Add examples in stdlib demonstrating the use syntax

Author: spastorino

library/core/src/clone.rs

@@ -196,6 +196,25 @@ pub macro Clone($item:item) {
 ///
 /// The `UseCloned` trait does not provide a method; instead, it indicates that
 /// `Clone::clone` is lightweight, and allows the use of the `.use` syntax.
+///
+/// ## .use postfix syntax
+///
+/// Values can be `.use`d by adding `.use` postfix to the value you want to use.
+///
+/// ```ignore (this won't work until we land use)
+/// fn foo(f: Foo) {
+///     // if `Foo` implements `Copy` f would be copied into x.
+///     // if `Foo` implements `UseCloned` f would be cloned into x.
+///     // otherwise f would be moved into x.
+///     let x = f.use;
+///     // ...
+/// }
+/// ```
+///
+/// ## use closures
+///
+/// Use closures allow captured values to be automatically used.
+/// This is similar to have a closure that you would call `.use` over each captured value.
 #[unstable(feature = "ergonomic_clones", issue = "132290")]
 #[cfg_attr(not(bootstrap), lang = "use_cloned")]
 pub trait UseCloned: Clone {

library/std/src/keyword_docs.rs

@@ -2121,8 +2121,8 @@ mod unsafe_keyword {}
 
 #[doc(keyword = "use")]
 //
-/// Import or rename items from other crates or modules, or specify precise capturing
-/// with `use<..>`.
+/// Import or rename items from other crates or modules, use values under ergonomic clones
+/// semantic, or specify precise capturing with `use<..>`.
 ///
 /// ## Importing items
 ///
@@ -2177,6 +2177,11 @@ mod unsafe_keyword {}
 /// The differences about paths and the `use` keyword between the 2015 and 2018 editions
 /// can also be found in the [Reference][ref-use-decls].
 ///
+/// ## Ergonomic clones
+///
+/// Use a values, copying its content if the value implements `Copy`, cloning the contents if the
+/// value implements `UseCloned` or moving it otherwise.
+///
 /// ## Precise capturing
 ///
 /// The `use<..>` syntax is used within certain `impl Trait` bounds to control which generic