From fb78a9bdc52551753d30acbbe84d52451ea09d56 Mon Sep 17 00:00:00 2001
From: Richard Berry <rjsberry@pm.me>
Date: Thu, 2 Mar 2023 09:45:29 +0000
Subject: [PATCH 1/2] Impl conversion from array to Vec

---
 CHANGELOG.md  |  1 +
 src/sealed.rs |  5 ++++
 src/vec.rs    | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 74 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3248ead1bc..57ce7f5f64 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added `format` macro.
 - Added `String::from_utf16`.
 - Added `is_full`, `recent_index`, `oldest`, and `oldest_index` to `HistoryBuffer`
+- Added infallible conversions from arrays to `Vec`.
 
 ### Changed
 
diff --git a/src/sealed.rs b/src/sealed.rs
index 674be808d0..5e4ef4bba4 100644
--- a/src/sealed.rs
+++ b/src/sealed.rs
@@ -3,6 +3,11 @@ pub(crate) const fn smaller_than<const N: usize, const MAX: usize>() {
     Assert::<N, MAX>::LESS;
 }
 
+#[allow(dead_code, path_statements, clippy::no_effect)]
+pub(crate) const fn greater_than_eq<const N: usize, const MIN: usize>() {
+    Assert::<N, MIN>::GREATER_EQ;
+}
+
 #[allow(dead_code, path_statements, clippy::no_effect)]
 pub(crate) const fn greater_than_eq_0<const N: usize>() {
     Assert::<N, 0>::GREATER_EQ;
diff --git a/src/vec.rs b/src/vec.rs
index 6d2a7cf692..f1605f854a 100644
--- a/src/vec.rs
+++ b/src/vec.rs
@@ -1,4 +1,11 @@
-use core::{cmp::Ordering, fmt, hash, iter::FromIterator, mem::MaybeUninit, ops, ptr, slice};
+use core::{
+    cmp::Ordering,
+    fmt, hash,
+    iter::FromIterator,
+    mem,
+    mem::{ManuallyDrop, MaybeUninit},
+    ops, ptr, slice,
+};
 
 /// A fixed capacity [`Vec`](https://doc.rust-lang.org/std/vec/struct.Vec.html).
 ///
@@ -85,6 +92,45 @@ impl<T, const N: usize> Vec<T, N> {
         Ok(v)
     }
 
+    /// Constructs a new vector with a fixed capacity of `N`, initializing
+    /// it with the provided array.
+    ///
+    /// The length of the provided array, `M` may be equal to _or_ less than
+    /// the capacity of the vector, `N`.
+    ///
+    /// If the length of the provided array is greater than the capacity of the
+    /// vector a compile-time error will be produced.
+    pub fn from_array<const M: usize>(src: [T; M]) -> Self {
+        // Const assert M >= 0
+        crate::sealed::greater_than_eq_0::<M>();
+        // Const assert N >= M
+        crate::sealed::greater_than_eq::<N, M>();
+
+        // We've got to copy `src`, but we're functionally moving it. Don't run
+        // any Drop code for T.
+        let src = ManuallyDrop::new(src);
+
+        if N == M {
+            Self {
+                len: N,
+                // NOTE(unsafe) ManuallyDrop<[T; M]> and [MaybeUninit<T>; N]
+                // have the same layout when N == M.
+                buffer: unsafe { mem::transmute_copy(&src) },
+            }
+        } else {
+            let mut v = Vec::<T, N>::new();
+
+            for (src_elem, dst_elem) in src.iter().zip(v.buffer.iter_mut()) {
+                // NOTE(unsafe) src element is not going to drop as src itself
+                // is wrapped in a ManuallyDrop.
+                dst_elem.write(unsafe { ptr::read(src_elem) });
+            }
+
+            v.len = M;
+            v
+        }
+    }
+
     /// Clones a vec into a new vec
     pub(crate) fn clone(&self) -> Self
     where
@@ -858,6 +904,12 @@ impl<T, const N: usize> Drop for Vec<T, N> {
     }
 }
 
+impl<T, const N: usize, const M: usize> From<[T; M]> for Vec<T, N> {
+    fn from(array: [T; M]) -> Self {
+        Self::from_array(array)
+    }
+}
+
 impl<'a, T: Clone, const N: usize> TryFrom<&'a [T]> for Vec<T, N> {
     type Error = ();
 
@@ -1533,6 +1585,21 @@ mod tests {
         assert!(Vec::<u8, 2>::from_slice(&[1, 2, 3]).is_err());
     }
 
+    #[test]
+    fn from_array() {
+        // Successful construction, N == M
+        let v: Vec<u8, 3> = Vec::from_array([1, 2, 3]);
+        assert_eq!(v, Vec::<u8, 3>::from([1, 2, 3]));
+        assert_eq!(v.len(), 3);
+        assert_eq!(v.as_slice(), &[1, 2, 3]);
+
+        // Successful construction, N > M
+        let v: Vec<u8, 4> = Vec::from_array([1, 2, 3]);
+        assert_eq!(v, Vec::<u8, 4>::from([1, 2, 3]));
+        assert_eq!(v.len(), 3);
+        assert_eq!(v.as_slice(), &[1, 2, 3]);
+    }
+
     #[test]
     fn starts_with() {
         let v: Vec<_, 8> = Vec::from_slice(b"ab").unwrap();

From 0a5fd1d9fcd4724434859371e9efac8db441e95c Mon Sep 17 00:00:00 2001
From: Richard Berry <rjsberry@pm.me>
Date: Thu, 2 Mar 2023 10:29:58 +0000
Subject: [PATCH 2/2] Add test for drop behavior on array to vec conversion

---
 src/vec.rs | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/src/vec.rs b/src/vec.rs
index f1605f854a..2df7d86c1c 100644
--- a/src/vec.rs
+++ b/src/vec.rs
@@ -1600,6 +1600,23 @@ mod tests {
         assert_eq!(v.as_slice(), &[1, 2, 3]);
     }
 
+    #[test]
+    fn from_array_no_drop() {
+        struct Drops(Option<u8>);
+
+        impl Drop for Drops {
+            fn drop(&mut self) {
+                self.0 = None;
+            }
+        }
+
+        let v: Vec<Drops, 3> = Vec::from([Drops(Some(1)), Drops(Some(2)), Drops(Some(3))]);
+
+        assert_eq!(v[0].0, Some(1));
+        assert_eq!(v[1].0, Some(2));
+        assert_eq!(v[2].0, Some(3));
+    }
+
     #[test]
     fn starts_with() {
         let v: Vec<_, 8> = Vec::from_slice(b"ab").unwrap();