Improve fill_struct_fields assist

[theduke]

The Intellij plugin generates literals for some types when filling struct fields.

If I remember correctly, this is String::new() for strings, Vec::new() for vectors, 0 / 0.0 for numbers, false for bools.
I like this behaviour since it let's me know the type, and provides a valid initialization.

I checked the code and I'm happy to implement this, but I wanted to discuss desired semantics first.

Eg, strings could also be implemented as "".to_string() instead of String::new(). Similarly, Vec could be vec![]. This would make it easier to provide your own values.

For other types, we could check if the type implements Default, and use T::default().

[kiljacken]

See, I have a weird feeling about this, as that would rarely be what you wanted. Assigning unit for each field ensures that you'll get an error, instead of leaving a field be some random default value. But that's just my take on it.

[theduke]

as that would rarely be what you wanted

I also had this concern, theoretically. But I found that (at least for me) it's not an issue.

The big benefit is that you immediately know the required type for many fields. Otherwise, you have to jump back to the field name and trigger the hover to learn the type, which costs a lot of time if you don't know the struct. Or you jump to the definition and copy/paste the fields with their type, which negates the whole point.

It could also be toggled with a config flag though, I guess.

[kiljacken]

Guess we just work differently and that's fair :) For cases where I don't already know what types go where, I especially love it filling with unit, as it forces me to go look at the type, and preferably the documentation, so I can actually know what goes in the fields, and not just the types.

In any case, it should probably be behind a configuration flag no matter what, as I could easily see lots of people preferring either way.

[davyzhang]

The Intellij plugin style of filling works great when writing unit tests where some parts of the fields are irrelevant, so I just want a quick fill

[eminence]

Here's another idea that I think could be useful. Imagine this code:

struct Person {
    name: String,
    age: u8,
    employed: bool,
}

let name: String = "Alice".to_string();
let age: u8 = 45;

let person = Person {
// <|> run "Fill struct fields" here
};

Today if you use the "Fill struct fields" assist, you'll get this:

let person = Person {
  name: (),
  age: (),
  employed: (),
}

But it would be neat if RA could recognize that there are two field names that have matching local variables of the same type, and auto-fill in the struct like this:

let person = Person {
  name,
  age,
  employed: (),
}

[agluszak]

As we've settled for using todo!() and I created a separate issue for @eminence 's sugestion, I think this one can be closed. @lnicola ?