Merge branch 'libcore_nofp'HEADmaster

1 files changed, 35 insertions, 2 deletions

diff --git a/libcore/clone.rs b/libcore/clone.rs
index a9c5684..c7a8fa8 100644
--- a/libcore/clone.rs
+++ b/libcore/clone.rs
@@ -46,9 +46,42 @@
 
 use marker::Sized;
 
-/// A common trait for cloning an object.
+/// A common trait for the ability to explicitly duplicate an object.
 ///
-/// This trait can be used with `#[derive]`.
+/// Differs from `Copy` in that `Copy` is implicit and extremely inexpensive, while
+/// `Clone` is always explicit and may or may not be expensive. In order to enforce
+/// these characteristics, Rust does not allow you to reimplement `Copy`, but you
+/// may reimplement `Clone` and run arbitrary code.
+///
+/// Since `Clone` is more general than `Copy`, you can automatically make anything
+/// `Copy` be `Clone` as well.
+///
+/// ## Derivable
+///
+/// This trait can be used with `#[derive]` if all fields are `Clone`. The `derive`d
+/// implementation of `clone()` calls `clone()` on each field.
+///
+/// ## How can I implement `Clone`?
+///
+/// Types that are `Copy` should have a trivial implementation of `Clone`. More formally:
+/// if `T: Copy`, `x: T`, and `y: &T`, then `let x = y.clone();` is equivalent to `let x = *y;`.
+/// Manual implementations should be careful to uphold this invariant; however, unsafe code
+/// must not rely on it to ensure memory safety.
+///
+/// An example is an array holding more than 32 elements of a type that is `Clone`; the standard
+/// library only implements `Clone` up until arrays of size 32. In this case, the implementation of
+/// `Clone` cannot be `derive`d, but can be implemented as:
+///
+/// ```
+/// #[derive(Copy)]
+/// struct Stats {
+///    frequencies: [i32; 100],
+/// }
+///
+/// impl Clone for Stats {
+///     fn clone(&self) -> Stats { *self }
+/// }
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Clone : Sized {
     /// Returns a copy of the value.