# Objective
Requires are currently more verbose than they need to be. People would
like to define inline component values. Additionally, the current
`#[require(Foo(custom_constructor))]` and `#[require(Foo(|| Foo(10))]`
syntax doesn't really make sense within the context of the Rust type
system. #18309 was an attempt to improve ergonomics for some cases, but
it came at the cost of even more weirdness / unintuitive behavior. Our
approach as a whole needs a rethink.
## Solution
Rework the `#[require()]` syntax to make more sense. This is a breaking
change, but I think it will make the system easier to learn, while also
improving ergonomics substantially:
```rust
#[derive(Component)]
#[require(
A, // this will use A::default()
B(1), // inline tuple-struct value
C { value: 1 }, // inline named-struct value
D::Variant, // inline enum variant
E::SOME_CONST, // inline associated const
F::new(1), // inline constructor
G = returns_g(), // an expression that returns G
H = SomethingElse::new(), // expression returns SomethingElse, where SomethingElse: Into<H>
)]
struct Foo;
```
## Migration Guide
Custom-constructor requires should use the new expression-style syntax:
```rust
// before
#[derive(Component)]
#[require(A(returns_a))]
struct Foo;
// after
#[derive(Component)]
#[require(A = returns_a())]
struct Foo;
```
Inline-closure-constructor requires should use the inline value syntax
where possible:
```rust
// before
#[derive(Component)]
#[require(A(|| A(10))]
struct Foo;
// after
#[derive(Component)]
#[require(A(10)]
struct Foo;
```
In cases where that is not possible, use the expression-style syntax:
```rust
// before
#[derive(Component)]
#[require(A(|| A(10))]
struct Foo;
// after
#[derive(Component)]
#[require(A = A(10)]
struct Foo;
```
---------
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: François Mockers <mockersf@gmail.com>
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