Use traits from where clauses for method resolution

E.g. if we have `T: some::Trait`, we can call methods from that trait without it
needing to be in scope.

crates/ra_hir/src/code_model.rs

@@ -847,16 +847,22 @@ impl Trait {
             .collect()
     }
 
-    /// Returns an iterator over the whole super trait hierarchy (not including
-    /// the trait itself). (This iterator may be infinite in case of circular
-    /// super trait dependencies, which are possible in malformed code.)
+    /// Returns an iterator over the whole super trait hierarchy (including the
+    /// trait itself).
     pub fn all_super_traits<'a>(
         self,
         db: &'a impl HirDatabase,
     ) -> impl Iterator<Item = Trait> + 'a {
-        self.direct_super_traits(db).into_iter().flat_map(move |t| {
+        self.all_super_traits_inner(db).unique()
+    }
+
+    fn all_super_traits_inner<'a>(
+        self,
+        db: &'a impl HirDatabase,
+    ) -> impl Iterator<Item = Trait> + 'a {
+        iter::once(self).chain(self.direct_super_traits(db).into_iter().flat_map(move |t| {
             iter::once(t).chain(Box::new(t.all_super_traits(db)) as Box<dyn Iterator<Item = Trait>>)
-        })
+        }))
     }
 
     pub fn associated_type_by_name(self, db: &impl DefDatabase, name: &Name) -> Option<TypeAlias> {
@@ -876,10 +882,7 @@ impl Trait {
         db: &impl HirDatabase,
         name: &Name,
     ) -> Option<TypeAlias> {
-        iter::once(self)
-            .chain(self.all_super_traits(db))
-            .unique()
-            .find_map(|t| t.associated_type_by_name(db, name))
+        self.all_super_traits(db).find_map(|t| t.associated_type_by_name(db, name))
     }
 
     pub(crate) fn trait_data(self, db: &impl DefDatabase) -> Arc<TraitData> {

crates/ra_hir/src/ty/method_resolution.rs

@@ -212,7 +212,13 @@ fn iterate_trait_method_candidates<T>(
     // FIXME: maybe put the trait_env behind a query (need to figure out good input parameters for that)
     let env = lower::trait_env(db, resolver);
     // if ty is `impl Trait` or `dyn Trait`, the trait doesn't need to be in scope
-    let traits = ty.value.inherent_trait().into_iter().chain(resolver.traits_in_scope(db));
+    let inherent_trait = ty.value.inherent_trait().into_iter();
+    // if we have `T: Trait` in the param env, the trait doesn't need to be in scope
+    let traits_from_env = env
+        .trait_predicates_for_self_ty(&ty.value)
+        .map(|tr| tr.trait_)
+        .flat_map(|t| t.all_super_traits(db));
+    let traits = inherent_trait.chain(traits_from_env).chain(resolver.traits_in_scope(db));
     'traits: for t in traits {
         let data = t.trait_data(db);
 

crates/ra_hir/src/ty/tests.rs

@@ -3660,8 +3660,7 @@ fn test<T: foo::Trait>(x: T) {
 }
 "#,
     );
-    // FIXME should be u32
-    assert_eq!(t, "{unknown}");
+    assert_eq!(t, "u32");
 }
 
 #[test]
@@ -3673,8 +3672,8 @@ mod foo {
         fn foo(&self) -> u32 {}
     }
 }
-trait Trait1: SuperTrait {}
-trait Trait2 where Self: SuperTrait {}
+trait Trait1: foo::SuperTrait {}
+trait Trait2 where Self: foo::SuperTrait {}
 
 fn test<T: Trait1, U: Trait2>(x: T, y: U) {
     x.foo();
@@ -3684,13 +3683,13 @@ fn test<T: Trait1, U: Trait2>(x: T, y: U) {
         @r###"
     [50; 54) 'self': &Self
     [63; 65) '{}': ()
-    [172; 173) 'x': T
-    [178; 179) 'y': U
-    [184; 213) '{     ...o(); }': ()
-    [190; 191) 'x': T
-    [190; 197) 'x.foo()': {unknown}
-    [203; 204) 'y': U
-    [203; 210) 'y.foo()': {unknown}
+    [182; 183) 'x': T
+    [188; 189) 'y': U
+    [194; 223) '{     ...o(); }': ()
+    [200; 201) 'x': T
+    [200; 207) 'x.foo()': {unknown}
+    [213; 214) 'y': U
+    [213; 220) 'y.foo()': {unknown}
     "###
     );
 }

crates/ra_hir/src/ty/traits.rs

@@ -96,6 +96,21 @@ pub struct TraitEnvironment {
     pub predicates: Vec<GenericPredicate>,
 }
 
+impl TraitEnvironment {
+    /// Returns trait refs with the given self type which are supposed to hold
+    /// in this trait env. E.g. if we are in `foo<T: SomeTrait>()`, this will
+    /// find that `T: SomeTrait` if we call it for `T`.
+    pub(crate) fn trait_predicates_for_self_ty<'a>(
+        &'a self,
+        ty: &'a Ty,
+    ) -> impl Iterator<Item = &'a TraitRef> + 'a {
+        self.predicates.iter().filter_map(move |pred| match pred {
+            GenericPredicate::Implemented(tr) if tr.self_ty() == ty => Some(tr),
+            _ => None,
+        })
+    }
+}
+
 /// Something (usually a goal), along with an environment.
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct InEnvironment<T> {