Distinguish elaboration vs supertrait expansion. Elaboration considers

all where-clauses, but supertrait considers a more limited
set. Supertrait expansion is suitable for virtual tables and associated
type inclusion, elaboration for more general reasoning.

Fixes rust-lang#20671.

Author: nikomatsakis

src/librustc/middle/traits/fulfill.rs

@@ -319,6 +319,7 @@ fn process_predicate<'a,'tcx>(selcx: &mut SelectionContext<'a,'tcx>,
      * type inference.
      */
 
+    debug!("process_predicate(obligation={})", obligation.repr(selcx.tcx()));
     let tcx = selcx.tcx();
     match obligation.predicate {
         ty::Predicate::Trait(ref data) => {
@@ -328,6 +329,7 @@ fn process_predicate<'a,'tcx>(selcx: &mut SelectionContext<'a,'tcx>,
                     false
                 }
                 Ok(Some(s)) => {
+                    debug!("process_predicate: new_obligations={}", s.repr(selcx.tcx()));
                     s.map_move_nested(|p| new_obligations.push(p));
                     true
                 }
@@ -398,6 +400,7 @@ fn process_predicate<'a,'tcx>(selcx: &mut SelectionContext<'a,'tcx>,
                    result.repr(tcx));
             match result {
                 Ok(Some(obligations)) => {
+                    debug!("process_predicate: new_obligations={}", obligations.repr(selcx.tcx()));
                     new_obligations.extend(obligations.into_iter());
                     true
                 }

src/librustc/middle/traits/mod.rs

@@ -43,11 +43,13 @@ pub use self::select::SelectionCache;
 pub use self::select::{MethodMatchResult, MethodMatched, MethodAmbiguous, MethodDidNotMatch};
 pub use self::select::{MethodMatchedData}; // intentionally don't export variants
 pub use self::util::elaborate_predicates;
+pub use self::util::elaborate_trait_refs;
 pub use self::util::get_vtable_index_of_object_method;
 pub use self::util::trait_ref_for_builtin_bound;
+pub use self::util::superpredicates;
 pub use self::util::supertraits;
 pub use self::util::Supertraits;
-pub use self::util::transitive_bounds;
+pub use self::util::supertraits_many;
 pub use self::util::upcast;
 
 mod coherence;

src/librustc/middle/traits/select.rs

@@ -1000,12 +1000,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         let caller_trait_refs: Vec<_> =
             self.param_env().caller_bounds.iter()
-            .filter_map(|o| o.to_opt_poly_trait_ref())
-            .collect();
+                                          .filter_map(|o| o.to_opt_poly_trait_ref())
+                                          .collect();
 
         let all_bounds =
-            util::transitive_bounds(
-                self.tcx(), &caller_trait_refs[..]);
+            util::elaborate_trait_refs(self.tcx(), &caller_trait_refs)
+            .filter_to_traits();
 
         let matching_bounds =
             all_bounds.filter(
@@ -1323,8 +1323,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             (&ImplCandidate(..), &ParamCandidate(..)) |
             (&ClosureCandidate(..), &ParamCandidate(..)) |
             (&FnPointerCandidate(..), &ParamCandidate(..)) |
-            (&BuiltinObjectCandidate(..), &ParamCandidate(_)) |
-            (&BuiltinCandidate(..), &ParamCandidate(..)) => {
+            (&BuiltinObjectCandidate(..), &ParamCandidate(_)) => {
                 // We basically prefer always prefer to use a
                 // where-clause over another option. Where clauses
                 // impose the burden of finding the exact match onto
@@ -1333,6 +1332,21 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 // #18453.
                 true
             }
+            (&ParamCandidate(..), &BuiltinCandidate(..)) => {
+                // Builtin candidates only trigger for things like
+                // `Sized` and `Copy` applied to structural types; in
+                // that case, the param candidate cannot be any
+                // different from the builtin rules, so prefer the
+                // builtin rules. It is important to do this because
+                // otherwise if you have
+                //
+                //     where (A, B) : Sized
+                //
+                // in your environment, for example, it will unduly
+                // influence matches of other tuples, forcing their
+                // argument types to be A and B.
+                true
+            }
             (&DefaultImplCandidate(_), _) => {
                 // Prefer other candidates over default implementations.
                 self.tcx().sess.bug(

src/librustc/middle/traits/util.rs

@@ -65,27 +65,32 @@ impl<'a,'tcx> PredicateSet<'a,'tcx> {
 
 ///////////////////////////////////////////////////////////////////////////
 // `Elaboration` iterator
-///////////////////////////////////////////////////////////////////////////
+//
+// "Elaboration" is the process of identifying all the predicates that
+// are implied by a source predicate. This is done by expanding out
+// where clauses declared on a trait. Example:
+//
+// ```
+// trait Foo<T> : Bar<T> where T : Baz { .. }
+// trait Bar<T> { .. }
+// trait Baz { .. }
+// ```
+//
+// Elaborating `X:Foo<Y>` would yield `[X:Foo<Y>, X:Bar<Y>, Y:Baz,
+// Y:Sized]`. If you only want the supertraits (`X:Bar<Y>`, here),
+// see the supertrait iterator below.
 
-/// "Elaboration" is the process of identifying all the predicates that
-/// are implied by a source predicate. Currently this basically means
-/// walking the "supertraits" and other similar assumptions. For
-/// example, if we know that `T : Ord`, the elaborator would deduce
-/// that `T : PartialOrd` holds as well. Similarly, if we have `trait
-/// Foo : 'static`, and we know that `T : Foo`, then we know that `T :
-/// 'static`.
+/// Elaborates a set of predicates into other predicates that are
+/// implied by the initial set.
 pub struct Elaborator<'cx, 'tcx:'cx> {
     tcx: &'cx ty::ctxt<'tcx>,
     stack: Vec<ty::Predicate<'tcx>>,
     visited: PredicateSet<'cx,'tcx>,
-}
 
-pub fn elaborate_trait_ref<'cx, 'tcx>(
-    tcx: &'cx ty::ctxt<'tcx>,
-    trait_ref: ty::PolyTraitRef<'tcx>)
-    -> Elaborator<'cx, 'tcx>
-{
-    elaborate_predicates(tcx, vec![trait_ref.as_predicate()])
+    /// If true, we only elaborate `A:Foo` to `A:Bar` where `Bar` is a
+    /// supertrait of `Foo`. Otherwise, we consider all where-clauses
+    /// declared on `Foo`.
+    supertraits_only: bool,
 }
 
 pub fn elaborate_trait_refs<'cx, 'tcx>(
@@ -101,15 +106,24 @@ pub fn elaborate_trait_refs<'cx, 'tcx>(
 
 pub fn elaborate_predicates<'cx, 'tcx>(
     tcx: &'cx ty::ctxt<'tcx>,
-    mut predicates: Vec<ty::Predicate<'tcx>>)
+    predicates: Vec<ty::Predicate<'tcx>>)
     -> Elaborator<'cx, 'tcx>
 {
-    let mut visited = PredicateSet::new(tcx);
-    predicates.retain(|pred| visited.insert(pred));
-    Elaborator { tcx: tcx, stack: predicates, visited: visited }
+    Elaborator::new(tcx, predicates, false)
 }
 
 impl<'cx, 'tcx> Elaborator<'cx, 'tcx> {
+    fn new(tcx: &'cx ty::ctxt<'tcx>,
+           mut predicates: Vec<ty::Predicate<'tcx>>,
+           supertraits_only: bool)
+           -> Elaborator<'cx, 'tcx>
+    {
+        let mut visited = PredicateSet::new(tcx);
+        predicates.retain(|pred| visited.insert(pred));
+        Elaborator { tcx: tcx, stack: predicates,
+                     visited: visited, supertraits_only: supertraits_only }
+    }
+
     pub fn filter_to_traits(self) -> FilterToTraits<Elaborator<'cx, 'tcx>> {
         FilterToTraits::new(self)
     }
@@ -118,7 +132,11 @@ impl<'cx, 'tcx> Elaborator<'cx, 'tcx> {
         match *predicate {
             ty::Predicate::Trait(ref data) => {
                 // Predicates declared on the trait.
-                let predicates = ty::lookup_super_predicates(self.tcx, data.def_id());
+                let predicates = if self.supertraits_only {
+                    ty::lookup_super_predicates(self.tcx, data.def_id())
+                } else {
+                    ty::lookup_predicates(self.tcx, data.def_id())
+                };
 
                 let mut predicates: Vec<_> =
                     predicates.predicates
@@ -191,27 +209,55 @@ impl<'cx, 'tcx> Iterator for Elaborator<'cx, 'tcx> {
 
 ///////////////////////////////////////////////////////////////////////////
 // Supertrait iterator
-///////////////////////////////////////////////////////////////////////////
+//
+// Expanding out the set of supertraits is a subset of the full elaboration
+// described above. The difference can be illustrated easily with this
+// example:
+//
+// ```
+// trait Foo<T> : Bar<T> where T : Baz { .. }
+// trait Bar<T> { .. }
+// trait Baz { .. }
+// ```
+//
+// Given the predicate `X:Foo<Y>`, supertrait expansion would yield
+// `[X:Foo<Y>, X:Bar<Y>]`, but the full elaboration would yield
+// `[X:Foo<Y>, X:Bar<Y>, Y:Baz, Y:Sized]`.
+//
+// Supertraits are the correct choice for vtables and objects (which
+// only contain supertraits). Another time when you want supertraits
+// is when elaborating the set of associated items contained by a
+// trait, which includes only those items defined in supertraits, not
+// arbitrary random traits.
 
 pub type Supertraits<'cx, 'tcx> = FilterToTraits<Elaborator<'cx, 'tcx>>;
 
 pub fn supertraits<'cx, 'tcx>(tcx: &'cx ty::ctxt<'tcx>,
                               trait_ref: ty::PolyTraitRef<'tcx>)
                               -> Supertraits<'cx, 'tcx>
 {
-    elaborate_trait_ref(tcx, trait_ref).filter_to_traits()
+    Elaborator::new(tcx, vec![trait_ref.as_predicate()], true).filter_to_traits()
 }
 
-pub fn transitive_bounds<'cx, 'tcx>(tcx: &'cx ty::ctxt<'tcx>,
-                                    bounds: &[ty::PolyTraitRef<'tcx>])
-                                    -> Supertraits<'cx, 'tcx>
+pub fn superpredicates<'cx, 'tcx>(tcx: &'cx ty::ctxt<'tcx>,
+                                  trait_refs: Vec<ty::Predicate<'tcx>>)
+                                  -> Elaborator<'cx, 'tcx>
 {
-    elaborate_trait_refs(tcx, bounds).filter_to_traits()
+    Elaborator::new(tcx, trait_refs, true)
+}
+
+pub fn supertraits_many<'cx, 'tcx>(tcx: &'cx ty::ctxt<'tcx>,
+                                   trait_refs: &[ty::PolyTraitRef<'tcx>])
+                                   -> Supertraits<'cx, 'tcx>
+{
+    let predicates = trait_refs.iter()
+                               .map(|trait_ref| trait_ref.as_predicate())
+                               .collect();
+    Elaborator::new(tcx, predicates, true).filter_to_traits()
 }
 
 ///////////////////////////////////////////////////////////////////////////
 // Other
-///////////////////////////////////////////////////////////////////////////
 
 /// A filter around an iterator of predicates that makes it yield up
 /// just trait references.
@@ -377,15 +423,17 @@ pub fn upcast<'tcx>(tcx: &ty::ctxt<'tcx>,
 pub fn get_vtable_index_of_object_method<'tcx>(tcx: &ty::ctxt<'tcx>,
                                                object_trait_ref: ty::PolyTraitRef<'tcx>,
                                                trait_def_id: ast::DefId,
-                                               method_offset_in_trait: uint) -> uint {
+                                               method_offset_in_trait: uint)
+                                               -> uint
+{
     // We need to figure the "real index" of the method in a
     // listing of all the methods of an object. We do this by
     // iterating down the supertraits of the object's trait until
     // we find the trait the method came from, counting up the
     // methods from them.
     let mut method_count = 0;
 
-    for bound_ref in transitive_bounds(tcx, &[object_trait_ref]) {
+    for bound_ref in supertraits(tcx, object_trait_ref) {
         if bound_ref.def_id() == trait_def_id {
             break;
         }

src/librustc/middle/ty.rs

@@ -5899,7 +5899,7 @@ pub fn each_bound_trait_and_supertraits<'tcx, F>(tcx: &ctxt<'tcx>,
                                                  -> bool where
     F: FnMut(PolyTraitRef<'tcx>) -> bool,
 {
-    for bound_trait_ref in traits::transitive_bounds(tcx, bounds) {
+    for bound_trait_ref in traits::supertraits_many(tcx, bounds) {
         if !f(bound_trait_ref) {
             return false;
         }
@@ -5928,7 +5928,7 @@ pub fn required_region_bounds<'tcx>(tcx: &ctxt<'tcx>,
 
     assert!(!erased_self_ty.has_escaping_regions());
 
-    traits::elaborate_predicates(tcx, predicates)
+    traits::superpredicates(tcx, predicates)
         .filter_map(|predicate| {
             match predicate {
                 ty::Predicate::Projection(..) |

src/librustc_typeck/astconv.rs

@@ -1059,7 +1059,7 @@ fn associated_path_def_to_ty<'tcx>(this: &AstConv<'tcx>,
     }
 
     let mut suitable_bounds: Vec<_> =
-        traits::transitive_bounds(tcx, &bounds)
+        traits::supertraits_many(tcx, &bounds)
         .filter(|b| this.trait_defines_associated_type_named(b.def_id(), assoc_name))
         .collect();
 

src/librustc_typeck/check/method/probe.rs

@@ -456,7 +456,9 @@ impl<'a,'tcx> ProbeContext<'a,'tcx> {
         debug!("elaborate_bounds(bounds={})", bounds.repr(self.tcx()));
 
         let tcx = self.tcx();
-        for bound_trait_ref in traits::transitive_bounds(tcx, bounds) {
+        for bound_trait_ref in
+            traits::elaborate_trait_refs(tcx, bounds).filter_map(|tr| tr.to_opt_poly_trait_ref())
+        {
             let (pos, method) = match trait_method(tcx,
                                                    bound_trait_ref.def_id(),
                                                    self.method_name) {

src/test/run-pass/trait-infer-from-where-clause.rs

@@ -0,0 +1,27 @@
+// Copyright 2013 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.
+
+// Tests a case where we need to consider the where-clauses of `Foo`,
+// and not just its supetraits, to infer that `A:Foo` holds.
+// Issue #20671.
+
+trait Foo<T> {
+    fn foo(&self) -> &T;
+}
+
+trait Bar<A> where A: Foo<Self> {
+    fn dummy(&self, a: A);
+}
+
+fn foobar<A, B: Bar<A>>(a: &A) -> &B {
+   a.foo()
+}
+
+fn main() { }