Handle stalling within ObligationForest.

It is simpler if `ObligationForest` does this itself, rather than the
caller having to manage it.

Author: nnethercote

compiler/rustc_data_structures/src/obligation_forest/mod.rs

@@ -42,7 +42,7 @@
 //!   now considered to be in error.
 //!
 //! When the call to `process_obligations` completes, you get back an `Outcome`,
-//! which includes three bits of information:
+//! which includes two bits of information:
 //!
 //! - `completed`: a list of obligations where processing was fully
 //!   completed without error (meaning that all transitive subobligations
@@ -53,13 +53,10 @@
 //!   all the obligations in `C` have been found completed.
 //! - `errors`: a list of errors that occurred and associated backtraces
 //!   at the time of error, which can be used to give context to the user.
-//! - `stalled`: if true, then none of the existing obligations were
-//!   *shallowly successful* (that is, no callback returned `Changed(_)`).
-//!   This implies that all obligations were either errors or returned an
-//!   ambiguous result, which means that any further calls to
-//!   `process_obligations` would simply yield back further ambiguous
-//!   results. This is used by the `FulfillmentContext` to decide when it
-//!   has reached a steady state.
+//!
+//! Upon completion, none of the existing obligations were *shallowly
+//! successful* (that is, no callback returned `Changed(_)`). This implies that
+//! all obligations were either errors or returned an ambiguous result.
 //!
 //! ### Implementation details
 //!
@@ -260,8 +257,6 @@ pub trait OutcomeTrait {
     type Obligation;
 
     fn new() -> Self;
-    fn mark_not_stalled(&mut self);
-    fn is_stalled(&self) -> bool;
     fn record_completed(&mut self, outcome: &Self::Obligation);
     fn record_error(&mut self, error: Self::Error);
 }
@@ -270,30 +265,14 @@ pub trait OutcomeTrait {
 pub struct Outcome<O, E> {
     /// Backtrace of obligations that were found to be in error.
     pub errors: Vec<Error<O, E>>,
-
-    /// If true, then we saw no successful obligations, which means
-    /// there is no point in further iteration. This is based on the
-    /// assumption that when trait matching returns `Error` or
-    /// `Unchanged`, those results do not affect environmental
-    /// inference state. (Note that if we invoke `process_obligations`
-    /// with no pending obligations, stalled will be true.)
-    pub stalled: bool,
 }
 
 impl<O, E> OutcomeTrait for Outcome<O, E> {
     type Error = Error<O, E>;
     type Obligation = O;
 
     fn new() -> Self {
-        Self { stalled: true, errors: vec![] }
-    }
-
-    fn mark_not_stalled(&mut self) {
-        self.stalled = false;
-    }
-
-    fn is_stalled(&self) -> bool {
-        self.stalled
+        Self { errors: vec![] }
     }
 
     fn record_completed(&mut self, _outcome: &Self::Obligation) {
@@ -415,10 +394,7 @@ impl<O: ForestObligation> ObligationForest<O> {
             .insert(node.obligation.as_cache_key());
     }
 
-    /// Performs a pass through the obligation list. This must
-    /// be called in a loop until `outcome.stalled` is false.
-    ///
-    /// This _cannot_ be unrolled (presently, at least).
+    /// Performs a fixpoint computation over the obligation list.
     #[inline(never)]
     pub fn process_obligations<P, OUT>(&mut self, processor: &mut P) -> OUT
     where
@@ -427,55 +403,66 @@ impl<O: ForestObligation> ObligationForest<O> {
     {
         let mut outcome = OUT::new();
 
-        // Note that the loop body can append new nodes, and those new nodes
-        // will then be processed by subsequent iterations of the loop.
-        //
-        // We can't use an iterator for the loop because `self.nodes` is
-        // appended to and the borrow checker would complain. We also can't use
-        // `for index in 0..self.nodes.len() { ... }` because the range would
-        // be computed with the initial length, and we would miss the appended
-        // nodes. Therefore we use a `while` loop.
-        let mut index = 0;
-        while let Some(node) = self.nodes.get_mut(index) {
-            // `processor.process_obligation` can modify the predicate within
-            // `node.obligation`, and that predicate is the key used for
-            // `self.active_cache`. This means that `self.active_cache` can get
-            // out of sync with `nodes`. It's not very common, but it does
-            // happen, and code in `compress` has to allow for it.
-            if node.state.get() != NodeState::Pending {
-                index += 1;
-                continue;
-            }
-
-            match processor.process_obligation(&mut node.obligation) {
-                ProcessResult::Unchanged => {
-                    // No change in state.
+        // Fixpoint computation: we repeat until the inner loop stalls.
+        loop {
+            let mut has_changed = false;
+
+            // Note that the loop body can append new nodes, and those new nodes
+            // will then be processed by subsequent iterations of the loop.
+            //
+            // We can't use an iterator for the loop because `self.nodes` is
+            // appended to and the borrow checker would complain. We also can't use
+            // `for index in 0..self.nodes.len() { ... }` because the range would
+            // be computed with the initial length, and we would miss the appended
+            // nodes. Therefore we use a `while` loop.
+            let mut index = 0;
+            while let Some(node) = self.nodes.get_mut(index) {
+                // `processor.process_obligation` can modify the predicate within
+                // `node.obligation`, and that predicate is the key used for
+                // `self.active_cache`. This means that `self.active_cache` can get
+                // out of sync with `nodes`. It's not very common, but it does
+                // happen, and code in `compress` has to allow for it.
+                if node.state.get() != NodeState::Pending {
+                    index += 1;
+                    continue;
                 }
-                ProcessResult::Changed(children) => {
-                    // We are not (yet) stalled.
-                    outcome.mark_not_stalled();
-                    node.state.set(NodeState::Success);
-
-                    for child in children {
-                        let st = self.register_obligation_at(child, Some(index));
-                        if let Err(()) = st {
-                            // Error already reported - propagate it
-                            // to our node.
-                            self.error_at(index);
+
+                match processor.process_obligation(&mut node.obligation) {
+                    ProcessResult::Unchanged => {
+                        // No change in state.
+                    }
+                    ProcessResult::Changed(children) => {
+                        // We are not (yet) stalled.
+                        has_changed = true;
+                        node.state.set(NodeState::Success);
+
+                        for child in children {
+                            let st = self.register_obligation_at(child, Some(index));
+                            if let Err(()) = st {
+                                // Error already reported - propagate it
+                                // to our node.
+                                self.error_at(index);
+                            }
                         }
                     }
+                    ProcessResult::Error(err) => {
+                        has_changed = true;
+                        outcome.record_error(Error { error: err, backtrace: self.error_at(index) });
+                    }
                 }
-                ProcessResult::Error(err) => {
-                    outcome.mark_not_stalled();
-                    outcome.record_error(Error { error: err, backtrace: self.error_at(index) });
-                }
+                index += 1;
+            }
+
+            // If unchanged, then we saw no successful obligations, which means
+            // there is no point in further iteration. This is based on the
+            // assumption that when trait matching returns `Error` or
+            // `Unchanged`, those results do not affect environmental inference
+            // state. (Note that this will occur if we invoke
+            // `process_obligations` with no pending obligations.)
+            if !has_changed {
+                break;
             }
-            index += 1;
-        }
 
-        // There's no need to perform marking, cycle processing and compression when nothing
-        // changed.
-        if !outcome.is_stalled() {
             self.mark_successes();
             self.process_cycles(processor);
             self.compress(|obl| outcome.record_completed(obl));

compiler/rustc_data_structures/src/obligation_forest/tests.rs

@@ -20,7 +20,6 @@ struct ClosureObligationProcessor<OF, BF, O, E> {
 struct TestOutcome<O, E> {
     pub completed: Vec<O>,
     pub errors: Vec<Error<O, E>>,
-    pub stalled: bool,
 }
 
 impl<O, E> OutcomeTrait for TestOutcome<O, E>
@@ -31,15 +30,7 @@ where
     type Obligation = O;
 
     fn new() -> Self {
-        Self { errors: vec![], stalled: false, completed: vec![] }
-    }
-
-    fn mark_not_stalled(&mut self) {
-        self.stalled = false;
-    }
-
-    fn is_stalled(&self) -> bool {
-        self.stalled
+        Self { errors: vec![], completed: vec![] }
     }
 
     fn record_completed(&mut self, outcome: &Self::Obligation) {

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -133,27 +133,16 @@ impl<'a, 'tcx> FulfillmentContext<'tcx> {
 
         let mut errors = Vec::new();
 
-        loop {
-            debug!("select: starting another iteration");
+        // Process pending obligations.
+        let outcome: Outcome<_, _> = self.predicates.process_obligations(&mut FulfillProcessor {
+            selcx,
+            register_region_obligations: self.register_region_obligations,
+        });
 
-            // Process pending obligations.
-            let outcome: Outcome<_, _> =
-                self.predicates.process_obligations(&mut FulfillProcessor {
-                    selcx,
-                    register_region_obligations: self.register_region_obligations,
-                });
-            debug!("select: outcome={:#?}", outcome);
+        // FIXME: if we kept the original cache key, we could mark projection
+        // obligations as complete for the projection cache here.
 
-            // FIXME: if we kept the original cache key, we could mark projection
-            // obligations as complete for the projection cache here.
-
-            errors.extend(outcome.errors.into_iter().map(to_fulfillment_error));
-
-            // If nothing new was added, no need to keep looping.
-            if outcome.stalled {
-                break;
-            }
-        }
+        errors.extend(outcome.errors.into_iter().map(to_fulfillment_error));
 
         debug!(
             "select({} predicates remaining, {} errors) done",