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.

[ghost]

I would like to pick this up.

[steveklabnik]

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

[ghost]

@steveklabnik sorry for take a while to process, please help me review document in PR #39096

[nikomatsakis]

What is the status of this work now? :)

[steveklabnik]

@nikomatsakis basically, until we have the bookshelf, we're not landing doc PRs until they're the last thing holding up stabilization. #39096 has some outstanding issues with it as well that need to be addressed before it's mergable.

[nikomatsakis]

@steveklabnik ok. To be clear, I think that a doc PR is indeed the only thing holding up stabilization here, so once those issues are addressed, I think we ought to merge.

[steveklabnik]

Okay. so, should we be doing that right now, to land stuff before the next release? or should we be waiting until after the branch? Has the next beta been branched?

[nikomatsakis]

@steveklabnik I don't care too much about whether this makes it into the next beta or not, we don't have to stress, but I would not delay either.

[ghost]

I haven't been available these days due to the big holiday in my country, so I couldn't reach the topic. At the moment, I have some troubles in work and cannot fix these problems, I will ask @phungleson for help. Thanks.

[ghost]

I'd like to help in this one, I don't know a lot but I'm willing to learn :)

[steveklabnik]

@z1mvader thank you! As far as docs go, #39459 has most of the work in it; maybe take a look at that and see if it makes sense to you? Leave a comment either way 😄

[est31]

As the changelog is created always when doing releases (at least this history seems to indicate that), I think nothing needs to be changed in the Rust git repo, right? So only Rust by example remains for this issue to be resolved?

[chriskrycho]

Though not required for this, I've gone ahead and made an example, so you can mark that item as done as well!

[steveklabnik]

we have both rust-lang/rust-by-example#844 and rust-lang/rust-by-example#837 so i think this is all done now!

[zeyangl]

I know this feature is already implemented and closed. I'm trying to get it to work but fails for some reason.

struct Color2
{
    pub r: i32,
}

fn handle_file()
{
    let tc = Color2{0};
}

fn main()
{
    println!("started...");
    handle_file();
}

extremely obvious code here. Whenever I compile and run it. I get this error:

error: expected identifier, found `0`
  --> src/main.rs:19:21
   |
19 |     let tc = Color2{0};
   |              ------ ^ expected identifier
   |              |
   |              while parsing this struct

I ran both 1.2.3 stable and nightly on this, same result.
Is there something really obvious that i am not getting?
Appreciate the help!

[Havvy]

Yes. The shorthand lets you substitute e.g. r: r inside of Color2{r: r} with just r so it'd be Color2{r}. It doesn't let you pass values anonymously, so you'd want to do Color2{r: 0} and not use the shorthand in your example, or if you really want to use the shorthand... let r = 0; let tc = Color2{ r };

[est31]

Adding to that, even though this works:

struct Tea(u32, u32);

fn main() {
    let t = Tea {
        0: 2,
        1: 3,
    };
}

This doesn't:

struct Tea(u32, u32);

fn main() {
    let 0 = 2;
    let 1 = 3;
    let t = Tea {
        0,
        1,
    };
}

[steveklabnik]

I'm trying to get it to work but fails for some reason.

Generally, users.rust-lang.org or StackOverflow are better places to get help than adding more comments on closed bugs! In this case, you got an answer though :)