From 18728185702c3f64c4e19220676f5e0d08c9885c Mon Sep 17 00:00:00 2001
From: Sergio Benitez <sb@sergio.bz>
Date: Fri, 16 Apr 2021 01:11:57 -0700
Subject: [PATCH] Support type generics in 'Responder' derive.

---
 core/codegen/src/derive/responder.rs |  7 +++++-
 core/codegen/src/lib.rs              | 37 ++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/core/codegen/src/derive/responder.rs b/core/codegen/src/derive/responder.rs
index 136656c3..6454f5aa 100644
--- a/core/codegen/src/derive/responder.rs
+++ b/core/codegen/src/derive/responder.rs
@@ -18,13 +18,18 @@ struct FieldAttr {
 
 pub fn derive_responder(input: proc_macro::TokenStream) -> TokenStream {
     DeriveGenerator::build_for(input, quote!(impl<'__r, '__o: '__r> ::rocket::response::Responder<'__r, '__o>))
-        .support(Support::Struct | Support::Enum | Support::Lifetime)
+        .support(Support::Struct | Support::Enum | Support::Lifetime | Support::Type)
         .replace_generic(1, 0)
+        .type_bound(quote!(::rocket::response::Responder<'__r, '__o>))
         .validator(ValidatorBuild::new()
             .input_validate(|_, i| match i.generics().lifetimes().count() > 1 {
                 true => Err(i.generics().span().error("only one lifetime is supported")),
                 false => Ok(())
             })
+            .input_validate(|_, i| match i.generics().type_params().count() > 1 {
+                true => Err(i.generics().span().error("only one type generic is supported")),
+                false => Ok(())
+            })
             .fields_validate(|_, fields| match fields.is_empty() {
                 true => return Err(fields.span().error("need at least one field")),
                 false => Ok(())
diff --git a/core/codegen/src/lib.rs b/core/codegen/src/lib.rs
index 2f4a2794..f2f83367 100644
--- a/core/codegen/src/lib.rs
+++ b/core/codegen/src/lib.rs
@@ -545,6 +545,8 @@ pub fn derive_from_form(input: TokenStream) -> TokenStream {
 /// }
 /// ```
 ///
+/// # Semantics
+///
 /// The derive generates an implementation of the [`Responder`] trait for the
 /// decorated enum or structure. The derive uses the _first_ field of a variant
 /// or structure to generate a [`Response`]. As such, the type of the first
@@ -580,6 +582,8 @@ pub fn derive_from_form(input: TokenStream) -> TokenStream {
 ///
 /// Decorating the first field with `#[response(ignore)]` has no effect.
 ///
+/// # Field Attribute
+///
 /// Additionally, the `response` attribute can be used on named structures and
 /// enum variants to override the status and/or content-type of the [`Response`]
 /// produced by the generated implementation. The `response` attribute used in
@@ -646,6 +650,39 @@ pub fn derive_from_form(input: TokenStream) -> TokenStream {
 /// [`Response`]: ../rocket/struct.Response.html
 /// [`Response::set_header()`]: ../rocket/response/struct.Response.html#method.set_header
 /// [`ContentType::parse_flexible()`]: ../rocket/http/struct.ContentType.html#method.parse_flexible
+///
+/// # Generics
+///
+/// The derive accepts at most one type generic and at most one lifetime
+/// generic. If a type generic is present, the generated implementation will
+/// require a bound of `Responder` for the generic. As such, the generic should
+/// be used as a `Responder`:
+///
+/// ```rust
+/// # #[macro_use] extern crate rocket;
+/// #[derive(Responder)]
+/// #[response(status = 404, content_type = "html")]
+/// struct NotFoundHtml<T>(T);
+/// ```
+///
+/// If a lifetime generic is present, it will be used as the second lifetime
+/// paramter `'o` parameter in `impl Responder<'r, 'o>`:
+///
+/// ```rust
+/// # #[macro_use] extern crate rocket;
+/// #[derive(Responder)]
+/// #[response(status = 404, content_type = "html")]
+/// struct NotFoundHtmlString<'o>(&'o str);
+/// ```
+///
+/// Both a type generic and lifetime generic may be used:
+///
+/// ```rust
+/// # #[macro_use] extern crate rocket;
+/// # use rocket::response::Responder;
+/// #[derive(Responder)]
+/// struct SomeResult<'o, T>(Result<T, &'o str>);
+/// ```
 #[proc_macro_derive(Responder, attributes(response))]
 pub fn derive_responder(input: TokenStream) -> TokenStream {
     emit!(derive::responder::derive_responder(input))