From c638a0b91faabdf78eb0c3dbb2bb826791f96be8 Mon Sep 17 00:00:00 2001
From: Expyron <5100376+Expyron@users.noreply.github.com>
Date: Sat, 18 Nov 2023 10:03:44 +0100
Subject: [PATCH] Add `from_const_with_len_unchecked`

---
 src/lib.rs   | 14 ++++++++++++++
 src/tests.rs | 12 ++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/src/lib.rs b/src/lib.rs
index f9a6c8a..cadb5d8 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2384,6 +2384,20 @@ impl<T, const N: usize> SmallVec<[T; N]> {
             data: SmallVecData::from_const(MaybeUninit::new(items)),
         }
     }
+
+    /// Constructs a new `SmallVec` on the stack from an array without
+    /// copying elements. Also sets the length. The user is responsible
+    /// for ensuring that `len <= N`.
+    /// 
+    /// This is a `const` version of [`SmallVec::from_buf_and_len_unchecked`] that is enabled by the feature `const_new`, with the limitation that it only works for arrays.
+    #[cfg_attr(docsrs, doc(cfg(feature = "const_new")))]
+    #[inline]
+    pub const unsafe fn from_const_with_len_unchecked(items: [T; N], len: usize) -> Self {
+        SmallVec {
+            capacity: len,
+            data: SmallVecData::from_const(MaybeUninit::new(items)),
+        }
+    }
 }
 
 #[cfg(feature = "const_generics")]
diff --git a/src/tests.rs b/src/tests.rs
index 936ad6f..3eab846 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -922,6 +922,12 @@ fn const_new() {
     assert_eq!(v.len(), 2);
     assert_eq!(v[0], 1);
     assert_eq!(v[1], 4);
+    let v = const_new_with_len();
+    assert_eq!(v.capacity(), 4);
+    assert_eq!(v.len(), 3);
+    assert_eq!(v[0], 2);
+    assert_eq!(v[1], 5);
+    assert_eq!(v[2], 7);
 }
 #[cfg(feature = "const_new")]
 const fn const_new_inner() -> SmallVec<[i32; 4]> {
@@ -935,6 +941,12 @@ const fn const_new_inline_sized() -> SmallVec<[i32; 4]> {
 const fn const_new_inline_args() -> SmallVec<[i32; 2]> {
     crate::smallvec_inline![1, 4]
 }
+#[cfg(feature = "const_new")]
+const fn const_new_with_len() -> SmallVec<[i32; 4]> {
+    unsafe {
+        SmallVec::<[i32; 4]>::from_const_with_len_unchecked([2, 5, 7, 0], 3)
+    }
+}
 
 #[test]
 fn empty_macro() {