1 files changed, 38 insertions, 2 deletions

diff --git a/crates/ra_hir_ty/src/method_resolution.rs b/crates/ra_hir_ty/src/method_resolution.rs
index df5901835..74b908c2e 100644
--- a/crates/ra_hir_ty/src/method_resolution.rs
+++ b/crates/ra_hir_ty/src/method_resolution.rs
@@ -20,7 +20,7 @@ use crate::{
     db::HirDatabase,
     primitive::{FloatBitness, Uncertain},
     utils::all_super_traits,
-    Canonical, InEnvironment, TraitEnvironment, TraitRef, Ty, TypeCtor, TypeWalk,
+    ApplicationTy, Canonical, InEnvironment, TraitEnvironment, TraitRef, Ty, TypeCtor, TypeWalk,
 };
 
 /// This is used as a key for indexing impls.
@@ -214,7 +214,7 @@ pub fn iterate_method_candidates<T>(
             // the methods by autoderef order of *receiver types*, not *self
             // types*.
 
-            let deref_chain: Vec<_> = autoderef::autoderef(db, Some(krate), ty).collect();
+            let deref_chain = autoderef_method_receiver(db, krate, ty);
             for i in 0..deref_chain.len() {
                 if let Some(result) = iterate_method_candidates_with_autoref(
                     &deref_chain[i..],
@@ -447,6 +447,25 @@ fn iterate_inherent_methods<T>(
     None
 }
 
+/// Returns the self type for the index trait call.
+pub fn resolve_indexing_op(
+    db: &impl HirDatabase,
+    ty: &Canonical<Ty>,
+    env: Arc<TraitEnvironment>,
+    krate: CrateId,
+    index_trait: TraitId,
+) -> Option<Canonical<Ty>> {
+    let ty = InEnvironment { value: ty.clone(), environment: env.clone() };
+    let deref_chain = autoderef_method_receiver(db, krate, ty);
+    for ty in deref_chain {
+        let goal = generic_implements_goal(db, env.clone(), index_trait, ty.clone());
+        if db.trait_solve(krate, goal).is_some() {
+            return Some(ty);
+        }
+    }
+    None
+}
+
 fn is_valid_candidate(
     db: &impl HirDatabase,
     name: Option<&Name>,
@@ -548,3 +567,20 @@ fn generic_implements_goal(
     let obligation = super::Obligation::Trait(trait_ref);
     Canonical { num_vars, value: InEnvironment::new(env, obligation) }
 }
+
+fn autoderef_method_receiver(
+    db: &impl HirDatabase,
+    krate: CrateId,
+    ty: InEnvironment<Canonical<Ty>>,
+) -> Vec<Canonical<Ty>> {
+    let mut deref_chain: Vec<_> = autoderef::autoderef(db, Some(krate), ty).collect();
+    // As a last step, we can do array unsizing (that's the only unsizing that rustc does for method receivers!)
+    if let Some(Ty::Apply(ApplicationTy { ctor: TypeCtor::Array, parameters })) =
+        deref_chain.last().map(|ty| &ty.value)
+    {
+        let num_vars = deref_chain.last().unwrap().num_vars;
+        let unsized_ty = Ty::apply(TypeCtor::Slice, parameters.clone());
+        deref_chain.push(Canonical { value: unsized_ty, num_vars })
+    }
+    deref_chain
+}