Do array unsizing for method receivers

It turns out rustc actually only unsizes array method receivers, so we don't
need to do any trait solving for this (at least for now).

Fixes #2670.

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..],
@@ -548,3 +548,20 @@ 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
+}

crates/ra_hir_ty/src/tests/method_resolution.rs

@@ -838,6 +838,24 @@ fn test() { (&S).foo()<|>; }
     assert_eq!(t, "u128");
 }
 
+#[test]
+fn method_resolution_unsize_array() {
+    let t = type_at(
+        r#"
+//- /main.rs
+#[lang = "slice"]
+impl<T> [T] {
+    fn len(&self) -> usize { loop {} }
+}
+fn test() {
+    let a = [1, 2, 3];
+    a.len()<|>;
+}
+"#,
+    );
+    assert_eq!(t, "usize");
+}
+
 #[test]
 fn method_resolution_trait_from_prelude() {
     let (db, pos) = TestDB::with_position(