From c14a9b0d7c42bce277b1d08be08ada93c6373a3d Mon Sep 17 00:00:00 2001
From: Yotam Ofek <yotam.ofek@gmail.com>
Date: Tue, 24 Aug 2021 19:16:18 +0300
Subject: [PATCH 1/2] Register inherent impls for builtin array type.

---
 crates/hir_ty/src/method_resolution.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/crates/hir_ty/src/method_resolution.rs b/crates/hir_ty/src/method_resolution.rs
index 3d233b1e20ab..eaeab33fc551 100644
--- a/crates/hir_ty/src/method_resolution.rs
+++ b/crates/hir_ty/src/method_resolution.rs
@@ -348,6 +348,7 @@ pub fn def_crates(
         }
         TyKind::Str => lang_item_crate!("str_alloc", "str"),
         TyKind::Slice(_) => lang_item_crate!("slice_alloc", "slice"),
+        TyKind::Array(..) => lang_item_crate!("array"),
         TyKind::Raw(Mutability::Not, _) => lang_item_crate!("const_ptr"),
         TyKind::Raw(Mutability::Mut, _) => lang_item_crate!("mut_ptr"),
         TyKind::Dyn(_) => {

From 3077ef9d44669e32ee446ea89adf9dbddf86c4a2 Mon Sep 17 00:00:00 2001
From: Yotam Ofek <yotam.ofek@gmail.com>
Date: Tue, 24 Aug 2021 19:17:34 +0300
Subject: [PATCH 2/2] Auto-deref arrays as arrays with unknown length.

---
 crates/hir_ty/src/method_resolution.rs       | 20 ++++++++++----
 crates/hir_ty/src/tests/method_resolution.rs | 29 ++++++++++++++++++++
 2 files changed, 44 insertions(+), 5 deletions(-)

diff --git a/crates/hir_ty/src/method_resolution.rs b/crates/hir_ty/src/method_resolution.rs
index eaeab33fc551..b3ab03927e50 100644
--- a/crates/hir_ty/src/method_resolution.rs
+++ b/crates/hir_ty/src/method_resolution.rs
@@ -16,6 +16,7 @@ use rustc_hash::{FxHashMap, FxHashSet};
 
 use crate::{
     autoderef,
+    consteval::{self, ConstExt},
     db::HirDatabase,
     from_foreign_def_id,
     primitive::{self, FloatTy, IntTy, UintTy},
@@ -955,12 +956,21 @@ fn autoderef_method_receiver(
 ) -> 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(TyKind::Array(parameters, _)) =
-        deref_chain.last().map(|ty| ty.value.kind(&Interner))
+    if let Some((TyKind::Array(parameters, array_len), binders)) =
+        deref_chain.last().map(|ty| (ty.value.kind(&Interner), &ty.binders))
     {
-        let kinds = deref_chain.last().unwrap().binders.clone();
-        let unsized_ty = TyKind::Slice(parameters.clone()).intern(&Interner);
-        deref_chain.push(Canonical { value: unsized_ty, binders: kinds })
+        let kinds = binders.clone();
+        let parameters = parameters.clone();
+
+        if !array_len.is_unknown() {
+            let unknown_array_len_ty =
+                TyKind::Array(parameters.clone(), consteval::usize_const(None)).intern(&Interner);
+            let kinds = kinds.clone();
+            deref_chain.push(Canonical { value: unknown_array_len_ty, binders: kinds });
+        }
+
+        let unsized_ty = TyKind::Slice(parameters).intern(&Interner);
+        deref_chain.push(Canonical { value: unsized_ty, binders: kinds });
     }
     deref_chain
 }
diff --git a/crates/hir_ty/src/tests/method_resolution.rs b/crates/hir_ty/src/tests/method_resolution.rs
index 0c5022730998..5f138b8f979a 100644
--- a/crates/hir_ty/src/tests/method_resolution.rs
+++ b/crates/hir_ty/src/tests/method_resolution.rs
@@ -1265,6 +1265,35 @@ fn f() {
     );
 }
 
+#[test]
+fn resolve_const_generic_array_methods() {
+    check_types(
+        r#"
+#[lang = "array"]
+impl<T, const N: usize> [T; N] {
+    pub fn map<F, U>(self, f: F) -> [U; N]
+    where
+        F: FnMut(T) -> U,
+    { loop {} }
+}
+
+#[lang = "slice"]
+impl<T> [T] {
+    pub fn map<F, U>(self, f: F) -> &[U]
+    where
+        F: FnMut(T) -> U,
+    { loop {} }
+}
+
+fn f() {
+    let v = [1, 2].map::<_, usize>(|x| -> x * 2);
+    v;
+  //^ [usize; _]
+}
+    "#,
+    );
+}
+
 #[test]
 fn skip_array_during_method_dispatch() {
     check_types(