Add docs for Vec::from functions

Part of #51430

library/alloc/src/vec/mod.rs

@@ -2712,6 +2712,13 @@ impl<T, A: Allocator> AsMut<[T]> for Vec<T, A> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Clone> From<&[T]> for Vec<T> {
+    /// Allocate a `Vec<T>` and fill it by cloning `s`'s items.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(Vec::from(&[1, 2, 3][..]), vec![1, 2, 3]);
+    /// ```
     #[cfg(not(test))]
     fn from(s: &[T]) -> Vec<T> {
         s.to_vec()
@@ -2724,6 +2731,13 @@ impl<T: Clone> From<&[T]> for Vec<T> {
 
 #[stable(feature = "vec_from_mut", since = "1.19.0")]
 impl<T: Clone> From<&mut [T]> for Vec<T> {
+    /// Allocate a `Vec<T>` and fill it by cloning `s`'s items.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(Vec::from(&mut [1, 2, 3][..]), vec![1, 2, 3]);
+    /// ```
     #[cfg(not(test))]
     fn from(s: &mut [T]) -> Vec<T> {
         s.to_vec()
@@ -2740,6 +2754,13 @@ impl<T, const N: usize> From<[T; N]> for Vec<T> {
     fn from(s: [T; N]) -> Vec<T> {
         <[T]>::into_vec(box s)
     }
+    /// Allocate a `Vec<T>` and move `s`'s items into it.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(Vec::from([1, 2, 3]), vec![1, 2, 3]);
+    /// ```
     #[cfg(test)]
     fn from(s: [T; N]) -> Vec<T> {
         crate::slice::into_vec(box s)
@@ -2751,6 +2772,20 @@ impl<'a, T> From<Cow<'a, [T]>> for Vec<T>
 where
     [T]: ToOwned<Owned = Vec<T>>,
 {
+    /// Convert a clone-on-write slice into a vector.
+    ///
+    /// If `s` already owns a `Vec<T>`, it will be returned directly.
+    /// If `s` is borrowing a slice, a new `Vec<T>` will be allocated and
+    /// filled by cloning `s`'s items into it.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use std::borrow::Cow;
+    /// let o: Cow<[i32]> = Cow::Owned(vec![1, 2, 3]);
+    /// let b: Cow<[i32]> = Cow::Borrowed(&[1, 2, 3]);
+    /// assert_eq!(Vec::from(o), Vec::from(b));
+    /// ```
     fn from(s: Cow<'a, [T]>) -> Vec<T> {
         s.into_owned()
     }
@@ -2760,6 +2795,15 @@ where
 #[cfg(not(test))]
 #[stable(feature = "vec_from_box", since = "1.18.0")]
 impl<T, A: Allocator> From<Box<[T], A>> for Vec<T, A> {
+    /// Convert a boxed slice into a vector.
+    /// No heap allocation is performed, and the items are not copied.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let b: Box<[i32]> = vec![1, 2, 3].into_boxed_slice();
+    /// assert_eq!(Vec::from(b), vec![1, 2, 3]);
+    /// ```
     fn from(s: Box<[T], A>) -> Self {
         let len = s.len();
         Self { buf: RawVec::from_box(s), len }
@@ -2770,13 +2814,30 @@ impl<T, A: Allocator> From<Box<[T], A>> for Vec<T, A> {
 #[cfg(not(test))]
 #[stable(feature = "box_from_vec", since = "1.20.0")]
 impl<T, A: Allocator> From<Vec<T, A>> for Box<[T], A> {
+    /// Convert a vector into a boxed slice.
+    ///
+    /// If `v` has excess capacity, its items will be moved into a
+    /// newly-allocated buffer with exactly the right capacity.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(Box::from(vec![1, 2, 3]), vec![1, 2, 3].into_boxed_slice());
+    /// ```
     fn from(v: Vec<T, A>) -> Self {
         v.into_boxed_slice()
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl From<&str> for Vec<u8> {
+    /// Allocate a `Vec<u8>` and fill it with a UTF-8 string.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// assert_eq!(Vec::from("123"), vec![b'1', b'2', b'3']);
+    /// ```
     fn from(s: &str) -> Vec<u8> {
         From::from(s.as_bytes())
     }