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Use the per-tree state to detect and permit DAGs (but not cyclic graphs)

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This commit is contained in:
Niko Matsakis 2016-02-01 16:08:34 -05:00
parent 37815fde39
commit 35d6efb232
1 changed files with 80 additions and 41 deletions
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121
src/librustc/middle/traits/fulfill.rs
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@ -36,6 +36,7 @@ pub struct GlobalFulfilledPredicates<'tcx> {
dep_graph: DepGraph,
}
#[derive(Debug)]
pub struct LocalFulfilledPredicates<'tcx> {
set: FnvHashSet<ty::Predicate<'tcx>>
}
@ -66,7 +67,8 @@ pub struct FulfillmentContext<'tcx> {
// A list of all obligations that have been registered with this
// fulfillment context.
predicates: ObligationForest<PendingPredicateObligation<'tcx>, ()>,
predicates: ObligationForest<PendingPredicateObligation<'tcx>,
LocalFulfilledPredicates<'tcx>>,
// A set of constraints that regionck must validate. Each
// constraint has the form `T:'a`, meaning "some type `T` must
@ -192,7 +194,7 @@ impl<'tcx> FulfillmentContext<'tcx> {
obligation: obligation,
stalled_on: vec![]
};
self.predicates.push_tree(obligation, ());
self.predicates.push_tree(obligation, LocalFulfilledPredicates::new());
}
pub fn region_obligations(&self,
@ -278,7 +280,8 @@ impl<'tcx> FulfillmentContext<'tcx> {
let outcome = {
let region_obligations = &mut self.region_obligations;
self.predicates.process_obligations(
|obligation, _tree, backtrace| process_predicate(selcx,
|obligation, tree, backtrace| process_predicate(selcx,
tree,
obligation,
backtrace,
region_obligations))
@ -315,61 +318,97 @@ impl<'tcx> FulfillmentContext<'tcx> {
/// Like `process_predicate1`, but wrap result into a pending predicate.
fn process_predicate<'a,'tcx>(selcx: &mut SelectionContext<'a,'tcx>,
tree_cache: &mut LocalFulfilledPredicates<'tcx>,
pending_obligation: &mut PendingPredicateObligation<'tcx>,
backtrace: Backtrace<PendingPredicateObligation<'tcx>>,
mut backtrace: Backtrace<PendingPredicateObligation<'tcx>>,
region_obligations: &mut NodeMap<Vec<RegionObligation<'tcx>>>)
-> Result<Option<Vec<PendingPredicateObligation<'tcx>>>,
FulfillmentErrorCode<'tcx>>
{
match process_predicate1(selcx, pending_obligation, backtrace, region_obligations) {
match process_predicate1(selcx, pending_obligation, backtrace.clone(), region_obligations) {
Ok(Some(v)) => {
// FIXME(#30977) the right thing to do here, I think, is to permit
// DAGs. That is, we should detect whenever this predicate
// has appeared somewhere in the current tree./ If it's a
// parent, that's a cycle, and we should either error out
// or consider it ok. But if it's NOT a parent, we can
// ignore it, since it will be proven (or not) separately.
// However, this is a touch tricky, so I'm doing something
// a bit hackier for now so that the `huge-struct.rs` passes.
// FIXME(#30977) The code below is designed to detect (and
// permit) DAGs, while still ensuring that the reasoning
// is acyclic. However, it does a few things
// suboptimally. For example, it refreshes type variables
// a lot, probably more than needed, but also less than
// you might want.
//
// - more than needed: I want to be very sure we don't
// accidentally treat a cycle as a DAG, so I am
// refreshing type variables as we walk the ancestors;
// but we are going to repeat this a lot, which is
// sort of silly, and it would be nicer to refresh
// them *in place* so that later predicate processing
// can benefit from the same work;
// - less than you might want: we only add items in the cache here,
// but maybe we learn more about type variables and could add them into
// the cache later on.
let tcx = selcx.tcx();
let retain_vec: Vec<_> = {
let mut dedup = FnvHashSet();
v.iter()
.map(|o| {
// Compute a little FnvHashSet for the ancestors. We only
// do this the first time that we care.
let mut cache = None;
let mut is_ancestor = |predicate: &ty::Predicate<'tcx>| {
if cache.is_none() {
let mut c = FnvHashSet();
for ancestor in backtrace.by_ref() {
// Ugh. This just feels ridiculously
// inefficient. But we need to compare
// predicates without being concerned about
// the vagaries of type inference, so for now
// just ensure that they are always
// up-to-date. (I suppose we could just use a
// snapshot and check if they are unifiable?)
let resolved_predicate =
selcx.infcx().resolve_type_vars_if_possible(
&ancestor.obligation.predicate);
c.insert(resolved_predicate);
}
cache = Some(c);
}
cache.as_ref().unwrap().contains(predicate)
};
let pending_predicate_obligations: Vec<_> =
v.into_iter()
.filter_map(|obligation| {
// Probably silly, but remove any inference
// variables. This is actually crucial to the
// ancestor check below, but it's not clear that
// it makes sense to ALWAYS do it.
let obligation = selcx.infcx().resolve_type_vars_if_possible(&obligation);
// Screen out obligations that we know globally
// are true. This should really be the DAG check
// mentioned above.
if tcx.fulfilled_predicates.borrow().check_duplicate(&o.predicate) {
return false;
if tcx.fulfilled_predicates.borrow().check_duplicate(&obligation.predicate) {
return None;
}
// If we see two siblings that are exactly the
// same, no need to add them twice.
if !dedup.insert(&o.predicate) {
return false;
// Check whether this obligation appears somewhere else in the tree.
if tree_cache.is_duplicate_or_add(&obligation.predicate) {
// If the obligation appears as a parent,
// allow it, because that is a cycle.
// Otherwise though we can just ignore
// it. Note that we have to be careful around
// inference variables here -- for the
// purposes of the ancestor check, we retain
// the invariant that all type variables are
// fully refreshed.
if !(&mut is_ancestor)(&obligation.predicate) {
return None;
}
}
true
Some(PendingPredicateObligation {
obligation: obligation,
stalled_on: vec![]
})
})
.collect()
};
let pending_predicate_obligations =
v.into_iter()
.zip(retain_vec)
.flat_map(|(o, retain)| {
if retain {
Some(PendingPredicateObligation {
obligation: o,
stalled_on: vec![]
})
} else {
None
}
})
.collect();
.collect();
Ok(Some(pending_predicate_obligations))
}