Re-introduce concept of projection cache 'completion'

Instead of clearing out the cache entirely, we store
the intermediate evaluation result into the cache entry.
This accomplishes several things:

* We avoid the performance hit associated with re-evaluating
  the sub-obligations
* We avoid causing issues with incremental compilation, since
  the final evaluation result is always the same
* We avoid affecting other uses of the same `InferCtxt` which
  might care about 'side effects' from processing the sub-obligations
  (e,g. region constraints). Only code that is specifically aware
   of the new 'complete' code is affected

compiler/rustc_infer/src/traits/project.rs

@@ -10,7 +10,7 @@ use rustc_data_structures::{
 };
 use rustc_middle::ty::{self, Ty};
 
-pub use rustc_middle::traits::Reveal;
+pub use rustc_middle::traits::{EvaluationResult, Reveal};
 
 pub(crate) type UndoLog<'tcx> =
     snapshot_map::UndoLog<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>;
@@ -92,7 +92,42 @@ pub enum ProjectionCacheEntry<'tcx> {
     Ambiguous,
     Recur,
     Error,
-    NormalizedTy(NormalizedTy<'tcx>),
+    NormalizedTy {
+        ty: NormalizedTy<'tcx>,
+        /// If we were able to successfully evaluate the
+        /// corresponding cache entry key during predicate
+        /// evaluation, then this field stores the final
+        /// result obtained from evaluating all of the projection
+        /// sub-obligations. During evaluation, we will skip
+        /// evaluating the cached sub-obligations in `ty`
+        /// if this field is set. Evaluation only
+        /// cares about the final result, so we don't
+        /// care about any region constraint side-effects
+        /// produced by evaluating the sub-boligations.
+        ///
+        /// Additionally, we will clear out the sub-obligations
+        /// entirely if we ever evaluate the cache entry (along
+        /// with all its sub obligations) to `EvaluatedToOk`.
+        /// This affects all users of the cache, not just evaluation.
+        /// Since a result of `EvaluatedToOk` means that there were
+        /// no region obligations that need to be tracked, it's
+        /// fine to forget about the sub-obligations - they
+        /// don't provide any additional information. However,
+        /// we do *not* discard any obligations when we see
+        /// `EvaluatedToOkModuloRegions` - we don't know
+        /// which sub-obligations may introduce region constraints,
+        /// so we keep them all to be safe.
+        ///
+        /// When we are not performing evaluation
+        /// (e.g. in `FulfillmentContext`), we ignore this field,
+        /// and always re-process the cached sub-obligations
+        /// (which may have been cleared out - see the above
+        /// paragraph).
+        /// This ensures that we do not lose any regions
+        /// constraints that arise from processing the
+        /// sub-obligations.
+        complete: Option<EvaluationResult>,
+    },
 }
 
 impl<'tcx> ProjectionCacheStorage<'tcx> {
@@ -149,10 +184,41 @@ impl<'tcx> ProjectionCache<'_, 'tcx> {
             debug!("Not overwriting Recur");
             return;
         }
-        let fresh_key = map.insert(key, ProjectionCacheEntry::NormalizedTy(value));
+        let fresh_key =
+            map.insert(key, ProjectionCacheEntry::NormalizedTy { ty: value, complete: None });
         assert!(!fresh_key, "never started projecting `{:?}`", key);
     }
 
+    /// Mark the relevant projection cache key as having its derived obligations
+    /// complete, so they won't have to be re-computed (this is OK to do in a
+    /// snapshot - if the snapshot is rolled back, the obligations will be
+    /// marked as incomplete again).
+    pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>, result: EvaluationResult) {
+        let mut map = self.map();
+        match map.get(&key) {
+            Some(&ProjectionCacheEntry::NormalizedTy { ref ty, complete: _ }) => {
+                info!("ProjectionCacheEntry::complete({:?}) - completing {:?}", key, ty);
+                let mut ty = ty.clone();
+                if result == EvaluationResult::EvaluatedToOk {
+                    ty.obligations = vec![];
+                }
+                map.insert(key, ProjectionCacheEntry::NormalizedTy { ty, complete: Some(result) });
+            }
+            ref value => {
+                // Type inference could "strand behind" old cache entries. Leave
+                // them alone for now.
+                info!("ProjectionCacheEntry::complete({:?}) - ignoring {:?}", key, value);
+            }
+        };
+    }
+
+    pub fn is_complete(&mut self, key: ProjectionCacheKey<'tcx>) -> Option<EvaluationResult> {
+        self.map().get(&key).and_then(|res| match res {
+            ProjectionCacheEntry::NormalizedTy { ty: _, complete } => *complete,
+            _ => None,
+        })
+    }
+
     /// Indicates that trying to normalize `key` resulted in
     /// ambiguity. No point in trying it again then until we gain more
     /// type information (in which case, the "fully resolved" key will