From 51827589d2a841730254893dd2948f2c8e9a99f7 Mon Sep 17 00:00:00 2001
From: Alex Vlasov <alex.m.vlasov@gmail.com>
Date: Thu, 17 Jun 2021 18:13:28 +0200
Subject: [PATCH 1/2] Initial implementation

---
 library/core/src/array/mod.rs | 16 ++++++++++++++++
 library/core/tests/array.rs   |  6 ++++++
 library/core/tests/lib.rs     |  1 +
 3 files changed, 23 insertions(+)

diff --git a/library/core/src/array/mod.rs b/library/core/src/array/mod.rs
index 37af3557fdd51..7c18a8467fad5 100644
--- a/library/core/src/array/mod.rs
+++ b/library/core/src/array/mod.rs
@@ -389,6 +389,22 @@ array_impl_default! {32, T T T T T T T T T T T T T T T T T T T T T T T T T T T T
 
 #[lang = "array"]
 impl<T, const N: usize> [T; N] {
+    /// Returns the number of elements in the array.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(array_len)]
+    /// let a: [usize; 3] = [1, 2, 3];
+    /// assert_eq!(a.len(), 3);
+    /// ```
+    #[doc(alias = "length")]
+    #[unstable(feature = "array_len", issue = "none")]
+    #[inline]
+    pub const fn len(&self) -> usize {
+        N
+    }
+
     /// Returns an array of the same size as `self`, with function `f` applied to each element
     /// in order.
     ///
diff --git a/library/core/tests/array.rs b/library/core/tests/array.rs
index 0ae625bdb68c6..0fe59c795d8d4 100644
--- a/library/core/tests/array.rs
+++ b/library/core/tests/array.rs
@@ -288,6 +288,12 @@ fn empty_array_is_always_default() {
     let _arr = <[DoesNotImplDefault; 0]>::default();
 }
 
+#[test]
+fn array_len() {
+    let a = [1, 2, 3];
+    assert_eq!(a.len(), 3);
+}
+
 #[test]
 fn array_map() {
     let a = [1, 2, 3];
diff --git a/library/core/tests/lib.rs b/library/core/tests/lib.rs
index 16051b3bc36c7..5527e1019792a 100644
--- a/library/core/tests/lib.rs
+++ b/library/core/tests/lib.rs
@@ -1,6 +1,7 @@
 #![feature(alloc_layout_extra)]
 #![feature(array_chunks)]
 #![feature(array_methods)]
+#![feature(array_len)]
 #![feature(array_map)]
 #![feature(array_windows)]
 #![feature(bool_to_option)]

From 189342d646acdc2e4d4b203a590c397dae4d0b83 Mon Sep 17 00:00:00 2001
From: Alex Vlasov <alex.m.vlasov@gmail.com>
Date: Sun, 20 Jun 2021 15:01:04 +0200
Subject: [PATCH 2/2] mark as insta-stable-ish

---
 library/core/src/array/mod.rs | 5 +++--
 library/core/tests/lib.rs     | 1 -
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/library/core/src/array/mod.rs b/library/core/src/array/mod.rs
index 7c18a8467fad5..7dec9328eebd3 100644
--- a/library/core/src/array/mod.rs
+++ b/library/core/src/array/mod.rs
@@ -394,12 +394,13 @@ impl<T, const N: usize> [T; N] {
     /// # Examples
     ///
     /// ```
-    /// #![feature(array_len)]
     /// let a: [usize; 3] = [1, 2, 3];
     /// assert_eq!(a.len(), 3);
     /// ```
+    #[cfg(not(bootstrap))]
     #[doc(alias = "length")]
-    #[unstable(feature = "array_len", issue = "none")]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_stable(feature = "array_len", since = "1.53.0")]
     #[inline]
     pub const fn len(&self) -> usize {
         N
diff --git a/library/core/tests/lib.rs b/library/core/tests/lib.rs
index 5527e1019792a..16051b3bc36c7 100644
--- a/library/core/tests/lib.rs
+++ b/library/core/tests/lib.rs
@@ -1,7 +1,6 @@
 #![feature(alloc_layout_extra)]
 #![feature(array_chunks)]
 #![feature(array_methods)]
-#![feature(array_len)]
 #![feature(array_map)]
 #![feature(array_windows)]
 #![feature(bool_to_option)]