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Auto merge of #86993 - jackh726:project-gat-binders, r=nikomatsakis
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Replace associated item bound vars with placeholders when projecting

Fixes #76407
Fixes #76826

Similar, but more limited, to #85499. This allows us to handle things like `for<'a> <T as Trait>::Assoc<'a>` but not `for<'a> <T as Trait<'a>>::Assoc`, unblocking GATs.

r? `@nikomatsakis`
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bors committed Jul 16, 2021
2 parents b1f8e27 + cf001dc commit 27e4205
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Showing 19 changed files with 671 additions and 33 deletions.
3 changes: 2 additions & 1 deletion compiler/rustc_middle/src/ty/layout.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2483,9 +2483,10 @@ impl<'tcx> ty::Instance<'tcx> {
// `src/test/ui/polymorphization/normalized_sig_types.rs`), and codegen not keeping
// track of a polymorphization `ParamEnv` to allow normalizing later.
let mut sig = match *ty.kind() {
ty::FnDef(def_id, substs) => tcx
ty::FnDef(def_id, substs) if tcx.sess.opts.debugging_opts.polymorphize => tcx
.normalize_erasing_regions(tcx.param_env(def_id), tcx.fn_sig(def_id))
.subst(tcx, substs),
ty::FnDef(def_id, substs) => tcx.fn_sig(def_id).subst(tcx, substs),
_ => unreachable!(),
};

Expand Down
340 changes: 339 additions & 1 deletion compiler/rustc_trait_selection/src/traits/project.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,8 @@ use rustc_middle::ty::subst::Subst;
use rustc_middle::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, WithConstness};
use rustc_span::symbol::sym;

use std::collections::BTreeMap;

pub use rustc_middle::traits::Reveal;

pub type PolyProjectionObligation<'tcx> = Obligation<'tcx, ty::PolyProjectionPredicate<'tcx>>;
Expand Down Expand Up @@ -296,6 +298,7 @@ struct AssocTypeNormalizer<'a, 'b, 'tcx> {
cause: ObligationCause<'tcx>,
obligations: &'a mut Vec<PredicateObligation<'tcx>>,
depth: usize,
universes: Vec<Option<ty::UniverseIndex>>,
}

impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
Expand All @@ -306,12 +309,18 @@ impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
depth: usize,
obligations: &'a mut Vec<PredicateObligation<'tcx>>,
) -> AssocTypeNormalizer<'a, 'b, 'tcx> {
AssocTypeNormalizer { selcx, param_env, cause, obligations, depth }
AssocTypeNormalizer { selcx, param_env, cause, obligations, depth, universes: vec![] }
}

fn fold<T: TypeFoldable<'tcx>>(&mut self, value: T) -> T {
let value = self.selcx.infcx().resolve_vars_if_possible(value);

assert!(
!value.has_escaping_bound_vars(),
"Normalizing {:?} without wrapping in a `Binder`",
value
);

if !value.has_projections() { value } else { value.fold_with(self) }
}
}
Expand All @@ -321,6 +330,16 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
self.selcx.tcx()
}

fn fold_binder<T: TypeFoldable<'tcx>>(
&mut self,
t: ty::Binder<'tcx, T>,
) -> ty::Binder<'tcx, T> {
self.universes.push(None);
let t = t.super_fold_with(self);
self.universes.pop();
t
}

fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
if !ty.has_projections() {
return ty;
Expand Down Expand Up @@ -396,6 +415,52 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
normalized_ty
}

ty::Projection(data) if !data.trait_ref(self.tcx()).has_escaping_bound_vars() => {
// Okay, so you thought the previous branch was hacky. Well, to
// extend upon this, when the *trait ref* doesn't have escaping
// bound vars, but the associated item *does* (can only occur
// with GATs), then we might still be able to project the type.
// For this, we temporarily replace the bound vars with
// placeholders. Note though, that in the case that we still
// can't project for whatever reason (e.g. self type isn't
// known enough), we *can't* register an obligation and return
// an inference variable (since then that obligation would have
// bound vars and that's a can of worms). Instead, we just
// give up and fall back to pretending like we never tried!

let infcx = self.selcx.infcx();
let (data, mapped_regions, mapped_types, mapped_consts) =
BoundVarReplacer::replace_bound_vars(infcx, &mut self.universes, data);
let normalized_ty = opt_normalize_projection_type(
self.selcx,
self.param_env,
data,
self.cause.clone(),
self.depth,
&mut self.obligations,
)
.ok()
.flatten()
.unwrap_or_else(|| ty);

let normalized_ty = PlaceholderReplacer::replace_placeholders(
infcx,
mapped_regions,
mapped_types,
mapped_consts,
&self.universes,
normalized_ty,
);
debug!(
?self.depth,
?ty,
?normalized_ty,
obligations.len = ?self.obligations.len(),
"AssocTypeNormalizer: normalized type"
);
normalized_ty
}

_ => ty,
}
}
Expand All @@ -410,6 +475,279 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
}
}

pub struct BoundVarReplacer<'me, 'tcx> {
infcx: &'me InferCtxt<'me, 'tcx>,
// These three maps track the bound variable that were replaced by placeholders. It might be
// nice to remove these since we already have the `kind` in the placeholder; we really just need
// the `var` (but we *could* bring that into scope if we were to track them as we pass them).
mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
// The current depth relative to *this* folding, *not* the entire normalization. In other words,
// the depth of binders we've passed here.
current_index: ty::DebruijnIndex,
// The `UniverseIndex` of the binding levels above us. These are optional, since we are lazy:
// we don't actually create a universe until we see a bound var we have to replace.
universe_indices: &'me mut Vec<Option<ty::UniverseIndex>>,
}

impl<'me, 'tcx> BoundVarReplacer<'me, 'tcx> {
/// Returns `Some` if we *were* able to replace bound vars. If there are any bound vars that
/// use a binding level above `universe_indices.len()`, we fail.
pub fn replace_bound_vars<T: TypeFoldable<'tcx>>(
infcx: &'me InferCtxt<'me, 'tcx>,
universe_indices: &'me mut Vec<Option<ty::UniverseIndex>>,
value: T,
) -> (
T,
BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
BTreeMap<ty::PlaceholderType, ty::BoundTy>,
BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
) {
let mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion> = BTreeMap::new();
let mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy> = BTreeMap::new();
let mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar> = BTreeMap::new();

let mut replacer = BoundVarReplacer {
infcx,
mapped_regions,
mapped_types,
mapped_consts,
current_index: ty::INNERMOST,
universe_indices,
};

let value = value.super_fold_with(&mut replacer);

(value, replacer.mapped_regions, replacer.mapped_types, replacer.mapped_consts)
}

fn universe_for(&mut self, debruijn: ty::DebruijnIndex) -> ty::UniverseIndex {
let infcx = self.infcx;
let index =
self.universe_indices.len() - debruijn.as_usize() + self.current_index.as_usize() - 1;
let universe = self.universe_indices[index].unwrap_or_else(|| {
for i in self.universe_indices.iter_mut().take(index + 1) {
*i = i.or_else(|| Some(infcx.create_next_universe()))
}
self.universe_indices[index].unwrap()
});
universe
}
}

impl TypeFolder<'tcx> for BoundVarReplacer<'_, 'tcx> {
fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
self.infcx.tcx
}

fn fold_binder<T: TypeFoldable<'tcx>>(
&mut self,
t: ty::Binder<'tcx, T>,
) -> ty::Binder<'tcx, T> {
self.current_index.shift_in(1);
let t = t.super_fold_with(self);
self.current_index.shift_out(1);
t
}

fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
match *r {
ty::ReLateBound(debruijn, _)
if debruijn.as_usize() + 1
> self.current_index.as_usize() + self.universe_indices.len() =>
{
bug!("Bound vars outside of `self.universe_indices`");
}
ty::ReLateBound(debruijn, br) if debruijn >= self.current_index => {
let universe = self.universe_for(debruijn);
let p = ty::PlaceholderRegion { universe, name: br.kind };
self.mapped_regions.insert(p.clone(), br);
self.infcx.tcx.mk_region(ty::RePlaceholder(p))
}
_ => r,
}
}

fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
match *t.kind() {
ty::Bound(debruijn, _)
if debruijn.as_usize() + 1
> self.current_index.as_usize() + self.universe_indices.len() =>
{
bug!("Bound vars outside of `self.universe_indices`");
}
ty::Bound(debruijn, bound_ty) if debruijn >= self.current_index => {
let universe = self.universe_for(debruijn);
let p = ty::PlaceholderType { universe, name: bound_ty.var };
self.mapped_types.insert(p.clone(), bound_ty);
self.infcx.tcx.mk_ty(ty::Placeholder(p))
}
_ if t.has_vars_bound_at_or_above(self.current_index) => t.super_fold_with(self),
_ => t,
}
}

fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
match *ct {
ty::Const { val: ty::ConstKind::Bound(debruijn, _), ty: _ }
if debruijn.as_usize() + 1
> self.current_index.as_usize() + self.universe_indices.len() =>
{
bug!("Bound vars outside of `self.universe_indices`");
}
ty::Const { val: ty::ConstKind::Bound(debruijn, bound_const), ty }
if debruijn >= self.current_index =>
{
let universe = self.universe_for(debruijn);
let p = ty::PlaceholderConst {
universe,
name: ty::BoundConst { var: bound_const, ty },
};
self.mapped_consts.insert(p.clone(), bound_const);
self.infcx.tcx.mk_const(ty::Const { val: ty::ConstKind::Placeholder(p), ty })
}
_ if ct.has_vars_bound_at_or_above(self.current_index) => ct.super_fold_with(self),
_ => ct,
}
}
}

// The inverse of `BoundVarReplacer`: replaces placeholders with the bound vars from which they came.
pub struct PlaceholderReplacer<'me, 'tcx> {
infcx: &'me InferCtxt<'me, 'tcx>,
mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
universe_indices: &'me Vec<Option<ty::UniverseIndex>>,
current_index: ty::DebruijnIndex,
}

impl<'me, 'tcx> PlaceholderReplacer<'me, 'tcx> {
pub fn replace_placeholders<T: TypeFoldable<'tcx>>(
infcx: &'me InferCtxt<'me, 'tcx>,
mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
universe_indices: &'me Vec<Option<ty::UniverseIndex>>,
value: T,
) -> T {
let mut replacer = PlaceholderReplacer {
infcx,
mapped_regions,
mapped_types,
mapped_consts,
universe_indices,
current_index: ty::INNERMOST,
};
value.super_fold_with(&mut replacer)
}
}

impl TypeFolder<'tcx> for PlaceholderReplacer<'_, 'tcx> {
fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
self.infcx.tcx
}

fn fold_binder<T: TypeFoldable<'tcx>>(
&mut self,
t: ty::Binder<'tcx, T>,
) -> ty::Binder<'tcx, T> {
if !t.has_placeholders() && !t.has_infer_regions() {
return t;
}
self.current_index.shift_in(1);
let t = t.super_fold_with(self);
self.current_index.shift_out(1);
t
}

fn fold_region(&mut self, r0: ty::Region<'tcx>) -> ty::Region<'tcx> {
let r1 = match r0 {
ty::ReVar(_) => self
.infcx
.inner
.borrow_mut()
.unwrap_region_constraints()
.opportunistic_resolve_region(self.infcx.tcx, r0),
_ => r0,
};

let r2 = match *r1 {
ty::RePlaceholder(p) => {
let replace_var = self.mapped_regions.get(&p);
match replace_var {
Some(replace_var) => {
let index = self
.universe_indices
.iter()
.position(|u| matches!(u, Some(pu) if *pu == p.universe))
.unwrap_or_else(|| bug!("Unexpected placeholder universe."));
let db = ty::DebruijnIndex::from_usize(
self.universe_indices.len() - index + self.current_index.as_usize() - 1,
);
self.tcx().mk_region(ty::ReLateBound(db, *replace_var))
}
None => r1,
}
}
_ => r1,
};

debug!(?r0, ?r1, ?r2, "fold_region");

r2
}

fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
match *ty.kind() {
ty::Placeholder(p) => {
let replace_var = self.mapped_types.get(&p);
match replace_var {
Some(replace_var) => {
let index = self
.universe_indices
.iter()
.position(|u| matches!(u, Some(pu) if *pu == p.universe))
.unwrap_or_else(|| bug!("Unexpected placeholder universe."));
let db = ty::DebruijnIndex::from_usize(
self.universe_indices.len() - index + self.current_index.as_usize() - 1,
);
self.tcx().mk_ty(ty::Bound(db, *replace_var))
}
None => ty,
}
}

_ if ty.has_placeholders() || ty.has_infer_regions() => ty.super_fold_with(self),
_ => ty,
}
}

fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
if let ty::Const { val: ty::ConstKind::Placeholder(p), ty } = *ct {
let replace_var = self.mapped_consts.get(&p);
match replace_var {
Some(replace_var) => {
let index = self
.universe_indices
.iter()
.position(|u| matches!(u, Some(pu) if *pu == p.universe))
.unwrap_or_else(|| bug!("Unexpected placeholder universe."));
let db = ty::DebruijnIndex::from_usize(
self.universe_indices.len() - index + self.current_index.as_usize() - 1,
);
self.tcx()
.mk_const(ty::Const { val: ty::ConstKind::Bound(db, *replace_var), ty })
}
None => ct,
}
} else {
ct.super_fold_with(self)
}
}
}

/// The guts of `normalize`: normalize a specific projection like `<T
/// as Trait>::Item`. The result is always a type (and possibly
/// additional obligations). If ambiguity arises, which implies that
Expand Down
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