let the loop iterate into the argument/return
// types appearing in the fn signature
}
ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => {
// All of the requirements on type parameters
// have already been checked for `impl Trait` in
// return position. We do need to check type-alias-impl-trait though.
if self.tcx().is_type_alias_impl_trait(def_id) {
let obligations = self.nominal_obligations(def_id, args);
self.out.extend(obligations);
}
}
ty::Alias(ty::Weak, ty::AliasTy { def_id, args, .. }) => {
let obligations = self.nominal_obligations(def_id, args);
self.out.extend(obligations);
}
ty::Dynamic(data, r, _) => {
// WfObject
//
// Here, we defer WF checking due to higher-ranked
// regions. This is perhaps not ideal.
self.from_object_ty(ty, data, r);
// FIXME(#27579) RFC also considers adding trait
// obligations that don't refer to Self and
// checking those
let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
if !defer_to_coercion {
let cause = self.cause(traits::WellFormed(None));
let component_traits = data.auto_traits().chain(data.principal_def_id());
let tcx = self.tcx();
self.out.extend(component_traits.map(|did| {
traits::Obligation::with_depth(
tcx,
cause.clone(),
depth,
param_env,
ty::Binder::dummy(ty::PredicateKind::ObjectSafe(did)),
)
}));
}
}
// Inference variables are the complicated case, since we don't
// know what type they are. We do two things:
//
// 1. Check if they have been resolved, and if so proceed with
// THAT type.
// 2. If not, we've at least simplified things (e.g., we went
// from `Vec<$0>: WF` to `$0: WF`), so we can
// register a pending obligation and keep
// moving. (Goal is that an "inductive hypothesis"
// is satisfied to ensure termination.)
// See also the comment on `fn obligations`, describing "livelock"
// prevention, which happens before this can be reached.
ty::Infer(_) => {
let cause = self.cause(traits::WellFormed(None));
self.out.push(traits::Obligation::with_depth(
self.tcx(),
cause,
self.recursion_depth,
param_env,
ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(
ty.into(),
))),
));
}
}
debug!(?self.out);
}
}
#[instrument(level = "debug", skip(self))]
fn nominal_obligations(
&mut self,
def_id: DefId,
args: GenericArgsRef<'tcx>,
) -> Vec<traits::PredicateObligation<'tcx>> {
let predicates = self.tcx().predicates_of(def_id);
let mut origins = vec![def_id; predicates.predicates.len()];
let mut head = predicates;
while let Some(parent) = head.parent {
head = self.tcx().predicates_of(parent);
origins.extend(iter::repeat(parent).take(head.predicates.len()));
}
let predicates = predicates.instantiate(self.tcx(), args);
trace!("{:#?}", predicates);
debug_assert_eq!(predicates.predicates.len(), origins.len());
iter::zip(predicates, origins.into_iter().rev())
.map(|((pred, span), origin_def_id)| {
let code = if span.is_dummy() {
traits::ItemObligation(origin_def_id)
} else {
traits::BindingObligation(origin_def_id, span)
};
let cause = self.cause(code);
traits::Obligation::with_depth(
self.tcx(),
cause,
self.recursion_depth,
self.param_env,
pred,
)
})
.filter(|pred| !pred.has_escaping_bound_vars())
.collect()
}
fn from_object_ty(
&mut self,
ty: Ty<'tcx>,
data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
region: ty::Region<'tcx>,
) {
// Imagine a type like this:
//
// trait Foo { }
// trait Bar<'c> : 'c { }
//
// &'b (Foo+'c+Bar<'d>)
// ^
//
// In this case, the following relationships must hold:
//
// 'b <= 'c
// 'd <= 'c
//
// The first conditions is due to the normal region pointer
// rules, which say that a reference cannot outlive its
// referent.
//
// The final condition may be a bit surprising. In particular,
// you may expect that it would have been `'c <= 'd`, since
// usually lifetimes of outer things are conservative
// approximations for inner things. However, it works somewhat
// differently with trait objects: here the idea is that if the
// user specifies a region bound (`'c`, in this case) it is the
// "master bound" that *implies* that bounds from other traits are
// all met. (Remember that *all bounds* in a type like
// `Foo+Bar+Zed` must be met, not just one, hence if we write
// `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
// 'y.)
//
// Note: in fact we only permit builtin traits, not `Bar<'d>`, I
// am looking forward to the future here.
if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
let implicit_bounds = object_region_bounds(self.tcx(), data);
let explicit_bound = region;
self.out.reserve(implicit_bounds.len());
for implicit_bound in implicit_bounds {
let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
let outlives =
ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
self.out.push(traits::Obligation::with_depth(
self.tcx(),
cause,
self.recursion_depth,
self.param_env,
outlives,
));
}
}
}
}
/// Given an object type like `SomeTrait + Send`, computes the lifetime
/// bounds that must hold on the elided self type. These are derived
/// from the declarations of `SomeTrait`, `Send`, and friends -- if
/// they declare `trait SomeTrait : 'static`, for example, then
/// `'static` would appear in the list. The hard work is done by
/// `infer::required_region_bounds`, see that for more information.
pub fn object_region_bounds<'tcx>(
tcx: TyCtxt<'tcx>,
existential_predicates: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
) -> Vec<ty::Region<'tcx>> {
// Since we don't actually *know* the self type for an object,
// this "open(err)" serves as a kind of dummy standin -- basically
// a placeholder type.
let open_ty = Ty::new_fresh(tcx, 0);
let predicates = existential_predicates.iter().filter_map(|predicate| {
if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
None
} else {
Some(predicate.with_self_ty(tcx, open_ty))
}
});
required_region_bounds(tcx, open_ty, predicates)
}
/// Given a set of predicates that apply to an object type, returns
/// the region bounds that the (erased) `Self` type must
/// outlive. Precisely *because* the `Self` type is erased, the
/// parameter `erased_self_ty` must be supplied to indicate what type
/// has been used to represent `Self` in the predicates
/// themselves. This should really be a unique type; `FreshTy(0)` is a
/// popular choice.
///
/// N.B., in some cases, particularly around higher-ranked bounds,
/// this function returns a kind of conservative approximation.
/// That is, all regions returned by this function are definitely
/// required, but there may be other region bounds that are not
/// returned, as well as requirements like `for<'a> T: 'a`.
///
/// Requires that trait definitions have been processed so that we can
/// elaborate predicates and walk supertraits.
#[instrument(skip(tcx, predicates), level = "debug", ret)]
pub(crate) fn required_region_bounds<'tcx>(
tcx: TyCtxt<'tcx>,
erased_self_ty: Ty<'tcx>,
predicates: impl Iterator<Item = ty::Clause<'tcx>>,
) -> Vec<ty::Region<'tcx>> {
assert!(!erased_self_ty.has_escaping_bound_vars());
traits::elaborate(tcx, predicates)
.filter_map(|pred| {
debug!(?pred);
match pred.kind().skip_binder() {
ty::ClauseKind::TypeOutlives(ty::OutlivesPredicate(ref t, ref r)) => {
// Search for a bound of the form `erased_self_ty
// : 'a`, but be wary of something like `for<'a>
// erased_self_ty : 'a` (we interpret a
// higher-ranked bound like that as 'static,
// though at present the code in `fulfill.rs`
// considers such bounds to be unsatisfiable, so
// it's kind of a moot point since you could never
// construct such an object, but this seems
// correct even if that code changes).
if t == &erased_self_ty && !r.has_escaping_bound_vars() {
Some(*r)
} else {
None
}
}
ty::ClauseKind::Trait(_)
| ty::ClauseKind::RegionOutlives(_)
| ty::ClauseKind::Projection(_)
| ty::ClauseKind::ConstArgHasType(_, _)
| ty::ClauseKind::WellFormed(_)
| ty::ClauseKind::ConstEvaluatable(_) => None,
}
})
.collect()
}