From 2c3f0123e9e3da95cf32ba789b11c522ae3c9afc Mon Sep 17 00:00:00 2001
From: Niko Matsakis <niko@alum.mit.edu>
Date: Tue, 22 Dec 2015 19:46:51 -0500
Subject: [PATCH] only insert global predicates into the global cache once
we've completely proven them to be true
---
src/librustc/middle/traits/fulfill.rs | 63 ++++++++++++---------------
src/librustc_driver/test.rs | 2 +-
2 files changed, 28 insertions(+), 37 deletions(-)
diff --git a/src/librustc/middle/traits/fulfill.rs b/src/librustc/middle/traits/fulfill.rs
index ca2a9f9ca0b..a13cc141608 100644
--- a/src/librustc/middle/traits/fulfill.rs
+++ b/src/librustc/middle/traits/fulfill.rs
@@ -86,8 +86,6 @@ pub struct FulfillmentContext<'tcx> {
// obligations (otherwise, it's easy to fail to walk to a
// particular node-id).
region_obligations: NodeMap<Vec<RegionObligation<'tcx>>>,
-
- pub errors_will_be_reported: bool,
}
#[derive(Clone)]
@@ -105,28 +103,11 @@ pub struct PendingPredicateObligation<'tcx> {
impl<'tcx> FulfillmentContext<'tcx> {
/// Creates a new fulfillment context.
- ///
- /// `errors_will_be_reported` indicates whether ALL errors that
- /// are generated by this fulfillment context will be reported to
- /// the end user. This is used to inform caching, because it
- /// allows us to conclude that traits that resolve successfully
- /// will in fact always resolve successfully (in particular, it
- /// guarantees that if some dependent obligation encounters a
- /// problem, compilation will be aborted). If you're not sure of
- /// the right value here, pass `false`, as that is the more
- /// conservative option.
- ///
- /// FIXME -- a better option would be to hold back on modifying
- /// the global cache until we know that all dependent obligations
- /// are also satisfied. In that case, we could actually remove
- /// this boolean flag, and we'd also avoid the problem of squelching
- /// duplicate errors that occur across fns.
- pub fn new(errors_will_be_reported: bool) -> FulfillmentContext<'tcx> {
+ pub fn new() -> FulfillmentContext<'tcx> {
FulfillmentContext {
duplicate_set: FulfilledPredicates::new(),
predicates: ObligationForest::new(),
region_obligations: NodeMap(),
- errors_will_be_reported: errors_will_be_reported,
}
}
@@ -250,23 +231,27 @@ impl<'tcx> FulfillmentContext<'tcx> {
tcx: &ty::ctxt<'tcx>,
predicate: &ty::Predicate<'tcx>)
-> bool {
- // This is a kind of dirty hack to allow us to avoid "rederiving"
- // things that we have already proven in other methods.
- //
- // The idea is that any predicate that doesn't involve type
- // parameters and which only involves the 'static region (and
- // no other regions) is universally solvable, since impls are global.
- //
- // This is particularly important since even if we have a
- // cache hit in the selection context, we still wind up
- // evaluating the 'nested obligations'. This cache lets us
- // skip those.
-
- if self.errors_will_be_reported && predicate.is_global() {
- tcx.fulfilled_predicates.borrow_mut().is_duplicate_or_add(predicate)
- } else {
- self.duplicate_set.is_duplicate_or_add(predicate)
+ // For "global" predicates -- that is, predicates that don't
+ // involve type parameters, inference variables, or regions
+ // other than 'static -- we can check the cache in the tcx,
+ // which allows us to leverage work from other threads. Note
+ // that we don't add anything to this cache yet (unlike the
+ // local cache). This is because the tcx cache maintains the
+ // invariant that it only contains things that have been
+ // proven, and we have not yet proven that `predicate` holds.
+ if predicate.is_global() && tcx.fulfilled_predicates.borrow().is_duplicate(predicate) {
+ return true;
}
+
+ // If `predicate` is not global, or not present in the tcx
+ // cache, we can still check for it in our local cache and add
+ // it if not present. Note that if we find this predicate in
+ // the local cache we can stop immediately, without reporting
+ // any errors, even though we don't know yet if it is
+ // true. This is because, while we don't yet know if the
+ // predicate holds, we know that this same fulfillment context
+ // already is in the process of finding out.
+ self.duplicate_set.is_duplicate_or_add(predicate)
}
/// Attempts to select obligations using `selcx`. If `only_new_obligations` is true, then it
@@ -294,6 +279,12 @@ impl<'tcx> FulfillmentContext<'tcx> {
debug!("select_where_possible: outcome={:?}", outcome);
+ // these are obligations that were proven to be true.
+ for pending_obligation in outcome.successful {
+ let predicate = &pending_obligation.obligation.predicate;
+ if predicate.is_global() {
+ selcx.tcx().fulfilled_predicates.borrow_mut()
+ .is_duplicate_or_add(predicate);
}
}
diff --git a/src/librustc_driver/test.rs b/src/librustc_driver/test.rs
index 8f3366eacb3..b19628baa88 100644
--- a/src/librustc_driver/test.rs
+++ b/src/librustc_driver/test.rs
@@ -139,7 +139,7 @@ fn test_env<F>(source_string: &str,
lang_items,
stability::Index::new(krate),
|tcx| {
- let infcx = infer::new_infer_ctxt(tcx, &tcx.tables, None, false);
+ let infcx = infer::new_infer_ctxt(tcx, &tcx.tables, None);
body(Env { infcx: &infcx });
let free_regions = FreeRegionMap::new();
infcx.resolve_regions_and_report_errors(&free_regions,