/
implicator.rs
447 lines (395 loc) · 16.8 KB
/
implicator.rs
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// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
// #![warn(deprecated_mode)]
use middle::infer::{InferCtxt, GenericKind};
use middle::subst::Substs;
use middle::traits;
use middle::ty::{self, RegionEscape, ToPolyTraitRef, ToPredicate, Ty};
use middle::ty_fold::{TypeFoldable, TypeFolder};
use syntax::ast;
use syntax::codemap::Span;
use util::common::ErrorReported;
use util::nodemap::FnvHashSet;
// Helper functions related to manipulating region types.
#[derive(Debug)]
pub enum Implication<'tcx> {
RegionSubRegion(Option<Ty<'tcx>>, ty::Region, ty::Region),
RegionSubGeneric(Option<Ty<'tcx>>, ty::Region, GenericKind<'tcx>),
RegionSubClosure(Option<Ty<'tcx>>, ty::Region, ast::DefId, &'tcx Substs<'tcx>),
Predicate(ast::DefId, ty::Predicate<'tcx>),
}
struct Implicator<'a, 'tcx: 'a> {
infcx: &'a InferCtxt<'a,'tcx>,
body_id: ast::NodeId,
stack: Vec<(ty::Region, Option<Ty<'tcx>>)>,
span: Span,
out: Vec<Implication<'tcx>>,
visited: FnvHashSet<Ty<'tcx>>,
}
/// This routine computes the well-formedness constraints that must hold for the type `ty` to
/// appear in a context with lifetime `outer_region`
pub fn implications<'a,'tcx>(
infcx: &'a InferCtxt<'a,'tcx>,
body_id: ast::NodeId,
ty: Ty<'tcx>,
outer_region: ty::Region,
span: Span)
-> Vec<Implication<'tcx>>
{
debug!("implications(body_id={}, ty={:?}, outer_region={:?})",
body_id,
ty,
outer_region);
let mut stack = Vec::new();
stack.push((outer_region, None));
let mut wf = Implicator { infcx: infcx,
body_id: body_id,
span: span,
stack: stack,
out: Vec::new(),
visited: FnvHashSet() };
wf.accumulate_from_ty(ty);
debug!("implications: out={:?}", wf.out);
wf.out
}
impl<'a, 'tcx> Implicator<'a, 'tcx> {
fn tcx(&self) -> &'a ty::ctxt<'tcx> {
self.infcx.tcx
}
fn accumulate_from_ty(&mut self, ty: Ty<'tcx>) {
debug!("accumulate_from_ty(ty={:?})",
ty);
// When expanding out associated types, we can visit a cyclic
// set of types. Issue #23003.
if !self.visited.insert(ty) {
return;
}
match ty.sty {
ty::TyBool |
ty::TyChar |
ty::TyInt(..) |
ty::TyUint(..) |
ty::TyFloat(..) |
ty::TyBareFn(..) |
ty::TyError |
ty::TyStr => {
// No borrowed content reachable here.
}
ty::TyClosure(def_id, substs) => {
let &(r_a, opt_ty) = self.stack.last().unwrap();
self.out.push(Implication::RegionSubClosure(opt_ty, r_a, def_id, substs));
}
ty::TyTrait(ref t) => {
let required_region_bounds =
object_region_bounds(self.tcx(), &t.principal, t.bounds.builtin_bounds);
self.accumulate_from_object_ty(ty, t.bounds.region_bound, required_region_bounds)
}
ty::TyEnum(def_id, substs) |
ty::TyStruct(def_id, substs) => {
let item_scheme = self.tcx().lookup_item_type(def_id);
self.accumulate_from_adt(ty, def_id, &item_scheme.generics, substs)
}
ty::TyArray(t, _) |
ty::TySlice(t) |
ty::TyRawPtr(ty::TypeAndMut { ty: t, .. }) |
ty::TyBox(t) => {
self.accumulate_from_ty(t)
}
ty::TyRef(r_b, mt) => {
self.accumulate_from_rptr(ty, *r_b, mt.ty);
}
ty::TyParam(p) => {
self.push_param_constraint_from_top(p);
}
ty::TyProjection(ref data) => {
// `<T as TraitRef<..>>::Name`
self.push_projection_constraint_from_top(data);
}
ty::TyTuple(ref tuptys) => {
for &tupty in tuptys {
self.accumulate_from_ty(tupty);
}
}
ty::TyInfer(_) => {
// This should not happen, BUT:
//
// Currently we uncover region relationships on
// entering the fn check. We should do this after
// the fn check, then we can call this case a bug().
}
}
}
fn accumulate_from_rptr(&mut self,
ty: Ty<'tcx>,
r_b: ty::Region,
ty_b: Ty<'tcx>) {
// We are walking down a type like this, and current
// position is indicated by caret:
//
// &'a &'b ty_b
// ^
//
// At this point, top of stack will be `'a`. We must
// require that `'a <= 'b`.
self.push_region_constraint_from_top(r_b);
// Now we push `'b` onto the stack, because it must
// constrain any borrowed content we find within `T`.
self.stack.push((r_b, Some(ty)));
self.accumulate_from_ty(ty_b);
self.stack.pop().unwrap();
}
/// Pushes a constraint that `r_b` must outlive the top region on the stack.
fn push_region_constraint_from_top(&mut self,
r_b: ty::Region) {
// Indicates that we have found borrowed content with a lifetime
// of at least `r_b`. This adds a constraint that `r_b` must
// outlive the region `r_a` on top of the stack.
//
// As an example, imagine walking a type like:
//
// &'a &'b T
// ^
//
// when we hit the inner pointer (indicated by caret), `'a` will
// be on top of stack and `'b` will be the lifetime of the content
// we just found. So we add constraint that `'a <= 'b`.
let &(r_a, opt_ty) = self.stack.last().unwrap();
self.push_sub_region_constraint(opt_ty, r_a, r_b);
}
/// Pushes a constraint that `r_a <= r_b`, due to `opt_ty`
fn push_sub_region_constraint(&mut self,
opt_ty: Option<Ty<'tcx>>,
r_a: ty::Region,
r_b: ty::Region) {
self.out.push(Implication::RegionSubRegion(opt_ty, r_a, r_b));
}
/// Pushes a constraint that `param_ty` must outlive the top region on the stack.
fn push_param_constraint_from_top(&mut self,
param_ty: ty::ParamTy) {
let &(region, opt_ty) = self.stack.last().unwrap();
self.push_param_constraint(region, opt_ty, param_ty);
}
/// Pushes a constraint that `projection_ty` must outlive the top region on the stack.
fn push_projection_constraint_from_top(&mut self,
projection_ty: &ty::ProjectionTy<'tcx>) {
let &(region, opt_ty) = self.stack.last().unwrap();
self.out.push(Implication::RegionSubGeneric(
opt_ty, region, GenericKind::Projection(projection_ty.clone())));
}
/// Pushes a constraint that `region <= param_ty`, due to `opt_ty`
fn push_param_constraint(&mut self,
region: ty::Region,
opt_ty: Option<Ty<'tcx>>,
param_ty: ty::ParamTy) {
self.out.push(Implication::RegionSubGeneric(
opt_ty, region, GenericKind::Param(param_ty)));
}
fn accumulate_from_adt(&mut self,
ty: Ty<'tcx>,
def_id: ast::DefId,
_generics: &ty::Generics<'tcx>,
substs: &Substs<'tcx>)
{
let predicates =
self.tcx().lookup_predicates(def_id).instantiate(self.tcx(), substs);
let predicates = match self.fully_normalize(&predicates) {
Ok(predicates) => predicates,
Err(ErrorReported) => { return; }
};
for predicate in predicates.predicates.as_slice() {
match *predicate {
ty::Predicate::Trait(ref data) => {
self.accumulate_from_assoc_types_transitive(data);
}
ty::Predicate::Equate(..) => { }
ty::Predicate::Projection(..) => { }
ty::Predicate::RegionOutlives(ref data) => {
match self.tcx().no_late_bound_regions(data) {
None => { }
Some(ty::OutlivesPredicate(r_a, r_b)) => {
self.push_sub_region_constraint(Some(ty), r_b, r_a);
}
}
}
ty::Predicate::TypeOutlives(ref data) => {
match self.tcx().no_late_bound_regions(data) {
None => { }
Some(ty::OutlivesPredicate(ty_a, r_b)) => {
self.stack.push((r_b, Some(ty)));
self.accumulate_from_ty(ty_a);
self.stack.pop().unwrap();
}
}
}
}
}
let obligations = predicates.predicates
.into_iter()
.map(|pred| Implication::Predicate(def_id, pred));
self.out.extend(obligations);
let variances = self.tcx().item_variances(def_id);
for (®ion, &variance) in substs.regions().iter().zip(&variances.regions) {
match variance {
ty::Contravariant | ty::Invariant => {
// If any data with this lifetime is reachable
// within, it must be at least contravariant.
self.push_region_constraint_from_top(region)
}
ty::Covariant | ty::Bivariant => { }
}
}
for (&ty, &variance) in substs.types.iter().zip(&variances.types) {
match variance {
ty::Covariant | ty::Invariant => {
// If any data of this type is reachable within,
// it must be at least covariant.
self.accumulate_from_ty(ty);
}
ty::Contravariant | ty::Bivariant => { }
}
}
}
/// Given that there is a requirement that `Foo<X> : 'a`, where
/// `Foo` is declared like `struct Foo<T> where T : SomeTrait`,
/// this code finds all the associated types defined in
/// `SomeTrait` (and supertraits) and adds a requirement that `<X
/// as SomeTrait>::N : 'a` (where `N` is some associated type
/// defined in `SomeTrait`). This rule only applies to
/// trait-bounds that are not higher-ranked, because we cannot
/// project out of a HRTB. This rule helps code using associated
/// types to compile, see Issue #22246 for an example.
fn accumulate_from_assoc_types_transitive(&mut self,
data: &ty::PolyTraitPredicate<'tcx>)
{
debug!("accumulate_from_assoc_types_transitive({:?})",
data);
for poly_trait_ref in traits::supertraits(self.tcx(), data.to_poly_trait_ref()) {
match self.tcx().no_late_bound_regions(&poly_trait_ref) {
Some(trait_ref) => { self.accumulate_from_assoc_types(trait_ref); }
None => { }
}
}
}
fn accumulate_from_assoc_types(&mut self,
trait_ref: ty::TraitRef<'tcx>)
{
debug!("accumulate_from_assoc_types({:?})",
trait_ref);
let trait_def_id = trait_ref.def_id;
let trait_def = self.tcx().lookup_trait_def(trait_def_id);
let assoc_type_projections: Vec<_> =
trait_def.associated_type_names
.iter()
.map(|&name| self.tcx().mk_projection(trait_ref.clone(), name))
.collect();
debug!("accumulate_from_assoc_types: assoc_type_projections={:?}",
assoc_type_projections);
let tys = match self.fully_normalize(&assoc_type_projections) {
Ok(tys) => { tys }
Err(ErrorReported) => { return; }
};
for ty in tys {
self.accumulate_from_ty(ty);
}
}
fn accumulate_from_object_ty(&mut self,
ty: Ty<'tcx>,
region_bound: ty::Region,
required_region_bounds: Vec<ty::Region>)
{
// 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.
// The content of this object type must outlive
// `bounds.region_bound`:
let r_c = region_bound;
self.push_region_constraint_from_top(r_c);
// And then, in turn, to be well-formed, the
// `region_bound` that user specified must imply the
// region bounds required from all of the trait types:
for &r_d in &required_region_bounds {
// Each of these is an instance of the `'c <= 'b`
// constraint above
self.out.push(Implication::RegionSubRegion(Some(ty), r_d, r_c));
}
}
fn fully_normalize<T>(&self, value: &T) -> Result<T,ErrorReported>
where T : TypeFoldable<'tcx> + ty::HasTypeFlags
{
let value =
traits::fully_normalize(self.infcx,
traits::ObligationCause::misc(self.span, self.body_id),
value);
match value {
Ok(value) => Ok(value),
Err(errors) => {
// I don't like reporting these errors here, but I
// don't know where else to report them just now. And
// I don't really expect errors to arise here
// frequently. I guess the best option would be to
// propagate them out.
traits::report_fulfillment_errors(self.infcx, &errors);
Err(ErrorReported)
}
}
}
}
/// 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
/// `ty::required_region_bounds`, see that for more information.
pub fn object_region_bounds<'tcx>(
tcx: &ty::ctxt<'tcx>,
principal: &ty::PolyTraitRef<'tcx>,
others: ty::BuiltinBounds)
-> Vec<ty::Region>
{
// Since we don't actually *know* the self type for an object,
// this "open(err)" serves as a kind of dummy standin -- basically
// a skolemized type.
let open_ty = tcx.mk_infer(ty::FreshTy(0));
// Note that we preserve the overall binding levels here.
assert!(!open_ty.has_escaping_regions());
let substs = tcx.mk_substs(principal.0.substs.with_self_ty(open_ty));
let trait_refs = vec!(ty::Binder(ty::TraitRef::new(principal.0.def_id, substs)));
let mut predicates = others.to_predicates(tcx, open_ty);
predicates.extend(trait_refs.iter().map(|t| t.to_predicate()));
tcx.required_region_bounds(open_ty, predicates)
}