Support #[derive(FromRequest)] on enums (#1009)

* Support `#[from_request(via(...))]` on enums

* Check `#[from_request]` on variants

* check for non enum/struct and clean up

* changelog

* changelog

* remove needless feature

* changelog ref

axum-macros/tests/from_request/fail/enum_from_request_ident_in_variant.rs

@@ -0,0 +1,12 @@
+use axum_macros::FromRequest;
+
+#[derive(FromRequest, Clone)]
+#[from_request(via(axum::Extension))]
+enum Extractor {
+    Foo {
+        #[from_request(via(axum::Extension))]
+        foo: (),
+    }
+}
+
+fn main() {}

axum-macros/tests/from_request/fail/enum_from_request_ident_in_variant.stderr

@@ -0,0 +1,5 @@
+error: `#[from_request(via(...))]` cannot be used inside variants
+ --> tests/from_request/fail/enum_from_request_ident_in_variant.rs:7:24
+  |
+7 |         #[from_request(via(axum::Extension))]
+  |                        ^^^

axum-macros/tests/from_request/fail/enum_from_request_on_variant.rs

@@ -0,0 +1,10 @@
+use axum_macros::FromRequest;
+
+#[derive(FromRequest, Clone)]
+#[from_request(via(axum::Extension))]
+enum Extractor {
+    #[from_request(via(axum::Extension))]
+    Foo,
+}
+
+fn main() {}

axum-macros/tests/from_request/fail/enum_from_request_on_variant.stderr

@@ -0,0 +1,5 @@
+error: `#[from_request(via(...))]` cannot be used on variants
+ --> tests/from_request/fail/enum_from_request_on_variant.rs:6:20
+  |
+6 |     #[from_request(via(axum::Extension))]
+  |                    ^^^

axum-macros/tests/from_request/fail/enum_no_via.rs

@@ -0,0 +1,6 @@
+use axum_macros::FromRequest;
+
+#[derive(FromRequest, Clone)]
+enum Extractor {}
+
+fn main() {}

axum-macros/tests/from_request/fail/enum_no_via.stderr

@@ -0,0 +1,7 @@
+error: missing `#[from_request(via(...))]`
+ --> tests/from_request/fail/enum_no_via.rs:3:10
+  |
+3 | #[derive(FromRequest, Clone)]
+  |          ^^^^^^^^^^^
+  |
+  = note: this error originates in the derive macro `FromRequest` (in Nightly builds, run with -Z macro-backtrace for more info)

axum-macros/tests/from_request/fail/enum_rejection_derive.rs

@@ -0,0 +1,7 @@
+use axum_macros::FromRequest;
+
+#[derive(FromRequest, Clone)]
+#[from_request(rejection_derive(!Error))]
+enum Extractor {}
+
+fn main() {}

axum-macros/tests/from_request/fail/enum_rejection_derive.stderr

@@ -0,0 +1,5 @@
+error: cannot use `rejection_derive` on enums
+ --> tests/from_request/fail/enum_rejection_derive.rs:4:16
+  |
+4 | #[from_request(rejection_derive(!Error))]
+  |                ^^^^^^^^^^^^^^^^

axum-macros/tests/from_request/fail/not_enum_or_struct.rs

@@ -0,0 +1,6 @@
+use axum_macros::FromRequest;
+
+#[derive(FromRequest)]
+union Extractor {}
+
+fn main() {}

axum-macros/tests/from_request/fail/not_enum_or_struct.stderr

@@ -0,0 +1,11 @@
+error: expected `struct` or `enum`
+ --> tests/from_request/fail/not_enum_or_struct.rs:4:1
+  |
+4 | union Extractor {}
+  | ^^^^^^^^^^^^^^^^^^
+
+error: unions cannot have zero fields
+ --> tests/from_request/fail/not_enum_or_struct.rs:4:1
+  |
+4 | union Extractor {}
+  | ^^^^^^^^^^^^^^^^^^