ahl/schema

schema -- A Generic Schema Derivation for Rust

This crate is pretty simple in concept: it exposes a trait, Schema, and a derive macro for that trait. The trait has a single method, Schema::schema(), that produces a struct of type syn::DeriveInput (which this crate re-exports) that represents the schema for the type on which schema() is invoked.

#[derive(Schema)]
struct MyType {
    a: u32,
}

If we println!("{:#?}", MyType::schema()) we get:

DeriveInput {
    attrs: [],
    vis: Inherited,
    ident: Ident(
        MyType,
    ),
    generics: Generics {
        lt_token: None,
        params: [],
        gt_token: None,
        where_clause: None,
    },
    data: Struct(
        DataStruct {
            struct_token: Struct,
            fields: Named(
                FieldsNamed {
                    brace_token: Brace,
                    named: [
                        Field {
                            attrs: [],
                            vis: Inherited,
                            ident: Some(
                                Ident(
                                    a,
                                ),
                            ),
                            colon_token: Some(
                                Colon,
                            ),
                            ty: Path(
                                TypePath {
                                    qself: None,
                                    path: Path {
                                        leading_colon: None,
                                        segments: [
                                            PathSegment {
                                                ident: Ident(
                                                    u32,
                                                ),
                                                arguments: None,
                                            },
                                        ],
                                    },
                                },
                            ),
                        },
                    ],
                },
            ),
            semi_token: None,
        },
    ),
}

---

FAQ

1. Ok, so what is this for?

Right. I'm not 100% sure. It seems potentially useful for developing or testing proc macros. It also seems useful in situations where one might make a proc macro but can't be bothered to deal with defining a trait on a bunch of base types.

2. Why did you make this?

Fair. I was doing some work with OpenAPI and JSON Schema and found it surprising I couldn't find a generic schema crate, something that provided a subset of Java reflection, say.

I found someone squatting on the name schema on crates.io. "Aha!" I thought, "I'll teach that evil squatter a lesson!" So I mailed him and he responded in the worst possible way: "okay, I sent an invite." Suddenly I was the evil squatter.

Months passed as they do.

I had the odd idea of a derive macro that parsed an item and then teleported that structure from proc macro context into program context. I usually want an actual use case to drive a project, but this seemed like a neat trick. It led to weird code like this (i.e. not exactly this):

impl Teleporter for TokenStream {
    fn teleport(&self) -> TokenStream {
        quote! {
            quote! {
                #self
            }
        }
    }

    //...
}

I'm not really sure what this crate is or if it's useful, but it was an interesting experiment. If you find it useful, have a different idea for how it should work, or just think schema should be a totally different crate: let me know.

3. I'm using it and...

Please tell me about what you're doing with it... you were saying...

4. ... I'm seeing an error on some type

Please file an issue. PRs welcome too.