Add core::mem::copy to complement core::mem::drop.

This is useful for combinators. I didn't add `clone` since you can already
use `Clone::clone` in its place; copy has no such corresponding function.

Author: jyn514

library/core/src/mem/mod.rs

@@ -964,6 +964,28 @@ pub const fn replace<T>(dest: &mut T, src: T) -> T {
 #[cfg_attr(not(test), rustc_diagnostic_item = "mem_drop")]
 pub fn drop<T>(_x: T) {}
 
+/// Bitwise-copies a value.
+///
+/// This function is not magic; it is literally defined as
+/// ```
+/// pub fn copy<T: Copy>(x: &T) -> T { *x }
+/// ```
+///
+/// It is useful when you want to pass a function pointer to a combinator, rather than defining a new closure.
+///
+/// Example:
+/// ```
+/// #![feature(mem_copy_fn)]
+/// use core::mem::copy;
+/// let result_from_ffi_function: Result<(), &i32> = Err(&1);
+/// let result_copied: Result<(), i32> = result_from_ffi_function.map_err(copy);
+/// ```
+#[inline]
+#[unstable(feature = "mem_copy_fn", issue = "98262")]
+pub fn copy<T: Copy>(x: &T) -> T {
+    *x
+}
+
 /// Interprets `src` as having type `&U`, and then reads `src` without moving
 /// the contained value.
 ///