document field init shorthand

[nikomatsakis]

RFC 1682 added "field init shorthand" (tracking issue is #37340), which allows you to use a shorthand in struct literals:

struct Foo { x } => struct Foo { x: x }

This is implemented on nightly and we would like to stabilize it, but documentation is required. Here is a list of places where the documentation would need to be updated, kindly prepared by @havvy:

• The grammar needs updating (PR: Update struct_expr grammar for field init shorthand. #38956)
• as would adding a section to the book and the new book.
• The reference would need a paragraph explaining the sugar.
• Additionally, we may want to add examples to Rust By Example showing the syntax.
• A changelog entry is needed. @Havvy proposes this short entry: "Added field init shorthand syntax when the field and variable names match."

I'm marking this as E-mentor, since updating these docs seems like a pretty good starter bug.

[steveklabnik]

yes, i am very happy to mentor anyone who needs help with this.

I would like to pick this up.

[steveklabnik]

@HnGiang great! Please let me know if you need any assistance.