95th / defaults

A better Default derive macro.

#[derive(Defaults)]

A better Default derive macro.

Examples:

You can provide default value for a non-default field.

#[derive(Defaults)]
pub struct Event {
    id: usize,
    name: String,
    #[def = "Instant::now()"]
    time: Instant,
}

You can also provide a different default value of an already default field.

#[derive(Defaults)]
pub struct Event {
    #[def = "4"]
    id: usize,
    name: String,
    #[def = "Instant::now()"]
    time: Instant,
}

You can even provide default value of enums:

#[derive(Defaults)]
#[def = "A"]
pub struct Classroom {
    A,
    B,
    C
}

But what about variants with data?

#[derive(Defaults)]
#[def = "A(100)"]
pub struct SomeEnum {
    A(usize),
    B { x: usize, y: usize },
    C
}

defaults's Issues

Decide if to either keep supporting methods or remove the restriction of having to add its own type in enum `def`

Currently we support method call in enum's def:

#[derive(Defaults)]
#[def = "foo()"]
enum Foo {
    A,
    B,
}

fn foo() -> Foo { ... }

But that means we need to write the type when directly specifying value:

#[derive(Defaults)]
#[def = "Foo::A"]
enum Foo {
    A,
    B,
}

Choose which one is required. Or find a third way.

Improve help message in case of Enum tuple variant

We produce a not-so-great error for incomplete tuple variant derive.

use defaults::Defaults;

#[derive(Defaults)]
#[def = "Foo::A"]
enum Foo {
    A(usize),
    B,
}

But it's mostly how rustc generates it.
One thing to do is improve this help:

error[E0308]: mismatched types
 --> $DIR/19-enums-tuple-variant-incomplete.rs:4:9
  |
4 | #[def = "Foo::A"]
  |         ^^^^^^^^
  |         |
  |         expected enum `Foo`, found fn item
  |         expected due to this
  |         help: use parentheses to instantiate this tuple variant: `"Foo::A"(_)`
5 | enum Foo {
6 |     A(usize),
  |     -------- fn(usize) -> Foo {Foo::A} defined here
  |
  = note: expected enum `Foo`
          found fn item `fn(usize) -> Foo {Foo::A}`

to without double quotes:

error[E0308]: mismatched types
 --> $DIR/19-enums-tuple-variant-incomplete.rs:4:9
  |
4 | #[def = "Foo::A"]
  |          ^^^^^^
  |          |
  |          expected enum `Foo`, found fn item
  |          expected due to this
  |          help: use parentheses to instantiate this tuple variant: `Foo::A(_)`
5 | enum Foo {
6 |     A(usize),
  |     -------- fn(usize) -> Foo {Foo::A} defined here
  |
  = note: expected enum `Foo`
          found fn item `fn(usize) -> Foo {Foo::A}`

To achieve this, we need to get span on string literal's interior.