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/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
     ///