Auto merge of #22028 - nikomatsakis:issue-22019-caching, r=aturon

Simplify cache selection by just using the local cache whenever there
are any where-clauses at all. This seems to be the simplest possible
rule and will (hopefully!) put an end to these annoying "cache leak"
bugs. Fixes #22019.

r? @aturon

Author: bors

src/librustc/middle/traits/doc.rs

@@ -434,87 +434,11 @@ attached to the `ParameterEnvironment` and the global cache attached
 to the `tcx`. We use the local cache whenever the result might depend
 on the where clauses that are in scope. The determination of which
 cache to use is done by the method `pick_candidate_cache` in
-`select.rs`.
-
-There are two cases where we currently use the local cache. The
-current rules are probably more conservative than necessary.
-
-### Trait references that involve parameter types
-
-The most obvious case where you need the local environment is
-when the trait reference includes parameter types. For example,
-consider the following function:
-
-    impl<T> Vec<T> {
-        fn foo(x: T)
-            where T : Foo
-        { ... }
-
-        fn bar(x: T)
-        { ... }
-    }
-
-If there is an obligation `T : Foo`, or `int : Bar<T>`, or whatever,
-clearly the results from `foo` and `bar` are potentially different,
-since the set of where clauses in scope are different.
-
-### Trait references with unbound variables when where clauses are in scope
-
-There is another less obvious interaction which involves unbound variables
-where *only* where clauses are in scope (no impls). This manifested as
-issue #18209 (`run-pass/trait-cache-issue-18209.rs`). Consider
-this snippet:
-
-```
-pub trait Foo {
-    fn load_from() -> Box<Self>;
-    fn load() -> Box<Self> {
-        Foo::load_from()
-    }
-}
-```
-
-The default method will incur an obligation `$0 : Foo` from the call
-to `load_from`. If there are no impls, this can be eagerly resolved to
-`VtableParam(Self : Foo)` and cached. Because the trait reference
-doesn't involve any parameters types (only the resolution does), this
-result was stored in the global cache, causing later calls to
-`Foo::load_from()` to get nonsense.
-
-To fix this, we always use the local cache if there are unbound
-variables and where clauses in scope. This is more conservative than
-necessary as far as I can tell. However, it still seems to be a simple
-rule and I observe ~99% hit rate on rustc, so it doesn't seem to hurt
-us in particular.
-
-Here is an example of the kind of subtle case that I would be worried
-about with a more complex rule (although this particular case works
-out ok). Imagine the trait reference doesn't directly reference a
-where clause, but the where clause plays a role in the winnowing
-phase. Something like this:
-
-```
-pub trait Foo<T> { ... }
-pub trait Bar { ... }
-impl<U,T:Bar> Foo<U> for T { ... } // Impl A
-impl Foo<char> for uint { ... }    // Impl B
-```
-
-Now, in some function, we have no where clauses in scope, and we have
-an obligation `$1 : Foo<$0>`. We might then conclude that `$0=char`
-and `$1=uint`: this is because for impl A to apply, `uint:Bar` would
-have to hold, and we know it does not or else the coherence check
-would have failed.  So we might enter into our global cache: `$1 :
-Foo<$0> => Impl B`.  Then we come along in a different scope, where a
-generic type `A` is around with the bound `A:Bar`. Now suddenly the
-impl is viable.
-
-The flaw in this imaginary DOOMSDAY SCENARIO is that we would not
-currently conclude that `$1 : Foo<$0>` implies that `$0 == uint` and
-`$1 == char`, even though it is true that (absent type parameters)
-there is no other type the user could enter. However, it is not
-*completely* implausible that we *could* draw this conclusion in the
-future; we wouldn't have to guess types, in particular, we could be
-led by the impls.
+`select.rs`. At the moment, we use a very simple, conservative rule:
+if there are any where-clauses in scope, then we use the local cache.
+We used to try and draw finer-grained distinctions, but that led to a
+serious of annoying and weird bugs like #22019 and #18290. This simple
+rule seems to be pretty clearly safe and also still retains a very
+high hit rate (~95% when compiling rustc).
 
 */

src/librustc/middle/traits/select.rs

@@ -705,14 +705,17 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         Ok(Some(candidate))
     }
 
-    fn pick_candidate_cache(&self,
-                            cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>)
-                            -> &SelectionCache<'tcx>
-    {
-        // High-level idea: we have to decide whether to consult the
-        // cache that is specific to this scope, or to consult the
-        // global cache. We want the cache that is specific to this
-        // scope whenever where clauses might affect the result.
+    fn pick_candidate_cache(&self) -> &SelectionCache<'tcx> {
+        // If there are any where-clauses in scope, then we always use
+        // a cache local to this particular scope. Otherwise, we
+        // switch to a global cache. We used to try and draw
+        // finer-grained distinctions, but that led to a serious of
+        // annoying and weird bugs like #22019 and #18290. This simple
+        // rule seems to be pretty clearly safe and also still retains
+        // a very high hit rate (~95% when compiling rustc).
+        if !self.param_env().caller_bounds.is_empty() {
+            return &self.param_env().selection_cache;
+        }
 
         // Avoid using the master cache during coherence and just rely
         // on the local cache. This effectively disables caching
@@ -725,28 +728,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return &self.param_env().selection_cache;
         }
 
-        // If the trait refers to any parameters in scope, then use
-        // the cache of the param-environment.
-        if
-            cache_fresh_trait_pred.0.input_types().iter().any(
-                |&t| ty::type_has_self(t) || ty::type_has_params(t))
-        {
-            return &self.param_env().selection_cache;
-        }
-
-        // If the trait refers to unbound type variables, and there
-        // are where clauses in scope, then use the local environment.
-        // If there are no where clauses in scope, which is a very
-        // common case, then we can use the global environment.
-        // See the discussion in doc.rs for more details.
-        if
-            !self.param_env().caller_bounds.is_empty() &&
-            cache_fresh_trait_pred.0.input_types().iter().any(
-                |&t| ty::type_has_ty_infer(t))
-        {
-            return &self.param_env().selection_cache;
-        }
-
         // Otherwise, we can use the global cache.
         &self.tcx().selection_cache
     }
@@ -755,7 +736,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                              cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>)
                              -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>>
     {
-        let cache = self.pick_candidate_cache(cache_fresh_trait_pred);
+        let cache = self.pick_candidate_cache();
         let hashmap = cache.hashmap.borrow();
         hashmap.get(&cache_fresh_trait_pred.0.trait_ref).map(|c| (*c).clone())
     }
@@ -764,7 +745,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                               cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
                               candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>)
     {
-        let cache = self.pick_candidate_cache(&cache_fresh_trait_pred);
+        let cache = self.pick_candidate_cache();
         let mut hashmap = cache.hashmap.borrow_mut();
         hashmap.insert(cache_fresh_trait_pred.0.trait_ref.clone(), candidate);
     }

src/test/run-pass/traits-issue-22019.rs

@@ -0,0 +1,41 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test an issue where global caching was causing free regions from
+// distinct scopes to be compared (`'g` and `'h`). The only important
+// thing is that compilation succeeds here.
+
+#![allow(missing_copy_implementations)]
+#![allow(unused_variables)]
+
+use std::borrow::ToOwned;
+
+pub struct CFGNode;
+
+pub type Node<'a> = &'a CFGNode;
+
+pub trait GraphWalk<'c, N> {
+    /// Returns all the nodes in this graph.
+    fn nodes(&'c self) where [N]:ToOwned<Vec<N>>;
+}
+
+impl<'g> GraphWalk<'g, Node<'g>> for u32
+{
+    fn nodes(&'g self) where [Node<'g>]:ToOwned<Vec<Node<'g>>>
+    { loop { } }
+}
+
+impl<'h> GraphWalk<'h, Node<'h>> for u64
+{
+    fn nodes(&'h self) where [Node<'h>]:ToOwned<Vec<Node<'h>>>
+    { loop { } }
+}
+
+fn main()  { }