Rollup merge of rust-lang#33277 - birkenfeld:fmt-named-dollar-args, r=steveklabnik

Fix std::fmt format spec: named args are allowed with "$" syntax

Author: Manishearth

src/libcollections/fmt.rs

@@ -333,7 +333,7 @@
 //! precision := count | '*'
 //! type := identifier | ''
 //! count := parameter | integer
-//! parameter := integer '$'
+//! parameter := argument '$'
 //! ```
 //!
 //! # Formatting Parameters
@@ -403,11 +403,12 @@
 //! println!("Hello {:5}!", "x");
 //! println!("Hello {:1$}!", "x", 5);
 //! println!("Hello {1:0$}!", 5, "x");
+//! println!("Hello {:width$}!", "x", width = 5);
 //! ```
 //!
 //! Referring to an argument with the dollar syntax does not affect the "next
-//! argument" counter, so it's usually a good idea to refer to all arguments by
-//! their position explicitly.
+//! argument" counter, so it's usually a good idea to refer to arguments by
+//! position, or use named arguments.
 //!
 //! ## Precision
 //!
@@ -426,7 +427,7 @@
 //!
 //!    the integer `N` itself is the precision.
 //!
-//! 2. An integer followed by dollar sign `.N$`:
+//! 2. An integer or name followed by dollar sign `.N$`:
 //!
 //!    use format *argument* `N` (which must be a `usize`) as the precision.
 //!
@@ -456,6 +457,10 @@
 //! // Hello {next arg (x)} is {arg 2 (0.01) with precision
 //! //                          specified in its predecessor (5)}
 //! println!("Hello {} is {2:.*}",   "x", 5, 0.01);
+//!
+//! // Hello {next arg (x)} is {arg "number" (0.01) with precision specified
+//! //                          in arg "prec" (5)}
+//! println!("Hello {} is {number:.prec$}", "x", prec = 5, number = 0.01);
 //! ```
 //!
 //! All print the same thing: