How to make sure that a Rust attribute proc macro runs before the compiler tries parsing the item?

I’m trying to make a macro that adds some extra syntax to struct type declarations.
For simplicity, here’s a toy example: a macro that replaces "function call"-style type declarations with normal ones.

#[my_macro]
struct Point {
  x: LiteralType("f32"),
  y: LiteralType("f32"),
}

should turn into

struct Point {
  x: f32,
  y: f32,
}

From the output of my code and cargo expand, I can tell that my code produces the correct syntax.
However, before that happens, the compiler has already tried parsing the LiteralType("f32") syntax, failed and emitted an error saying as much, and now the whole build fails.

I think I need to somehow force the compiler to run my macro’s code to completion before trying to parse it according to the regular Rust rules. I think it might be possible to do using functional macros, because those can accept syntax that’s much more unusual, but I’d prefer to stick with attribute macros because those provide a secondary input for attributes.

>Solution :

You cannot do that with attribute macros, they always validate their items.

But as you said, you can do that with function-like macros.