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core/codegen/src/derive/from_param.rs

@@ -11,21 +11,22 @@ pub fn derive_from_param(input: proc_macro::TokenStream) -> TokenStream {
         .validator(ValidatorBuild::new()
                    .fields_validate(|_, fields| {
                        if !fields.is_empty() {
-                           return Err(fields.span().error("Only empty enums are accepted"));
+                           return Err(
+                               fields.span().error("Only enums without data fields are supported")
+                           );
                        }
                        Ok(())
                    })
         )
         .inner_mapper(MapperBuild::new()
                       .enum_map(|_, data| {
-                          let mut matches = vec![];
-
-                          for field in data.variants() {
+                          let matches = data.variants().map(|field| {
                               let field_name = &field;
-                              matches.push(quote!(
+                              quote!(
                                   stringify!(#field_name) => Ok(Self::#field_name),
-                              ))
-                          }
+                              )
+
+                          });
 
                           quote! {
                               type Error = &'a str;

core/codegen/src/lib.rs

@@ -777,20 +777,26 @@ pub fn derive_from_form(input: TokenStream) -> TokenStream {
 /// Derive for the [`FromParam`] trait.
 ///
 /// The [`FromParam`] derive can be applied to enums with nullary
-/// (zero-length) fields:
+/// (zero-length) fields. To implement FromParam, the function matches each variant
+/// to its stringified field name (case sensitive):
 ///
 /// ```rust
 /// # #[macro_use] extern crate rocket;
 /// #
 /// use rocket::request::FromParam;
 ///
-/// #[derive(FromParam)]
+/// #[derive(FromParam, Debug, PartialEq)]
 /// enum MyParam {
 ///     A,
 ///     B,
 /// }
 ///
 /// assert_eq!(MyParam::from_param("A").unwrap(), MyParam::A);
+/// assert_eq!(MyParam::from_param("B").unwrap(), MyParam::B);
+/// assert!(MyParam::from_param("a").is_err());
+/// assert!(MyParam::from_param("b").is_err());
+/// assert!(MyParam::from_param("c").is_err());
+/// assert!(MyParam::from_param("C").is_err());
 /// ```
 ///
 /// Now `MyParam` can be used in an endpoint and will accept either `A` or `B`.

core/codegen/tests/from_param.rs

@@ -15,5 +15,5 @@ fn derive_from_param() {
     assert_eq!(test2, Test::Test2);
 
     let test3 = Test::from_param("not_test");
-    assert!(test3.is_err())
+    assert!(test3.is_err());
 }

core/codegen/tests/ui-fail-nightly/from_param.stderr

@@ -1,57 +1,53 @@
 error: named structs are not supported
- --> tests/ui-fail-stable/from_param.rs:4:1
+ --> tests/ui-fail-nightly/from_param.rs:4:1
   |
 4 | / struct Foo1 {
 5 | |     a: String
 6 | | }
   | |_^
-
-error: [note] error occurred while deriving `FromParam`
- --> tests/ui-fail-stable/from_param.rs:3:10
+  |
+note: error occurred while deriving `FromParam`
+ --> tests/ui-fail-nightly/from_param.rs:3:10
   |
 3 | #[derive(FromParam)]
   |          ^^^^^^^^^
-  |
   = note: this error originates in the derive macro `FromParam` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: named structs are not supported
- --> tests/ui-fail-stable/from_param.rs:9:1
+ --> tests/ui-fail-nightly/from_param.rs:9:1
   |
 9 | struct Foo2 {}
   | ^^^^^^^^^^^^^^
-
-error: [note] error occurred while deriving `FromParam`
- --> tests/ui-fail-stable/from_param.rs:8:10
+  |
+note: error occurred while deriving `FromParam`
+ --> tests/ui-fail-nightly/from_param.rs:8:10
   |
 8 | #[derive(FromParam)]
   |          ^^^^^^^^^
-  |
   = note: this error originates in the derive macro `FromParam` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: Only empty enums are accepted
-  --> tests/ui-fail-stable/from_param.rs:13:6
+error: Only enums without data fields are supported
+  --> tests/ui-fail-nightly/from_param.rs:13:6
    |
 13 |     A(String),
    |      ^^^^^^^^
-
-error: [note] error occurred while deriving `FromParam`
-  --> tests/ui-fail-stable/from_param.rs:11:10
+   |
+note: error occurred while deriving `FromParam`
+  --> tests/ui-fail-nightly/from_param.rs:11:10
    |
 11 | #[derive(FromParam)]
    |          ^^^^^^^^^
-   |
    = note: this error originates in the derive macro `FromParam` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: tuple structs are not supported
-  --> tests/ui-fail-stable/from_param.rs:18:1
+  --> tests/ui-fail-nightly/from_param.rs:18:1
    |
 18 | struct Foo4(usize);
    | ^^^^^^^^^^^^^^^^^^^
-
-error: [note] error occurred while deriving `FromParam`
-  --> tests/ui-fail-stable/from_param.rs:17:10
+   |
+note: error occurred while deriving `FromParam`
+  --> tests/ui-fail-nightly/from_param.rs:17:10
    |
 17 | #[derive(FromParam)]
    |          ^^^^^^^^^
-   |
    = note: this error originates in the derive macro `FromParam` (in Nightly builds, run with -Z macro-backtrace for more info)

core/codegen/tests/ui-fail-stable/from_param.stderr

@@ -28,7 +28,7 @@ error: [note] error occurred while deriving `FromParam`
   |
   = note: this error originates in the derive macro `FromParam` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: Only empty enums are accepted
+error: Only enums without data fields are supported
   --> tests/ui-fail-stable/from_param.rs:13:6
    |
 13 |     A(String),