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//! Code that handles "type-outlives" constraints like `T: 'a`. This
//! is based on the `push_outlives_components` function defined in rustc_infer,
//! but it adds a bit of heuristics on top, in particular to deal with
//! associated types and projections.
//!
//! When we process a given `T: 'a` obligation, we may produce two
//! kinds of constraints for the region inferencer:
//!
//! - Relationships between inference variables and other regions.
//! For example, if we have `&'?0 u32: 'a`, then we would produce
//! a constraint that `'a <= '?0`.
//! - "Verifys" that must be checked after inferencing is done.
//! For example, if we know that, for some type parameter `T`,
//! `T: 'a + 'b`, and we have a requirement that `T: '?1`,
//! then we add a "verify" that checks that `'?1 <= 'a || '?1 <= 'b`.
//! - Note the difference with the previous case: here, the region
//! variable must be less than something else, so this doesn't
//! affect how inference works (it finds the smallest region that
//! will do); it's just a post-condition that we have to check.
//!
//! **The key point is that once this function is done, we have
//! reduced all of our "type-region outlives" obligations into relationships
//! between individual regions.**
//!
//! One key input to this function is the set of "region-bound pairs".
//! These are basically the relationships between type parameters and
//! regions that are in scope at the point where the outlives
//! obligation was incurred. **When type-checking a function,
//! particularly in the face of closures, this is not known until
//! regionck runs!** This is because some of those bounds come
//! from things we have yet to infer.
//!
//! Consider:
//!
//! ```
//! fn bar<T>(a: T, b: impl for<'a> Fn(&'a T)) {}
//! fn foo<T>(x: T) {
//! bar(x, |y| { /* ... */})
//! // ^ closure arg
//! }
//! ```
//!
//! Here, the type of `y` may involve inference variables and the
//! like, and it may also contain implied bounds that are needed to
//! type-check the closure body (e.g., here it informs us that `T`
//! outlives the late-bound region `'a`).
//!
//! Note that by delaying the gathering of implied bounds until all
//! inference information is known, we may find relationships between
//! bound regions and other regions in the environment. For example,
//! when we first check a closure like the one expected as argument
//! to `foo`:
//!
//! ```
//! fn foo<U, F: for<'a> FnMut(&'a U)>(_f: F) {}
//! ```
//!
//! the type of the closure's first argument would be `&'a ?U`. We
//! might later infer `?U` to something like `&'b u32`, which would
//! imply that `'b: 'a`.
use crate::infer::outlives::components::{push_outlives_components, Component};
use crate::infer::outlives::env::OutlivesEnvironment;
use crate::infer::outlives::env::RegionBoundPairs;
use crate::infer::outlives::verify::VerifyBoundCx;
use crate::infer::{
self, GenericKind, InferCtxt, RegionObligation, SubregionOrigin, UndoLog, VerifyBound,
};
use crate::traits::{ObligationCause, ObligationCauseCode};
use rustc_data_structures::undo_log::UndoLogs;
use rustc_hir::def_id::LocalDefId;
use rustc_middle::ty::subst::GenericArgKind;
use rustc_middle::ty::{self, Region, Ty, TyCtxt, TypeVisitable};
use smallvec::smallvec;
impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
/// Registers that the given region obligation must be resolved
/// from within the scope of `body_id`. These regions are enqueued
/// and later processed by regionck, when full type information is
/// available (see `region_obligations` field for more
/// information).
#[instrument(level = "debug", skip(self))]
pub fn register_region_obligation(&self, obligation: RegionObligation<'tcx>) {
let mut inner = self.inner.borrow_mut();
inner.undo_log.push(UndoLog::PushRegionObligation);
inner.region_obligations.push(obligation);
}
pub fn register_region_obligation_with_cause(
&self,
sup_type: Ty<'tcx>,
sub_region: Region<'tcx>,
cause: &ObligationCause<'tcx>,
) {
debug!(?sup_type, ?sub_region, ?cause);
let origin = SubregionOrigin::from_obligation_cause(cause, || {
infer::RelateParamBound(
cause.span,
sup_type,
match cause.code().peel_derives() {
ObligationCauseCode::BindingObligation(_, span)
| ObligationCauseCode::ExprBindingObligation(_, span, ..) => Some(*span),
_ => None,
},
)
});
self.register_region_obligation(RegionObligation { sup_type, sub_region, origin });
}
/// Trait queries just want to pass back type obligations "as is"
pub fn take_registered_region_obligations(&self) -> Vec<RegionObligation<'tcx>> {
std::mem::take(&mut self.inner.borrow_mut().region_obligations)
}
/// NOTE: Prefer using [`InferCtxt::check_region_obligations_and_report_errors`]
/// instead of calling this directly.
///
/// Process the region obligations that must be proven (during
/// `regionck`) for the given `body_id`, given information about
/// the region bounds in scope and so forth. This function must be
/// invoked for all relevant body-ids before region inference is
/// done (or else an assert will fire).
///
/// See the `region_obligations` field of `InferCtxt` for some
/// comments about how this function fits into the overall expected
/// flow of the inferencer. The key point is that it is
/// invoked after all type-inference variables have been bound --
/// towards the end of regionck. This also ensures that the
/// region-bound-pairs are available (see comments above regarding
/// closures).
///
/// # Parameters
///
/// - `region_bound_pairs_map`: the set of region bounds implied by
/// the parameters and where-clauses. In particular, each pair
/// `('a, K)` in this list tells us that the bounds in scope
/// indicate that `K: 'a`, where `K` is either a generic
/// parameter like `T` or a projection like `T::Item`.
/// - `param_env` is the parameter environment for the enclosing function.
/// - `body_id` is the body-id whose region obligations are being
/// processed.
#[instrument(level = "debug", skip(self, region_bound_pairs))]
pub fn process_registered_region_obligations(
&self,
region_bound_pairs: &RegionBoundPairs<'tcx>,
param_env: ty::ParamEnv<'tcx>,
) {
assert!(
!self.in_snapshot.get(),
"cannot process registered region obligations in a snapshot"
);
let my_region_obligations = self.take_registered_region_obligations();
for RegionObligation { sup_type, sub_region, origin } in my_region_obligations {
debug!(
"process_registered_region_obligations: sup_type={:?} sub_region={:?} origin={:?}",
sup_type, sub_region, origin
);
let sup_type = self.resolve_vars_if_possible(sup_type);
let outlives =
&mut TypeOutlives::new(self, self.tcx, ®ion_bound_pairs, None, param_env);
outlives.type_must_outlive(origin, sup_type, sub_region);
}
}
/// Processes registered region obliations and resolves regions, reporting
/// any errors if any were raised. Prefer using this function over manually
/// calling `resolve_regions_and_report_errors`.
pub fn check_region_obligations_and_report_errors(
&self,
generic_param_scope: LocalDefId,
outlives_env: &OutlivesEnvironment<'tcx>,
) {
self.process_registered_region_obligations(
outlives_env.region_bound_pairs(),
outlives_env.param_env,
);
self.resolve_regions_and_report_errors(generic_param_scope, outlives_env)
}
}
/// The `TypeOutlives` struct has the job of "lowering" a `T: 'a`
/// obligation into a series of `'a: 'b` constraints and "verify"s, as
/// described on the module comment. The final constraints are emitted
/// via a "delegate" of type `D` -- this is usually the `infcx`, which
/// accrues them into the `region_obligations` code, but for NLL we
/// use something else.
pub struct TypeOutlives<'cx, 'tcx, D>
where
D: TypeOutlivesDelegate<'tcx>,
{
// See the comments on `process_registered_region_obligations` for the meaning
// of these fields.
delegate: D,
tcx: TyCtxt<'tcx>,
verify_bound: VerifyBoundCx<'cx, 'tcx>,
}
pub trait TypeOutlivesDelegate<'tcx> {
fn push_sub_region_constraint(
&mut self,
origin: SubregionOrigin<'tcx>,
a: ty::Region<'tcx>,
b: ty::Region<'tcx>,
);
fn push_verify(
&mut self,
origin: SubregionOrigin<'tcx>,
kind: GenericKind<'tcx>,
a: ty::Region<'tcx>,
bound: VerifyBound<'tcx>,
);
}
impl<'cx, 'tcx, D> TypeOutlives<'cx, 'tcx, D>
where
D: TypeOutlivesDelegate<'tcx>,
{
pub fn new(
delegate: D,
tcx: TyCtxt<'tcx>,
region_bound_pairs: &'cx RegionBoundPairs<'tcx>,
implicit_region_bound: Option<ty::Region<'tcx>>,
param_env: ty::ParamEnv<'tcx>,
) -> Self {
Self {
delegate,
tcx,
verify_bound: VerifyBoundCx::new(
tcx,
region_bound_pairs,
implicit_region_bound,
param_env,
),
}
}
/// Adds constraints to inference such that `T: 'a` holds (or
/// reports an error if it cannot).
///
/// # Parameters
///
/// - `origin`, the reason we need this constraint
/// - `ty`, the type `T`
/// - `region`, the region `'a`
#[instrument(level = "debug", skip(self))]
pub fn type_must_outlive(
&mut self,
origin: infer::SubregionOrigin<'tcx>,
ty: Ty<'tcx>,
region: ty::Region<'tcx>,
) {
assert!(!ty.has_escaping_bound_vars());
let mut components = smallvec![];
push_outlives_components(self.tcx, ty, &mut components);
self.components_must_outlive(origin, &components, region);
}
fn components_must_outlive(
&mut self,
origin: infer::SubregionOrigin<'tcx>,
components: &[Component<'tcx>],
region: ty::Region<'tcx>,
) {
for component in components.iter() {
let origin = origin.clone();
match component {
Component::Region(region1) => {
self.delegate.push_sub_region_constraint(origin, region, *region1);
}
Component::Param(param_ty) => {
self.param_ty_must_outlive(origin, region, *param_ty);
}
Component::Projection(projection_ty) => {
self.projection_must_outlive(origin, region, *projection_ty);
}
Component::EscapingProjection(subcomponents) => {
self.components_must_outlive(origin, &subcomponents, region);
}
Component::UnresolvedInferenceVariable(v) => {
// ignore this, we presume it will yield an error
// later, since if a type variable is not resolved by
// this point it never will be
self.tcx.sess.delay_span_bug(
origin.span(),
&format!("unresolved inference variable in outlives: {:?}", v),
);
}
}
}
}
fn param_ty_must_outlive(
&mut self,
origin: infer::SubregionOrigin<'tcx>,
region: ty::Region<'tcx>,
param_ty: ty::ParamTy,
) {
debug!(
"param_ty_must_outlive(region={:?}, param_ty={:?}, origin={:?})",
region, param_ty, origin
);
let generic = GenericKind::Param(param_ty);
let verify_bound = self.verify_bound.generic_bound(generic);
self.delegate.push_verify(origin, generic, region, verify_bound);
}
#[instrument(level = "debug", skip(self))]
fn projection_must_outlive(
&mut self,
origin: infer::SubregionOrigin<'tcx>,
region: ty::Region<'tcx>,
projection_ty: ty::ProjectionTy<'tcx>,
) {
// This case is thorny for inference. The fundamental problem is
// that there are many cases where we have choice, and inference
// doesn't like choice (the current region inference in
// particular). :) First off, we have to choose between using the
// OutlivesProjectionEnv, OutlivesProjectionTraitDef, and
// OutlivesProjectionComponent rules, any one of which is
// sufficient. If there are no inference variables involved, it's
// not hard to pick the right rule, but if there are, we're in a
// bit of a catch 22: if we picked which rule we were going to
// use, we could add constraints to the region inference graph
// that make it apply, but if we don't add those constraints, the
// rule might not apply (but another rule might). For now, we err
// on the side of adding too few edges into the graph.
// Compute the bounds we can derive from the trait definition.
// These are guaranteed to apply, no matter the inference
// results.
let trait_bounds: Vec<_> =
self.verify_bound.projection_declared_bounds_from_trait(projection_ty).collect();
debug!(?trait_bounds);
// Compute the bounds we can derive from the environment. This
// is an "approximate" match -- in some cases, these bounds
// may not apply.
let mut approx_env_bounds =
self.verify_bound.projection_approx_declared_bounds_from_env(projection_ty);
debug!("projection_must_outlive: approx_env_bounds={:?}", approx_env_bounds);
// Remove outlives bounds that we get from the environment but
// which are also deducible from the trait. This arises (cc
// #55756) in cases where you have e.g., `<T as Foo<'a>>::Item:
// 'a` in the environment but `trait Foo<'b> { type Item: 'b
// }` in the trait definition.
approx_env_bounds.retain(|bound_outlives| {
// OK to skip binder because we only manipulate and compare against other
// values from the same binder. e.g. if we have (e.g.) `for<'a> <T as Trait<'a>>::Item: 'a`
// in `bound`, the `'a` will be a `^1` (bound, debruijn index == innermost) region.
// If the declaration is `trait Trait<'b> { type Item: 'b; }`, then `projection_declared_bounds_from_trait`
// will be invoked with `['b => ^1]` and so we will get `^1` returned.
let bound = bound_outlives.skip_binder();
match *bound.0.kind() {
ty::Projection(projection_ty) => self
.verify_bound
.projection_declared_bounds_from_trait(projection_ty)
.all(|r| r != bound.1),
_ => panic!("expected only projection types from env, not {:?}", bound.0),
}
});
// If declared bounds list is empty, the only applicable rule is
// OutlivesProjectionComponent. If there are inference variables,
// then, we can break down the outlives into more primitive
// components without adding unnecessary edges.
//
// If there are *no* inference variables, however, we COULD do
// this, but we choose not to, because the error messages are less
// good. For example, a requirement like `T::Item: 'r` would be
// translated to a requirement that `T: 'r`; when this is reported
// to the user, it will thus say "T: 'r must hold so that T::Item:
// 'r holds". But that makes it sound like the only way to fix
// the problem is to add `T: 'r`, which isn't true. So, if there are no
// inference variables, we use a verify constraint instead of adding
// edges, which winds up enforcing the same condition.
let needs_infer = projection_ty.needs_infer();
if approx_env_bounds.is_empty() && trait_bounds.is_empty() && needs_infer {
debug!("projection_must_outlive: no declared bounds");
for k in projection_ty.substs {
match k.unpack() {
GenericArgKind::Lifetime(lt) => {
self.delegate.push_sub_region_constraint(origin.clone(), region, lt);
}
GenericArgKind::Type(ty) => {
self.type_must_outlive(origin.clone(), ty, region);
}
GenericArgKind::Const(_) => {
// Const parameters don't impose constraints.
}
}
}
return;
}
// If we found a unique bound `'b` from the trait, and we
// found nothing else from the environment, then the best
// action is to require that `'b: 'r`, so do that.
//
// This is best no matter what rule we use:
//
// - OutlivesProjectionEnv: these would translate to the requirement that `'b:'r`
// - OutlivesProjectionTraitDef: these would translate to the requirement that `'b:'r`
// - OutlivesProjectionComponent: this would require `'b:'r`
// in addition to other conditions
if !trait_bounds.is_empty()
&& trait_bounds[1..]
.iter()
.map(|r| Some(*r))
.chain(
// NB: The environment may contain `for<'a> T: 'a` style bounds.
// In that case, we don't know if they are equal to the trait bound
// or not (since we don't *know* whether the environment bound even applies),
// so just map to `None` here if there are bound vars, ensuring that
// the call to `all` will fail below.
approx_env_bounds.iter().map(|b| b.map_bound(|b| b.1).no_bound_vars()),
)
.all(|b| b == Some(trait_bounds[0]))
{
let unique_bound = trait_bounds[0];
debug!("projection_must_outlive: unique trait bound = {:?}", unique_bound);
debug!("projection_must_outlive: unique declared bound appears in trait ref");
self.delegate.push_sub_region_constraint(origin, region, unique_bound);
return;
}
// Fallback to verifying after the fact that there exists a
// declared bound, or that all the components appearing in the
// projection outlive; in some cases, this may add insufficient
// edges into the inference graph, leading to inference failures
// even though a satisfactory solution exists.
let generic = GenericKind::Projection(projection_ty);
let verify_bound = self.verify_bound.generic_bound(generic);
debug!("projection_must_outlive: pushing {:?}", verify_bound);
self.delegate.push_verify(origin, generic, region, verify_bound);
}
}
impl<'cx, 'tcx> TypeOutlivesDelegate<'tcx> for &'cx InferCtxt<'cx, 'tcx> {
fn push_sub_region_constraint(
&mut self,
origin: SubregionOrigin<'tcx>,
a: ty::Region<'tcx>,
b: ty::Region<'tcx>,
) {
self.sub_regions(origin, a, b)
}
fn push_verify(
&mut self,
origin: SubregionOrigin<'tcx>,
kind: GenericKind<'tcx>,
a: ty::Region<'tcx>,
bound: VerifyBound<'tcx>,
) {
self.verify_generic_bound(origin, kind, a, bound)
}
}