Auto merge of #51895 - nikomatsakis:move-self-trait-predicate-to-items, r=scalexm

Move self trait predicate to items

This is a "reimagination" of @tmandry's PR #50183. The main effect is described in this comment from one of the commits:

---

Before we had the following results for `predicates_of`:

```rust
trait Foo { // predicates_of: Self: Foo
  fn bar(); // predicates_of: Self: Foo (inherited from trait)
}
```

Now we have removed the `Self: Foo` from the trait. However, we still
add it to the trait ITEM. This is because when people do things like
`<T as Foo>::bar()`, they still need to prove that `T: Foo`, and
having it in the `predicates_of` seems to be the cleanest way to
ensure that happens right now (otherwise, we'd need special case code
in various places):

```rust
trait Foo { // predicates_of: []
  fn bar(); // predicates_of: Self: Foo
}
```

However, we sometimes want to get the list of *just* the predicates
truly defined on a trait item (e.g., for chalk, but also for a few
other bits of code). For that, we define `predicates_defined_on`,
which does not contain the `Self: Foo` predicate yet, and we plumb
that through metadata and so forth.

---

I'm assigning @eddyb as the main reviewer, but I thought I might delegate to scalexm for this one in any case. I also want to post an alternative that I'll leave in the comments; it occurred to me as I was writing. =)

r? @eddyb
cc @scalexm @tmandry @leodasvacas

Author: bors

src/librustc/dep_graph/dep_node.rs

@@ -504,6 +504,7 @@ define_dep_nodes!( <'tcx>
     [] GenericsOfItem(DefId),
     [] PredicatesOfItem(DefId),
     [] ExplicitPredicatesOfItem(DefId),
+    [] PredicatesDefinedOnItem(DefId),
     [] InferredOutlivesOf(DefId),
     [] InferredOutlivesCrate(CrateNum),
     [] SuperPredicatesOfItem(DefId),

src/librustc/traits/object_safety.rs

@@ -23,7 +23,6 @@ use hir::def_id::DefId;
 use lint;
 use traits;
 use ty::{self, Ty, TyCtxt, TypeFoldable};
-use ty::subst::Substs;
 use ty::util::ExplicitSelf;
 use std::borrow::Cow;
 use syntax::ast;
@@ -173,10 +172,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         trait_def_id: DefId,
         supertraits_only: bool) -> bool
     {
-        let trait_ref = ty::Binder::dummy(ty::TraitRef {
-            def_id: trait_def_id,
-            substs: Substs::identity_for_item(self, trait_def_id)
-        });
+        let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(self, trait_def_id));
         let predicates = if supertraits_only {
             self.super_predicates_of(trait_def_id)
         } else {
@@ -391,10 +387,9 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
 
                     // Compute supertraits of current trait lazily.
                     if supertraits.is_none() {
-                        let trait_ref = ty::Binder::bind(ty::TraitRef {
-                            def_id: trait_def_id,
-                            substs: Substs::identity_for_item(self, trait_def_id)
-                        });
+                        let trait_ref = ty::Binder::bind(
+                            ty::TraitRef::identity(self, trait_def_id),
+                        );
                         supertraits = Some(traits::supertraits(self, trait_ref).collect());
                     }
 

src/librustc/traits/select.rs

@@ -1237,6 +1237,11 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
         let mut candidates: Vec<EvaluatedCandidate> =
             candidates?.into_iter().filter_map(|c| c).collect();
 
+        debug!("winnowed to {} candidates for {:?}: {:?}",
+               candidates.len(),
+               stack,
+               candidates);
+
         // If there are STILL multiple candidate, we can further
         // reduce the list by dropping duplicates -- including
         // resolving specializations.

src/librustc/ty/mod.rs

@@ -2862,8 +2862,8 @@ fn param_env<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     }
     // Compute the bounds on Self and the type parameters.
 
-    let bounds = tcx.predicates_of(def_id).instantiate_identity(tcx);
-    let predicates = bounds.predicates;
+    let InstantiatedPredicates { predicates } =
+        tcx.predicates_of(def_id).instantiate_identity(tcx);
 
     // Finally, we have to normalize the bounds in the environment, in
     // case they contain any associated type projections. This process

src/librustc/ty/query/mod.rs

@@ -101,11 +101,39 @@ define_queries! { <'tcx>
     [] fn type_of: TypeOfItem(DefId) -> Ty<'tcx>,
 
     /// Maps from the def-id of an item (trait/struct/enum/fn) to its
-    /// associated generics and predicates.
+    /// associated generics.
     [] fn generics_of: GenericsOfItem(DefId) -> &'tcx ty::Generics,
+
+    /// Maps from the def-id of an item (trait/struct/enum/fn) to the
+    /// predicates (where clauses) that must be proven true in order
+    /// to reference it. This is almost always the "predicates query"
+    /// that you want.
+    ///
+    /// `predicates_of` builds on `predicates_defined_on` -- in fact,
+    /// it is almost always the same as that query, except for the
+    /// case of traits. For traits, `predicates_of` contains
+    /// an additional `Self: Trait<...>` predicate that users don't
+    /// actually write. This reflects the fact that to invoke the
+    /// trait (e.g., via `Default::default`) you must supply types
+    /// that actually implement the trait. (However, this extra
+    /// predicate gets in the way of some checks, which are intended
+    /// to operate over only the actual where-clauses written by the
+    /// user.)
     [] fn predicates_of: PredicatesOfItem(DefId) -> ty::GenericPredicates<'tcx>,
+
+    /// Maps from the def-id of an item (trait/struct/enum/fn) to the
+    /// predicates (where clauses) directly defined on it. This is
+    /// equal to the `explicit_predicates_of` predicates plus the
+    /// `inferred_outlives_of` predicates.
+    [] fn predicates_defined_on: PredicatesDefinedOnItem(DefId) -> ty::GenericPredicates<'tcx>,
+
+    /// Returns the predicates written explicit by the user.
     [] fn explicit_predicates_of: ExplicitPredicatesOfItem(DefId) -> ty::GenericPredicates<'tcx>,
 
+    /// Returns the inferred outlives predicates (e.g., for `struct
+    /// Foo<'a, T> { x: &'a T }`, this would return `T: 'a`).
+    [] fn inferred_outlives_of: InferredOutlivesOf(DefId) -> Lrc<Vec<ty::Predicate<'tcx>>>,
+
     /// Maps from the def-id of a trait to the list of
     /// super-predicates. This is a subset of the full list of
     /// predicates. We store these in a separate map because we must
@@ -141,9 +169,6 @@ define_queries! { <'tcx>
     /// (inferred) variance.
     [] fn variances_of: ItemVariances(DefId) -> Lrc<Vec<ty::Variance>>,
 
-    /// Maps from def-id of a type to its (inferred) outlives.
-    [] fn inferred_outlives_of: InferredOutlivesOf(DefId) -> Lrc<Vec<ty::Predicate<'tcx>>>,
-
     /// Maps from def-id of a type to its (inferred) outlives.
     [] fn inferred_outlives_crate: InferredOutlivesCrate(CrateNum)
         -> Lrc<ty::CratePredicatesMap<'tcx>>,

src/librustc/ty/query/plumbing.rs

@@ -1074,6 +1074,7 @@ pub fn force_from_dep_node<'a, 'gcx, 'lcx>(tcx: TyCtxt<'a, 'gcx, 'lcx>,
         DepKind::TypeOfItem => { force!(type_of, def_id!()); }
         DepKind::GenericsOfItem => { force!(generics_of, def_id!()); }
         DepKind::PredicatesOfItem => { force!(predicates_of, def_id!()); }
+        DepKind::PredicatesDefinedOnItem => { force!(predicates_defined_on, def_id!()); }
         DepKind::ExplicitPredicatesOfItem => { force!(explicit_predicates_of, def_id!()); }
         DepKind::InferredOutlivesOf => { force!(inferred_outlives_of, def_id!()); }
         DepKind::InferredOutlivesCrate => { force!(inferred_outlives_crate, LOCAL_CRATE); }

src/librustc/ty/sty.rs

@@ -614,6 +614,15 @@ impl<'tcx> TraitRef<'tcx> {
         TraitRef { def_id: def_id, substs: substs }
     }
 
+    /// Returns a TraitRef of the form `P0: Foo<P1..Pn>` where `Pi`
+    /// are the parameters defined on trait.
+    pub fn identity<'a, 'gcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>, def_id: DefId) -> TraitRef<'tcx> {
+        TraitRef {
+            def_id,
+            substs: Substs::identity_for_item(tcx, def_id),
+        }
+    }
+
     pub fn self_ty(&self) -> Ty<'tcx> {
         self.substs.type_at(0)
     }

src/librustc/ty/util.rs

@@ -518,10 +518,25 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         result
     }
 
+    /// True if `def_id` refers to a closure (e.g., `|x| x * 2`). Note
+    /// that closures have a def-id, but the closure *expression* also
+    /// has a `HirId` that is located within the context where the
+    /// closure appears (and, sadly, a corresponding `NodeId`, since
+    /// those are not yet phased out). The parent of the closure's
+    /// def-id will also be the context where it appears.
     pub fn is_closure(self, def_id: DefId) -> bool {
         self.def_key(def_id).disambiguated_data.data == DefPathData::ClosureExpr
     }
 
+    /// True if `def_id` refers to a trait (e.g., `trait Foo { ... }`).
+    pub fn is_trait(self, def_id: DefId) -> bool {
+        if let DefPathData::Trait(_) = self.def_key(def_id).disambiguated_data.data {
+            true
+        } else {
+            false
+        }
+    }
+
     /// True if this def-id refers to the implicit constructor for
     /// a tuple struct like `struct Foo(u32)`.
     pub fn is_struct_constructor(self, def_id: DefId) -> bool {

src/librustc_metadata/cstore_impl.rs

@@ -107,6 +107,7 @@ provide! { <'tcx> tcx, def_id, other, cdata,
         tcx.alloc_generics(cdata.get_generics(def_id.index, tcx.sess))
     }
     predicates_of => { cdata.get_predicates(def_id.index, tcx) }
+    predicates_defined_on => { cdata.get_predicates_defined_on(def_id.index, tcx) }
     super_predicates_of => { cdata.get_super_predicates(def_id.index, tcx) }
     trait_def => {
         tcx.alloc_trait_def(cdata.get_trait_def(def_id.index, tcx.sess))

src/librustc_metadata/decoder.rs

@@ -563,6 +563,13 @@ impl<'a, 'tcx> CrateMetadata {
         self.entry(item_id).predicates.unwrap().decode((self, tcx))
     }
 
+    pub fn get_predicates_defined_on(&self,
+                                   item_id: DefIndex,
+                                   tcx: TyCtxt<'a, 'tcx, 'tcx>)
+                                   -> ty::GenericPredicates<'tcx> {
+        self.entry(item_id).predicates_defined_on.unwrap().decode((self, tcx))
+    }
+
     pub fn get_super_predicates(&self,
                                 item_id: DefIndex,
                                 tcx: TyCtxt<'a, 'tcx, 'tcx>)