rustc_trait_selection/traits/auto_trait.rs
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//! Support code for rustdoc and external tools.
//! You really don't want to be using this unless you need to.
use std::collections::VecDeque;
use std::iter;
use rustc_data_structures::fx::{FxIndexMap, FxIndexSet, IndexEntry};
use rustc_data_structures::unord::UnordSet;
use rustc_infer::infer::DefineOpaqueTypes;
use rustc_middle::ty::{Region, RegionVid};
use tracing::debug;
use super::*;
use crate::errors::UnableToConstructConstantValue;
use crate::infer::region_constraints::{Constraint, RegionConstraintData};
use crate::traits::project::ProjectAndUnifyResult;
// FIXME(twk): this is obviously not nice to duplicate like that
#[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
pub enum RegionTarget<'tcx> {
Region(Region<'tcx>),
RegionVid(RegionVid),
}
#[derive(Default, Debug, Clone)]
pub struct RegionDeps<'tcx> {
pub larger: FxIndexSet<RegionTarget<'tcx>>,
pub smaller: FxIndexSet<RegionTarget<'tcx>>,
}
pub enum AutoTraitResult<A> {
ExplicitImpl,
PositiveImpl(A),
NegativeImpl,
}
pub struct AutoTraitInfo<'cx> {
pub full_user_env: ty::ParamEnv<'cx>,
pub region_data: RegionConstraintData<'cx>,
pub vid_to_region: FxIndexMap<ty::RegionVid, ty::Region<'cx>>,
}
pub struct AutoTraitFinder<'tcx> {
tcx: TyCtxt<'tcx>,
}
impl<'tcx> AutoTraitFinder<'tcx> {
pub fn new(tcx: TyCtxt<'tcx>) -> Self {
AutoTraitFinder { tcx }
}
/// Makes a best effort to determine whether and under which conditions an auto trait is
/// implemented for a type. For example, if you have
///
/// ```
/// struct Foo<T> { data: Box<T> }
/// ```
///
/// then this might return that `Foo<T>: Send` if `T: Send` (encoded in the AutoTraitResult
/// type). The analysis attempts to account for custom impls as well as other complex cases.
/// This result is intended for use by rustdoc and other such consumers.
///
/// (Note that due to the coinductive nature of Send, the full and correct result is actually
/// quite simple to generate. That is, when a type has no custom impl, it is Send iff its field
/// types are all Send. So, in our example, we might have that `Foo<T>: Send` if `Box<T>: Send`.
/// But this is often not the best way to present to the user.)
///
/// Warning: The API should be considered highly unstable, and it may be refactored or removed
/// in the future.
pub fn find_auto_trait_generics<A>(
&self,
ty: Ty<'tcx>,
typing_env: ty::TypingEnv<'tcx>,
trait_did: DefId,
mut auto_trait_callback: impl FnMut(AutoTraitInfo<'tcx>) -> A,
) -> AutoTraitResult<A> {
let tcx = self.tcx;
let trait_ref = ty::TraitRef::new(tcx, trait_did, [ty]);
let (infcx, orig_env) = tcx.infer_ctxt().build_with_typing_env(typing_env);
let mut selcx = SelectionContext::new(&infcx);
for polarity in [ty::PredicatePolarity::Positive, ty::PredicatePolarity::Negative] {
let result = selcx.select(&Obligation::new(
tcx,
ObligationCause::dummy(),
orig_env,
ty::TraitPredicate { trait_ref, polarity },
));
if let Ok(Some(ImplSource::UserDefined(_))) = result {
debug!("find_auto_trait_generics({trait_ref:?}): manual impl found, bailing out");
// If an explicit impl exists, it always takes priority over an auto impl
return AutoTraitResult::ExplicitImpl;
}
}
let (infcx, orig_env) = tcx.infer_ctxt().build_with_typing_env(typing_env);
let mut fresh_preds = FxIndexSet::default();
// Due to the way projections are handled by SelectionContext, we need to run
// evaluate_predicates twice: once on the original param env, and once on the result of
// the first evaluate_predicates call.
//
// The problem is this: most of rustc, including SelectionContext and traits::project,
// are designed to work with a concrete usage of a type (e.g., Vec<u8>
// fn<T>() { Vec<T> }. This information will generally never change - given
// the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
// If we're unable to prove that 'T' implements a particular trait, we're done -
// there's nothing left to do but error out.
//
// However, synthesizing an auto trait impl works differently. Here, we start out with
// a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
// with - and progressively discover the conditions we need to fulfill for it to
// implement a certain auto trait. This ends up breaking two assumptions made by trait
// selection and projection:
//
// * We can always cache the result of a particular trait selection for the lifetime of
// an InfCtxt
// * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
// SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
//
// We fix the first assumption by manually clearing out all of the InferCtxt's caches
// in between calls to SelectionContext.select. This allows us to keep all of the
// intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
// them between calls.
//
// We fix the second assumption by reprocessing the result of our first call to
// evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
// pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
// traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
// SelectionContext to return it back to us.
let Some((new_env, user_env)) =
self.evaluate_predicates(&infcx, trait_did, ty, orig_env, orig_env, &mut fresh_preds)
else {
return AutoTraitResult::NegativeImpl;
};
let (full_env, full_user_env) = self
.evaluate_predicates(&infcx, trait_did, ty, new_env, user_env, &mut fresh_preds)
.unwrap_or_else(|| {
panic!("Failed to fully process: {ty:?} {trait_did:?} {orig_env:?}")
});
debug!(
"find_auto_trait_generics({:?}): fulfilling \
with {:?}",
trait_ref, full_env
);
// At this point, we already have all of the bounds we need. FulfillmentContext is used
// to store all of the necessary region/lifetime bounds in the InferContext, as well as
// an additional sanity check.
let ocx = ObligationCtxt::new(&infcx);
ocx.register_bound(ObligationCause::dummy(), full_env, ty, trait_did);
let errors = ocx.select_all_or_error();
if !errors.is_empty() {
panic!("Unable to fulfill trait {trait_did:?} for '{ty:?}': {errors:?}");
}
let outlives_env = OutlivesEnvironment::new(full_env);
let _ = infcx.process_registered_region_obligations(&outlives_env, |ty, _| Ok(ty));
let region_data =
infcx.inner.borrow_mut().unwrap_region_constraints().region_constraint_data().clone();
let vid_to_region = self.map_vid_to_region(®ion_data);
let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
AutoTraitResult::PositiveImpl(auto_trait_callback(info))
}
/// The core logic responsible for computing the bounds for our synthesized impl.
///
/// To calculate the bounds, we call `SelectionContext.select` in a loop. Like
/// `FulfillmentContext`, we recursively select the nested obligations of predicates we
/// encounter. However, whenever we encounter an `UnimplementedError` involving a type
/// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular
/// type implements an auto trait, Unimplemented errors tell us what conditions need to be met.
///
/// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key
/// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete
/// user code. According, it considers all possible ways that a `Predicate` could be met, which
/// isn't always what we want for a synthesized impl. For example, given the predicate `T:
/// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T:
/// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`,
/// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up.
/// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl
/// like this:
/// ```ignore (illustrative)
/// impl<T> Send for Foo<T> where T: IntoIterator
/// ```
/// While it might be technically true that Foo implements Send where `T: IntoIterator`,
/// the bound is overly restrictive - it's really only necessary that `T: Iterator`.
///
/// For this reason, `evaluate_predicates` handles predicates with type variables specially.
/// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately
/// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later
/// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator`
/// needs to hold.
///
/// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever
/// constructed once for a given type. As part of the construction process, the `ParamEnv` will
/// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
/// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
/// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate`, or
/// else `SelectionContext` will choke on the missing predicates. However, this should never
/// show up in the final synthesized generics: we don't want our generated docs page to contain
/// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
/// separate `user_env`, which only holds the predicates that will actually be displayed to the
/// user.
fn evaluate_predicates(
&self,
infcx: &InferCtxt<'tcx>,
trait_did: DefId,
ty: Ty<'tcx>,
param_env: ty::ParamEnv<'tcx>,
user_env: ty::ParamEnv<'tcx>,
fresh_preds: &mut FxIndexSet<ty::Predicate<'tcx>>,
) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
let tcx = infcx.tcx;
// Don't try to process any nested obligations involving predicates
// that are already in the `ParamEnv` (modulo regions): we already
// know that they must hold.
for predicate in param_env.caller_bounds() {
fresh_preds.insert(self.clean_pred(infcx, predicate.as_predicate()));
}
let mut select = SelectionContext::new(infcx);
let mut already_visited = UnordSet::new();
let mut predicates = VecDeque::new();
predicates.push_back(ty::Binder::dummy(ty::TraitPredicate {
trait_ref: ty::TraitRef::new(infcx.tcx, trait_did, [ty]),
// Auto traits are positive
polarity: ty::PredicatePolarity::Positive,
}));
let computed_preds = param_env.caller_bounds().iter().map(|c| c.as_predicate());
let mut user_computed_preds: FxIndexSet<_> =
user_env.caller_bounds().iter().map(|c| c.as_predicate()).collect();
let mut new_env = param_env;
let dummy_cause = ObligationCause::dummy();
while let Some(pred) = predicates.pop_front() {
if !already_visited.insert(pred) {
continue;
}
// Call `infcx.resolve_vars_if_possible` to see if we can
// get rid of any inference variables.
let obligation = infcx.resolve_vars_if_possible(Obligation::new(
tcx,
dummy_cause.clone(),
new_env,
pred,
));
let result = select.poly_select(&obligation);
match result {
Ok(Some(ref impl_source)) => {
// If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
// we immediately bail out, since it's impossible for us to continue.
if let ImplSource::UserDefined(ImplSourceUserDefinedData {
impl_def_id, ..
}) = impl_source
{
// Blame 'tidy' for the weird bracket placement.
if infcx.tcx.impl_polarity(*impl_def_id) != ty::ImplPolarity::Positive {
debug!(
"evaluate_nested_obligations: found explicit negative impl\
{:?}, bailing out",
impl_def_id
);
return None;
}
}
let obligations = impl_source.borrow_nested_obligations().iter().cloned();
if !self.evaluate_nested_obligations(
ty,
obligations,
&mut user_computed_preds,
fresh_preds,
&mut predicates,
&mut select,
) {
return None;
}
}
Ok(None) => {}
Err(SelectionError::Unimplemented) => {
if self.is_param_no_infer(pred.skip_binder().trait_ref.args) {
already_visited.remove(&pred);
self.add_user_pred(&mut user_computed_preds, pred.upcast(self.tcx));
predicates.push_back(pred);
} else {
debug!(
"evaluate_nested_obligations: `Unimplemented` found, bailing: \
{:?} {:?} {:?}",
ty,
pred,
pred.skip_binder().trait_ref.args
);
return None;
}
}
_ => panic!("Unexpected error for '{ty:?}': {result:?}"),
};
let normalized_preds =
elaborate(tcx, computed_preds.clone().chain(user_computed_preds.iter().cloned()));
new_env = ty::ParamEnv::new(
tcx.mk_clauses_from_iter(normalized_preds.filter_map(|p| p.as_clause())),
param_env.reveal(),
);
}
let final_user_env = ty::ParamEnv::new(
tcx.mk_clauses_from_iter(user_computed_preds.into_iter().filter_map(|p| p.as_clause())),
user_env.reveal(),
);
debug!(
"evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
'{:?}'",
ty, trait_did, new_env, final_user_env
);
Some((new_env, final_user_env))
}
/// This method is designed to work around the following issue:
/// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
/// progressively building a `ParamEnv` based on the results we get.
/// However, our usage of `SelectionContext` differs from its normal use within the compiler,
/// in that we capture and re-reprocess predicates from `Unimplemented` errors.
///
/// This can lead to a corner case when dealing with region parameters.
/// During our selection loop in `evaluate_predicates`, we might end up with
/// two trait predicates that differ only in their region parameters:
/// one containing a HRTB lifetime parameter, and one containing a 'normal'
/// lifetime parameter. For example:
/// ```ignore (illustrative)
/// T as MyTrait<'a>
/// T as MyTrait<'static>
/// ```
/// If we put both of these predicates in our computed `ParamEnv`, we'll
/// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
///
/// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
/// Our end goal is to generate a user-visible description of the conditions
/// under which a type implements an auto trait. A trait predicate involving
/// a HRTB means that the type needs to work with any choice of lifetime,
/// not just one specific lifetime (e.g., `'static`).
fn add_user_pred(
&self,
user_computed_preds: &mut FxIndexSet<ty::Predicate<'tcx>>,
new_pred: ty::Predicate<'tcx>,
) {
let mut should_add_new = true;
user_computed_preds.retain(|&old_pred| {
if let (
ty::PredicateKind::Clause(ty::ClauseKind::Trait(new_trait)),
ty::PredicateKind::Clause(ty::ClauseKind::Trait(old_trait)),
) = (new_pred.kind().skip_binder(), old_pred.kind().skip_binder())
{
if new_trait.def_id() == old_trait.def_id() {
let new_args = new_trait.trait_ref.args;
let old_args = old_trait.trait_ref.args;
if !new_args.types().eq(old_args.types()) {
// We can't compare lifetimes if the types are different,
// so skip checking `old_pred`.
return true;
}
for (new_region, old_region) in
iter::zip(new_args.regions(), old_args.regions())
{
match (*new_region, *old_region) {
// If both predicates have an `ReBound` (a HRTB) in the
// same spot, we do nothing.
(ty::ReBound(_, _), ty::ReBound(_, _)) => {}
(ty::ReBound(_, _), _) | (_, ty::ReVar(_)) => {
// One of these is true:
// The new predicate has a HRTB in a spot where the old
// predicate does not (if they both had a HRTB, the previous
// match arm would have executed). A HRBT is a 'stricter'
// bound than anything else, so we want to keep the newer
// predicate (with the HRBT) in place of the old predicate.
//
// OR
//
// The old predicate has a region variable where the new
// predicate has some other kind of region. An region
// variable isn't something we can actually display to a user,
// so we choose their new predicate (which doesn't have a region
// variable).
//
// In both cases, we want to remove the old predicate,
// from `user_computed_preds`, and replace it with the new
// one. Having both the old and the new
// predicate in a `ParamEnv` would confuse `SelectionContext`.
//
// We're currently in the predicate passed to 'retain',
// so we return `false` to remove the old predicate from
// `user_computed_preds`.
return false;
}
(_, ty::ReBound(_, _)) | (ty::ReVar(_), _) => {
// This is the opposite situation as the previous arm.
// One of these is true:
//
// The old predicate has a HRTB lifetime in a place where the
// new predicate does not.
//
// OR
//
// The new predicate has a region variable where the old
// predicate has some other type of region.
//
// We want to leave the old
// predicate in `user_computed_preds`, and skip adding
// new_pred to `user_computed_params`.
should_add_new = false
}
_ => {}
}
}
}
}
true
});
if should_add_new {
user_computed_preds.insert(new_pred);
}
}
/// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
/// to each other, we match `ty::RegionVid`s to `ty::Region`s.
fn map_vid_to_region<'cx>(
&self,
regions: &RegionConstraintData<'cx>,
) -> FxIndexMap<ty::RegionVid, ty::Region<'cx>> {
let mut vid_map = FxIndexMap::<RegionTarget<'cx>, RegionDeps<'cx>>::default();
let mut finished_map = FxIndexMap::default();
for (constraint, _) in ®ions.constraints {
match constraint {
&Constraint::VarSubVar(r1, r2) => {
{
let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
deps1.larger.insert(RegionTarget::RegionVid(r2));
}
let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
deps2.smaller.insert(RegionTarget::RegionVid(r1));
}
&Constraint::RegSubVar(region, vid) => {
{
let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
deps1.larger.insert(RegionTarget::RegionVid(vid));
}
let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
deps2.smaller.insert(RegionTarget::Region(region));
}
&Constraint::VarSubReg(vid, region) => {
finished_map.insert(vid, region);
}
&Constraint::RegSubReg(r1, r2) => {
{
let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
deps1.larger.insert(RegionTarget::Region(r2));
}
let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
deps2.smaller.insert(RegionTarget::Region(r1));
}
}
}
while !vid_map.is_empty() {
let target = *vid_map.keys().next().unwrap();
let deps = vid_map.swap_remove(&target).unwrap();
for smaller in deps.smaller.iter() {
for larger in deps.larger.iter() {
match (smaller, larger) {
(&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
if let IndexEntry::Occupied(v) = vid_map.entry(*smaller) {
let smaller_deps = v.into_mut();
smaller_deps.larger.insert(*larger);
smaller_deps.larger.swap_remove(&target);
}
if let IndexEntry::Occupied(v) = vid_map.entry(*larger) {
let larger_deps = v.into_mut();
larger_deps.smaller.insert(*smaller);
larger_deps.smaller.swap_remove(&target);
}
}
(&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
finished_map.insert(v1, r1);
}
(&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
// Do nothing; we don't care about regions that are smaller than vids.
}
(&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
if let IndexEntry::Occupied(v) = vid_map.entry(*smaller) {
let smaller_deps = v.into_mut();
smaller_deps.larger.insert(*larger);
smaller_deps.larger.swap_remove(&target);
}
if let IndexEntry::Occupied(v) = vid_map.entry(*larger) {
let larger_deps = v.into_mut();
larger_deps.smaller.insert(*smaller);
larger_deps.smaller.swap_remove(&target);
}
}
}
}
}
}
finished_map
}
fn is_param_no_infer(&self, args: GenericArgsRef<'tcx>) -> bool {
self.is_of_param(args.type_at(0)) && !args.types().any(|t| t.has_infer_types())
}
pub fn is_of_param(&self, ty: Ty<'tcx>) -> bool {
match ty.kind() {
ty::Param(_) => true,
ty::Alias(ty::Projection, p) => self.is_of_param(p.self_ty()),
_ => false,
}
}
fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'tcx>) -> bool {
if let Some(ty) = p.term().skip_binder().as_type() {
matches!(ty.kind(), ty::Alias(ty::Projection, proj) if proj == &p.skip_binder().projection_term.expect_ty(self.tcx))
} else {
false
}
}
fn evaluate_nested_obligations(
&self,
ty: Ty<'_>,
nested: impl Iterator<Item = PredicateObligation<'tcx>>,
computed_preds: &mut FxIndexSet<ty::Predicate<'tcx>>,
fresh_preds: &mut FxIndexSet<ty::Predicate<'tcx>>,
predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,
selcx: &mut SelectionContext<'_, 'tcx>,
) -> bool {
let dummy_cause = ObligationCause::dummy();
for obligation in nested {
let is_new_pred =
fresh_preds.insert(self.clean_pred(selcx.infcx, obligation.predicate));
// Resolve any inference variables that we can, to help selection succeed
let predicate = selcx.infcx.resolve_vars_if_possible(obligation.predicate);
// We only add a predicate as a user-displayable bound if
// it involves a generic parameter, and doesn't contain
// any inference variables.
//
// Displaying a bound involving a concrete type (instead of a generic
// parameter) would be pointless, since it's always true
// (e.g. u8: Copy)
// Displaying an inference variable is impossible, since they're
// an internal compiler detail without a defined visual representation
//
// We check this by calling is_of_param on the relevant types
// from the various possible predicates
let bound_predicate = predicate.kind();
match bound_predicate.skip_binder() {
ty::PredicateKind::Clause(ty::ClauseKind::Trait(p)) => {
// Add this to `predicates` so that we end up calling `select`
// with it. If this predicate ends up being unimplemented,
// then `evaluate_predicates` will handle adding it the `ParamEnv`
// if possible.
predicates.push_back(bound_predicate.rebind(p));
}
ty::PredicateKind::Clause(ty::ClauseKind::Projection(p)) => {
let p = bound_predicate.rebind(p);
debug!(
"evaluate_nested_obligations: examining projection predicate {:?}",
predicate
);
// As described above, we only want to display
// bounds which include a generic parameter but don't include
// an inference variable.
// Additionally, we check if we've seen this predicate before,
// to avoid rendering duplicate bounds to the user.
if self.is_param_no_infer(p.skip_binder().projection_term.args)
&& !p.term().skip_binder().has_infer_types()
&& is_new_pred
{
debug!(
"evaluate_nested_obligations: adding projection predicate \
to computed_preds: {:?}",
predicate
);
// Under unusual circumstances, we can end up with a self-referential
// projection predicate. For example:
// <T as MyType>::Value == <T as MyType>::Value
// Not only is displaying this to the user pointless,
// having it in the ParamEnv will cause an issue if we try to call
// poly_project_and_unify_type on the predicate, since this kind of
// predicate will normally never end up in a ParamEnv.
//
// For these reasons, we ignore these weird predicates,
// ensuring that we're able to properly synthesize an auto trait impl
if self.is_self_referential_projection(p) {
debug!(
"evaluate_nested_obligations: encountered a projection
predicate equating a type with itself! Skipping"
);
} else {
self.add_user_pred(computed_preds, predicate);
}
}
// There are three possible cases when we project a predicate:
//
// 1. We encounter an error. This means that it's impossible for
// our current type to implement the auto trait - there's bound
// that we could add to our ParamEnv that would 'fix' this kind
// of error, as it's not caused by an unimplemented type.
//
// 2. We successfully project the predicate (Ok(Some(_))), generating
// some subobligations. We then process these subobligations
// like any other generated sub-obligations.
//
// 3. We receive an 'ambiguous' result (Ok(None))
// If we were actually trying to compile a crate,
// we would need to re-process this obligation later.
// However, all we care about is finding out what bounds
// are needed for our type to implement a particular auto trait.
// We've already added this obligation to our computed ParamEnv
// above (if it was necessary). Therefore, we don't need
// to do any further processing of the obligation.
//
// Note that we *must* try to project *all* projection predicates
// we encounter, even ones without inference variable.
// This ensures that we detect any projection errors,
// which indicate that our type can *never* implement the given
// auto trait. In that case, we will generate an explicit negative
// impl (e.g. 'impl !Send for MyType'). However, we don't
// try to process any of the generated subobligations -
// they contain no new information, since we already know
// that our type implements the projected-through trait,
// and can lead to weird region issues.
//
// Normally, we'll generate a negative impl as a result of encountering
// a type with an explicit negative impl of an auto trait
// (for example, raw pointers have !Send and !Sync impls)
// However, through some **interesting** manipulations of the type
// system, it's actually possible to write a type that never
// implements an auto trait due to a projection error, not a normal
// negative impl error. To properly handle this case, we need
// to ensure that we catch any potential projection errors,
// and turn them into an explicit negative impl for our type.
debug!("Projecting and unifying projection predicate {:?}", predicate);
match project::poly_project_and_unify_term(selcx, &obligation.with(self.tcx, p))
{
ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
debug!(
"evaluate_nested_obligations: Unable to unify predicate \
'{:?}' '{:?}', bailing out",
ty, e
);
return false;
}
ProjectAndUnifyResult::Recursive => {
debug!("evaluate_nested_obligations: recursive projection predicate");
return false;
}
ProjectAndUnifyResult::Holds(v) => {
// We only care about sub-obligations
// when we started out trying to unify
// some inference variables. See the comment above
// for more information
if p.term().skip_binder().has_infer_types() {
if !self.evaluate_nested_obligations(
ty,
v.into_iter(),
computed_preds,
fresh_preds,
predicates,
selcx,
) {
return false;
}
}
}
ProjectAndUnifyResult::FailedNormalization => {
// It's ok not to make progress when have no inference variables -
// in that case, we were only performing unification to check if an
// error occurred (which would indicate that it's impossible for our
// type to implement the auto trait).
// However, we should always make progress (either by generating
// subobligations or getting an error) when we started off with
// inference variables
if p.term().skip_binder().has_infer_types() {
panic!("Unexpected result when selecting {ty:?} {obligation:?}")
}
}
}
}
ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(binder)) => {
let binder = bound_predicate.rebind(binder);
selcx.infcx.region_outlives_predicate(&dummy_cause, binder)
}
ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(binder)) => {
let binder = bound_predicate.rebind(binder);
match (
binder.no_bound_vars(),
binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
) {
(None, Some(t_a)) => {
selcx.infcx.register_region_obligation_with_cause(
t_a,
selcx.infcx.tcx.lifetimes.re_static,
&dummy_cause,
);
}
(Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
selcx.infcx.register_region_obligation_with_cause(
t_a,
r_b,
&dummy_cause,
);
}
_ => {}
};
}
ty::PredicateKind::ConstEquate(c1, c2) => {
let evaluate = |c: ty::Const<'tcx>| {
if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
let ct = super::try_evaluate_const(
selcx.infcx,
c,
obligation.param_env,
);
if let Err(EvaluateConstErr::InvalidConstParamTy(_)) = ct {
self.tcx.dcx().emit_err(UnableToConstructConstantValue {
span: self.tcx.def_span(unevaluated.def),
unevaluated,
});
}
ct
} else {
Ok(c)
}
};
match (evaluate(c1), evaluate(c2)) {
(Ok(c1), Ok(c2)) => {
match selcx.infcx.at(&obligation.cause, obligation.param_env).eq(DefineOpaqueTypes::Yes,c1, c2)
{
Ok(_) => (),
Err(_) => return false,
}
}
_ => return false,
}
}
// There's not really much we can do with these predicates -
// we start out with a `ParamEnv` with no inference variables,
// and these don't correspond to adding any new bounds to
// the `ParamEnv`.
ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(..))
| ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(..))
| ty::PredicateKind::NormalizesTo(..)
| ty::PredicateKind::AliasRelate(..)
| ty::PredicateKind::DynCompatible(..)
| ty::PredicateKind::Subtype(..)
// FIXME(generic_const_exprs): you can absolutely add this as a where clauses
| ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(..))
| ty::PredicateKind::Coerce(..)
| ty::PredicateKind::Clause(ty::ClauseKind::HostEffect(..)) => {}
ty::PredicateKind::Ambiguous => return false,
};
}
true
}
pub fn clean_pred(
&self,
infcx: &InferCtxt<'tcx>,
p: ty::Predicate<'tcx>,
) -> ty::Predicate<'tcx> {
infcx.freshen(p)
}
}