Include Snippets/Templates to completion

[mickaelistria]

Some IDEs (VSCode, Eclipse IDE) do add support for templates as completion items. See for instance https://github.com/rust-lang-nursery/rls-vscode/blob/50a4e37bc88310ae1dda88c0a399cebe4408a8c0/snippets/rust.json which contains the snippets provided by VSCode. It seems like this feature shouldn't be specific to an IDE or the other and that it could be unified/shared in common place, which could be the language server itself. I suggest RLS should try to incorporate such a snippet/template mechanism for completion items.

[Boereck]

When the basic infrastructure is in place, further possibilities could be considered:

• user defined snippets
• snippets provided by external crates
• using snippet repositories (e.g. for snippets from the Rust Cookbook)

Ideally snippets should also be able to add external crate dependencies.

[kevincox]

I think a great option would be to use the type information to auto-generate snippets such as:

• Function arguments.
• Struct fields.

For example some IDEs provide an option to fill in struct fields. It would be very cool if this was supported. I think most of the value of these Language Server snippets comes from the possible dynanacism.

struct Foo {
  a: String,
  b: u32,
}

fn new() -> Foo {
  Foo {
    a: "".to_string(),
    b: 0,
  }
}

You can imagine that I selected the completion after typing (part of) Foo and selecting the completion the fields were auto-filled. Then you should be able to jump to the "default" values to easily modify them. There would need to be some logic for what the "default" value of an arbitrary type should be but even handling common ones would be amazing. The biggest win here is that you don't have to remember the field names or types.

[kevincox]

For example a lot of the templates in this file are just expanding function or macro arguments.

https://github.com/rust-lang/rls-vscode/blob/master/snippets/rust.json

[lijinpei]

And snippet is part of LSP' Completetion Request feature https://microsoft.github.io/language-server-protocol/specification#textDocument_completion

[lijinpei]

When trying to complete the struct:

struct Foo {
    ba1: i32,
    ba2: i32,
}

RLS generates the following response,

{"label":"Foo","kind":7,"detail":"struct Foo","insertText":"Foo","insertTextFormat":2}

Looks like we can pr racer to provide more sophisticated snippets https://github.com/rust-lang/rls/blob/0d6f53e1a4adbaf7d83cdc0cb54720203fcb522e/rls/src/actions/requests.rs#L284 https://github.com/racer-rust/racer/blob/eebd594bcc0382499c830130d384773d3edd686e/src/racer/snippets.rs#L33

[Xanewok]

cc #894