Rollup merge of #89869 - kpreid:from-doc, r=yaahc

Add documentation to more `From::from` implementations.

For users looking at documentation through IDE popups, this gives them relevant information rather than the generic trait documentation wording “Performs the conversion”. For users reading the documentation for a specific type for any reason, this informs them when the conversion may allocate or copy significant memory versus when it is always a move or cheap copy.

Notes on specific cases:
* The new documentation for `From<T> for T` explains that it is not a conversion at all.
* Also documented `impl<T, U> Into<U> for T where U: From<T>`, the other central blanket implementation of conversion.
* The new documentation for construction of maps and sets from arrays of keys mentions the handling of duplicates. Future work could be to do this for *all* code paths that convert an iterable to a map or set.
* I did not add documentation to conversions of a specific error type to a more general error type.
* I did not add documentation to unstable code.

This change was prepared by searching for the text "From<... for" and so may have missed some cases that for whatever reason did not match. I also looked for `Into` impls but did not find any worth documenting by the above criteria.

library/alloc/src/collections/btree/map.rs

@@ -2052,6 +2052,8 @@ where
 
 #[stable(feature = "std_collections_from_array", since = "1.56.0")]
 impl<K: Ord, V, const N: usize> From<[(K, V); N]> for BTreeMap<K, V> {
+    /// Converts a `[(K, V); N]` into a `BTreeMap<(K, V)>`.
+    ///
     /// ```
     /// use std::collections::BTreeMap;
     ///

library/alloc/src/collections/btree/set.rs

@@ -1097,6 +1097,8 @@ impl<T: Ord> FromIterator<T> for BTreeSet<T> {
 
 #[stable(feature = "std_collections_from_array", since = "1.56.0")]
 impl<T: Ord, const N: usize> From<[T; N]> for BTreeSet<T> {
+    /// Converts a `[T; N]` into a `BTreeSet<T>`.
+    ///
     /// ```
     /// use std::collections::BTreeSet;
     ///

library/alloc/src/collections/linked_list.rs

@@ -1953,6 +1953,8 @@ impl<T: Hash> Hash for LinkedList<T> {
 
 #[stable(feature = "std_collections_from_array", since = "1.56.0")]
 impl<T, const N: usize> From<[T; N]> for LinkedList<T> {
+    /// Converts a `[T; N]` into a `LinkedList<T>`.
+    ///
     /// ```
     /// use std::collections::LinkedList;
     ///

library/alloc/src/collections/vec_deque/mod.rs

@@ -3049,6 +3049,8 @@ impl<T, A: Allocator> From<VecDeque<T, A>> for Vec<T, A> {
 
 #[stable(feature = "std_collections_from_array", since = "1.56.0")]
 impl<T, const N: usize> From<[T; N]> for VecDeque<T> {
+    /// Converts a `[T; N]` into a `VecDeque<T>`.
+    ///
     /// ```
     /// use std::collections::VecDeque;
     ///

library/alloc/src/vec/mod.rs

@@ -2906,17 +2906,18 @@ impl<T: Clone> From<&mut [T]> for Vec<T> {
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "vec_from_array", since = "1.44.0")]
 impl<T, const N: usize> From<[T; N]> for Vec<T> {
-    #[cfg(not(test))]
-    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(not(test))]
+    fn from(s: [T; N]) -> Vec<T> {
+        <[T]>::into_vec(box s)
+    }
+
     #[cfg(test)]
     fn from(s: [T; N]) -> Vec<T> {
         crate::slice::into_vec(box s)

library/core/src/cell.rs

@@ -315,6 +315,7 @@ impl<T: Ord + Copy> Ord for Cell<T> {
 #[stable(feature = "cell_from", since = "1.12.0")]
 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 impl<T> const From<T> for Cell<T> {
+    /// Creates a new `Cell<T>` containing the given value.
     fn from(t: T) -> Cell<T> {
         Cell::new(t)
     }
@@ -1244,6 +1245,7 @@ impl<T: ?Sized + Ord> Ord for RefCell<T> {
 #[stable(feature = "cell_from", since = "1.12.0")]
 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 impl<T> const From<T> for RefCell<T> {
+    /// Creates a new `RefCell<T>` containing the given value.
     fn from(t: T) -> RefCell<T> {
         RefCell::new(t)
     }
@@ -1979,6 +1981,7 @@ impl<T: Default> Default for UnsafeCell<T> {
 #[stable(feature = "cell_from", since = "1.12.0")]
 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 impl<T> const From<T> for UnsafeCell<T> {
+    /// Creates a new `UnsafeCell<T>` containing the given value.
     fn from(t: T) -> UnsafeCell<T> {
         UnsafeCell::new(t)
     }

library/core/src/convert/mod.rs

@@ -538,6 +538,10 @@ impl<T, U> const Into<U> for T
 where
     U: ~const From<T>,
 {
+    /// Calls `U::from(self)`.
+    ///
+    /// That is, this conversion is whatever the implementation of
+    /// <code>[From]&lt;T&gt; for U</code> chooses to do.
     fn into(self) -> U {
         U::from(self)
     }
@@ -547,6 +551,7 @@ where
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 impl<T> const From<T> for T {
+    /// Returns the argument unchanged.
     fn from(t: T) -> T {
         t
     }

library/core/src/lazy.rs

@@ -75,6 +75,7 @@ impl<T: Eq> Eq for OnceCell<T> {}
 
 #[unstable(feature = "once_cell", issue = "74465")]
 impl<T> const From<T> for OnceCell<T> {
+    /// Creates a new `OnceCell<T>` which already contains the given `value`.
     fn from(value: T) -> Self {
         OnceCell { inner: UnsafeCell::new(Some(value)) }
     }

library/core/src/ptr/non_null.rs

@@ -721,6 +721,9 @@ impl<T: ?Sized> const From<Unique<T>> for NonNull<T> {
 #[stable(feature = "nonnull", since = "1.25.0")]
 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 impl<T: ?Sized> const From<&mut T> for NonNull<T> {
+    /// Converts a `&mut T` to a `NonNull<T>`.
+    ///
+    /// This conversion is safe and infallible since references cannot be null.
     #[inline]
     fn from(reference: &mut T) -> Self {
         // SAFETY: A mutable reference cannot be null.
@@ -731,6 +734,9 @@ impl<T: ?Sized> const From<&mut T> for NonNull<T> {
 #[stable(feature = "nonnull", since = "1.25.0")]
 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 impl<T: ?Sized> const From<&T> for NonNull<T> {
+    /// Converts a `&T` to a `NonNull<T>`.
+    ///
+    /// This conversion is safe and infallible since references cannot be null.
     #[inline]
     fn from(reference: &T) -> Self {
         // SAFETY: A reference cannot be null, so the conditions for

library/core/src/ptr/unique.rs

@@ -178,6 +178,9 @@ impl<T: ?Sized> fmt::Pointer for Unique<T> {
 
 #[unstable(feature = "ptr_internals", issue = "none")]
 impl<T: ?Sized> const From<&mut T> for Unique<T> {
+    /// Converts a `&mut T` to a `Unique<T>`.
+    ///
+    /// This conversion is infallible since references cannot be null.
     #[inline]
     fn from(reference: &mut T) -> Self {
         // SAFETY: A mutable reference cannot be null

library/core/src/sync/atomic.rs

@@ -1295,6 +1295,7 @@ impl const From<bool> for AtomicBool {
 #[stable(feature = "atomic_from", since = "1.23.0")]
 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 impl<T> const From<*mut T> for AtomicPtr<T> {
+    /// Converts a `*mut T` into an `AtomicPtr<T>`.
     #[inline]
     fn from(p: *mut T) -> Self {
         Self::new(p)

library/core/src/task/poll.rs

@@ -243,7 +243,7 @@ impl<T, E> Poll<Option<Result<T, E>>> {
 #[stable(feature = "futures_api", since = "1.36.0")]
 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 impl<T> const From<T> for Poll<T> {
-    /// Convert to a `Ready` variant.
+    /// Moves the value into a [`Poll::Ready`] to make a `Poll<T>`.
     ///
     /// # Example
     ///

library/std/src/ffi/c_str.rs

@@ -871,6 +871,8 @@ impl Borrow<CStr> for CString {
 
 #[stable(feature = "cstring_from_cow_cstr", since = "1.28.0")]
 impl<'a> From<Cow<'a, CStr>> for CString {
+    /// Converts a `Cow<'a, CStr>` into a `CString`, by copying the contents if they are
+    /// borrowed.
     #[inline]
     fn from(s: Cow<'a, CStr>) -> Self {
         s.into_owned()
@@ -879,6 +881,8 @@ impl<'a> From<Cow<'a, CStr>> for CString {
 
 #[stable(feature = "box_from_c_str", since = "1.17.0")]
 impl From<&CStr> for Box<CStr> {
+    /// Converts a `&CStr` into a `Box<CStr>`,
+    /// by copying the contents into a newly allocated [`Box`].
     fn from(s: &CStr) -> Box<CStr> {
         let boxed: Box<[u8]> = Box::from(s.to_bytes_with_nul());
         unsafe { Box::from_raw(Box::into_raw(boxed) as *mut CStr) }
@@ -887,6 +891,8 @@ impl From<&CStr> for Box<CStr> {
 
 #[stable(feature = "box_from_cow", since = "1.45.0")]
 impl From<Cow<'_, CStr>> for Box<CStr> {
+    /// Converts a `Cow<'a, CStr>` into a `Box<CStr>`,
+    /// by copying the contents if they are borrowed.
     #[inline]
     fn from(cow: Cow<'_, CStr>) -> Box<CStr> {
         match cow {
@@ -984,6 +990,8 @@ impl From<CString> for Arc<CStr> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<&CStr> for Arc<CStr> {
+    /// Converts a `&CStr` into a `Arc<CStr>`,
+    /// by copying the contents into a newly allocated [`Arc`].
     #[inline]
     fn from(s: &CStr) -> Arc<CStr> {
         let arc: Arc<[u8]> = Arc::from(s.to_bytes_with_nul());
@@ -1004,6 +1012,8 @@ impl From<CString> for Rc<CStr> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<&CStr> for Rc<CStr> {
+    /// Converts a `&CStr` into a `Rc<CStr>`,
+    /// by copying the contents into a newly allocated [`Rc`].
     #[inline]
     fn from(s: &CStr) -> Rc<CStr> {
         let rc: Rc<[u8]> = Rc::from(s.to_bytes_with_nul());
@@ -1530,6 +1540,7 @@ impl ToOwned for CStr {
 
 #[stable(feature = "cstring_asref", since = "1.7.0")]
 impl From<&CStr> for CString {
+    /// Copies the contents of the `&CStr` into a newly allocated `CString`.
     fn from(s: &CStr) -> CString {
         s.to_owned()
     }

library/std/src/ffi/os_str.rs

@@ -452,6 +452,8 @@ impl From<String> for OsString {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized + AsRef<OsStr>> From<&T> for OsString {
+    /// Copies any value implementing <code>[AsRef]&lt;[OsStr]&gt;</code>
+    /// into a newly allocated [`OsString`].
     fn from(s: &T) -> OsString {
         s.as_ref().to_os_string()
     }
@@ -942,6 +944,7 @@ impl OsStr {
 
 #[stable(feature = "box_from_os_str", since = "1.17.0")]
 impl From<&OsStr> for Box<OsStr> {
+    /// Copies the string into a newly allocated <code>[Box]&lt;[OsStr]&gt;</code>.
     #[inline]
     fn from(s: &OsStr) -> Box<OsStr> {
         let rw = Box::into_raw(s.inner.into_box()) as *mut OsStr;
@@ -951,6 +954,8 @@ impl From<&OsStr> for Box<OsStr> {
 
 #[stable(feature = "box_from_cow", since = "1.45.0")]
 impl From<Cow<'_, OsStr>> for Box<OsStr> {
+    /// Converts a `Cow<'a, OsStr>` into a <code>[Box]&lt;[OsStr]&gt;</code>,
+    /// by copying the contents if they are borrowed.
     #[inline]
     fn from(cow: Cow<'_, OsStr>) -> Box<OsStr> {
         match cow {
@@ -1000,6 +1005,7 @@ impl From<OsString> for Arc<OsStr> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<&OsStr> for Arc<OsStr> {
+    /// Copies the string into a newly allocated <code>[Arc]&lt;[OsStr]&gt;</code>.
     #[inline]
     fn from(s: &OsStr) -> Arc<OsStr> {
         let arc = s.inner.into_arc();
@@ -1020,6 +1026,7 @@ impl From<OsString> for Rc<OsStr> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<&OsStr> for Rc<OsStr> {
+    /// Copies the string into a newly allocated <code>[Rc]&lt;[OsStr]&gt;</code>.
     #[inline]
     fn from(s: &OsStr) -> Rc<OsStr> {
         let rc = s.inner.into_rc();
@@ -1029,6 +1036,7 @@ impl From<&OsStr> for Rc<OsStr> {
 
 #[stable(feature = "cow_from_osstr", since = "1.28.0")]
 impl<'a> From<OsString> for Cow<'a, OsStr> {
+    /// Moves the string into a [`Cow::Owned`].
     #[inline]
     fn from(s: OsString) -> Cow<'a, OsStr> {
         Cow::Owned(s)
@@ -1037,6 +1045,7 @@ impl<'a> From<OsString> for Cow<'a, OsStr> {
 
 #[stable(feature = "cow_from_osstr", since = "1.28.0")]
 impl<'a> From<&'a OsStr> for Cow<'a, OsStr> {
+    /// Converts the string reference into a [`Cow::Borrowed`].
     #[inline]
     fn from(s: &'a OsStr) -> Cow<'a, OsStr> {
         Cow::Borrowed(s)
@@ -1045,6 +1054,7 @@ impl<'a> From<&'a OsStr> for Cow<'a, OsStr> {
 
 #[stable(feature = "cow_from_osstr", since = "1.28.0")]
 impl<'a> From<&'a OsString> for Cow<'a, OsStr> {
+    /// Converts the string reference into a [`Cow::Borrowed`].
     #[inline]
     fn from(s: &'a OsString) -> Cow<'a, OsStr> {
         Cow::Borrowed(s.as_os_str())
@@ -1053,6 +1063,8 @@ impl<'a> From<&'a OsString> for Cow<'a, OsStr> {
 
 #[stable(feature = "osstring_from_cow_osstr", since = "1.28.0")]
 impl<'a> From<Cow<'a, OsStr>> for OsString {
+    /// Converts a `Cow<'a, OsStr>` into an [`OsString`],
+    /// by copying the contents if they are borrowed.
     #[inline]
     fn from(s: Cow<'a, OsStr>) -> Self {
         s.into_owned()

library/std/src/path.rs

@@ -1600,7 +1600,7 @@ impl From<Cow<'_, Path>> for Box<Path> {
 
 #[stable(feature = "path_buf_from_box", since = "1.18.0")]
 impl From<Box<Path>> for PathBuf {
-    /// Converts a `Box<Path>` into a `PathBuf`
+    /// Converts a <code>[Box]&lt;[Path]&gt;</code> into a [`PathBuf`].
     ///
     /// This conversion does not allocate or copy memory.
     #[inline]
@@ -1611,7 +1611,7 @@ impl From<Box<Path>> for PathBuf {
 
 #[stable(feature = "box_from_path_buf", since = "1.20.0")]
 impl From<PathBuf> for Box<Path> {
-    /// Converts a `PathBuf` into a `Box<Path>`
+    /// Converts a [`PathBuf`] into a <code>[Box]&lt;[Path]&gt;</code>.
     ///
     /// This conversion currently should not allocate memory,
     /// but this behavior is not guaranteed on all platforms or in all future versions.
@@ -1631,7 +1631,7 @@ impl Clone for Box<Path> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized + AsRef<OsStr>> From<&T> for PathBuf {
-    /// Converts a borrowed `OsStr` to a `PathBuf`.
+    /// Converts a borrowed [`OsStr`] to a [`PathBuf`].
     ///
     /// Allocates a [`PathBuf`] and copies the data into it.
     #[inline]

library/std/src/process.rs

@@ -1277,7 +1277,7 @@ impl fmt::Debug for Stdio {
 
 #[stable(feature = "stdio_from", since = "1.20.0")]
 impl From<ChildStdin> for Stdio {
-    /// Converts a `ChildStdin` into a `Stdio`
+    /// Converts a [`ChildStdin`] into a [`Stdio`].
     ///
     /// # Examples
     ///
@@ -1306,7 +1306,7 @@ impl From<ChildStdin> for Stdio {
 
 #[stable(feature = "stdio_from", since = "1.20.0")]
 impl From<ChildStdout> for Stdio {
-    /// Converts a `ChildStdout` into a `Stdio`
+    /// Converts a [`ChildStdout`] into a [`Stdio`].
     ///
     /// # Examples
     ///
@@ -1335,7 +1335,7 @@ impl From<ChildStdout> for Stdio {
 
 #[stable(feature = "stdio_from", since = "1.20.0")]
 impl From<ChildStderr> for Stdio {
-    /// Converts a `ChildStderr` into a `Stdio`
+    /// Converts a [`ChildStderr`] into a [`Stdio`].
     ///
     /// # Examples
     ///
@@ -1366,7 +1366,7 @@ impl From<ChildStderr> for Stdio {
 
 #[stable(feature = "stdio_from", since = "1.20.0")]
 impl From<fs::File> for Stdio {
-    /// Converts a `File` into a `Stdio`
+    /// Converts a [`File`](fs::File) into a [`Stdio`].
     ///
     /// # Examples
     ///