Allow deriving MakeSchema on simple structs!

schemars_derive/.gitignore

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+/target
+**/*.rs.bk
+Cargo.lock

schemars_derive/Cargo.toml

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+[package]
+name = "schemars_derive"
+version = "0.1.0"
+authors = ["Graham Esau <gesau@hotmail.co.uk>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[lib]
+proc-macro = true
+
+[dependencies]
+proc-macro2 = "0.4"
+quote = "0.6.13"
+syn = { version = "0.15.22", features = ["visit"] } # do we need visit?
+serde_derive_internals = "0.24.1"

schemars_derive/src/lib.rs

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+#[macro_use]
+extern crate quote;
+#[macro_use]
+extern crate syn;
+
+extern crate proc_macro;
+extern crate proc_macro2;
+
+use proc_macro2::TokenStream;
+use syn::spanned::Spanned;
+use syn::{Data, DeriveInput, Fields};
+
+#[proc_macro_derive(MakeSchema, attributes(schemars, serde))]
+pub fn derive_make_schema(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+
+    let name = input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    let fn_contents = match input.data {
+        Data::Struct(data) => struct_implementation(&data.fields),
+        _ => unimplemented!("Only structs work so far!"),
+    };
+
+    let impl_block = quote! {
+        #[automatically_derived]
+        impl #impl_generics schemars::make_schema::MakeSchema for #name #ty_generics #where_clause {
+            fn make_schema(gen: &mut schemars::SchemaGenerator) -> schemars::Schema {
+                let mut o = schemars::SchemaObject {
+                    ..Default::default()
+                };
+                #fn_contents
+                o.into()
+            }
+        };
+    };
+    proc_macro::TokenStream::from(impl_block)
+}
+
+fn struct_implementation(fields: &Fields) -> TokenStream {
+    match fields {
+        Fields::Named(ref fields) => {
+            let recurse = fields.named.iter().map(|f| {
+                let name = (&f.ident).as_ref().map(ToString::to_string);
+                let ty = &f.ty;
+                quote_spanned! {f.span()=>
+                    o.properties.insert(#name.to_owned(), gen.subschema_for::<#ty>());
+                }
+            });
+            quote! {
+                #(#recurse)*
+            }
+        }
+        _ => unimplemented!("Only named fields work so far!"),
+    }
+}