Allow customising error types

num_enum/src/lib.rs

@@ -21,10 +21,11 @@ pub trait FromPrimitive: Sized {
 
 pub trait TryFromPrimitive: Sized {
     type Primitive: Copy + Eq + fmt::Debug;
+    type Error;
 
     const NAME: &'static str;
 
-    fn try_from_primitive(number: Self::Primitive) -> Result<Self, TryFromPrimitiveError<Self>>;
+    fn try_from_primitive(number: Self::Primitive) -> Result<Self, Self::Error>;
 }
 
 pub trait UnsafeFromPrimitive: Sized {
@@ -56,6 +57,12 @@ pub struct TryFromPrimitiveError<Enum: TryFromPrimitive> {
     pub number: Enum::Primitive,
 }
 
+impl<Enum: TryFromPrimitive> TryFromPrimitiveError<Enum> {
+    pub fn new(number: Enum::Primitive) -> Self {
+        Self { number }
+    }
+}
+
 impl<Enum: TryFromPrimitive> fmt::Debug for TryFromPrimitiveError<Enum> {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         fmt.debug_struct("TryFromPrimitiveError")

num_enum/tests/renamed_num_enum.rs

@@ -1,3 +1,5 @@
+use std::process::Stdio;
+
 #[test]
 fn no_std() {
     assert!(::std::process::Command::new("cargo")
@@ -9,6 +11,8 @@ fn no_std() {
                 "/../renamed_num_enum/Cargo.toml",
             ),
         ])
+        .stdout(Stdio::inherit())
+        .stderr(Stdio::inherit())
         .status()
         .unwrap()
         .success())
@@ -27,6 +31,8 @@ fn std() {
             "--features",
             "std",
         ])
+        .stdout(Stdio::inherit())
+        .stderr(Stdio::inherit())
         .status()
         .unwrap()
         .success())

num_enum/tests/try_build/compile_fail/custom_error_type_parsing.rs

@@ -0,0 +1,56 @@
+#[derive(num_enum::TryFromPrimitive)]
+#[num_enum(error_type(name = CustomError))]
+#[repr(u8)]
+enum MissingConstructor {
+    Zero,
+    One,
+    Two,
+}
+
+#[derive(num_enum::TryFromPrimitive)]
+#[num_enum(error_type(constructor = CustomError::new))]
+#[repr(u8)]
+enum MissingName {
+    Zero,
+    One,
+    Two,
+}
+
+#[derive(num_enum::TryFromPrimitive)]
+#[num_enum(error_type(name = CustomError, constructor = CustomError::new, extra = something))]
+#[repr(u8)]
+enum ExtraAttr {
+    Zero,
+    One,
+    Two,
+}
+
+#[derive(num_enum::TryFromPrimitive)]
+#[num_enum(error_type(name = CustomError, constructor = CustomError::new), error_type(name = CustomError, constructor = CustomError::new))]
+#[repr(u8)]
+enum TwoErrorTypes {
+    Zero,
+    One,
+    Two,
+}
+
+#[derive(num_enum::TryFromPrimitive)]
+#[num_enum(error_type(name = CustomError, constructor = CustomError::new))]
+#[num_enum(error_type(name = CustomError, constructor = CustomError::new))]
+#[repr(u8)]
+enum TwoAttrs {
+    Zero,
+    One,
+    Two,
+}
+
+struct CustomError {}
+
+impl CustomError {
+    fn new(_: u8) -> CustomError {
+        CustomError{}
+    }
+}
+
+fn main() {
+}

num_enum/tests/try_build/compile_fail/custom_error_type_parsing.stderr

@@ -0,0 +1,29 @@
+error: num_enum error_type attribute requires `constructor` value
+ --> tests/try_build/compile_fail/custom_error_type_parsing.rs:2:12
+  |
+2 | #[num_enum(error_type(name = CustomError))]
+  |            ^^^^^^^^^^
+
+error: num_enum error_type attribute requires `name` value
+  --> tests/try_build/compile_fail/custom_error_type_parsing.rs:11:12
+   |
+11 | #[num_enum(error_type(constructor = CustomError::new))]
+   |            ^^^^^^^^^^
+
+error: expected `name` or `constructor`
+  --> tests/try_build/compile_fail/custom_error_type_parsing.rs:20:75
+   |
+20 | #[num_enum(error_type(name = CustomError, constructor = CustomError::new, extra = something))]
+   |                                                                           ^^^^^
+
+error: num_enum attribute must have at most one error_type
+  --> tests/try_build/compile_fail/custom_error_type_parsing.rs:29:76
+   |
+29 | #[num_enum(error_type(name = CustomError, constructor = CustomError::new), error_type(name = CustomError, constructor = CustomError::new))]
+   |                                                                            ^^^^^^^^^^
+
+error: At most one num_enum error_type attribute may be specified
+  --> tests/try_build/compile_fail/custom_error_type_parsing.rs:39:1
+   |
+39 | #[num_enum(error_type(name = CustomError, constructor = CustomError::new))]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

num_enum/tests/try_from_primitive.rs

@@ -502,6 +502,47 @@ fn try_from_primitive_number() {
     assert_eq!(try_from, Ok(Enum::Whatever));
 }
 
+#[test]
+fn custom_error() {
+    #[derive(Debug, Eq, PartialEq, TryFromPrimitive)]
+    #[num_enum(error_type(name = CustomError, constructor = CustomError::new))]
+    #[repr(u8)]
+    enum FirstNumber {
+        Zero,
+        One,
+        Two,
+    }
+
+    #[derive(Debug, Eq, PartialEq, TryFromPrimitive)]
+    #[num_enum(error_type(constructor = CustomError::new, name = CustomError))]
+    #[repr(u8)]
+    enum SecondNumber {
+        Zero,
+        One,
+        Two,
+    }
+
+    #[derive(Debug, PartialEq, Eq)]
+    struct CustomError {
+        bad_value: u8,
+    }
+
+    impl CustomError {
+        fn new(value: u8) -> CustomError {
+            CustomError { bad_value: value }
+        }
+    }
+
+    let zero: Result<FirstNumber, _> = 0u8.try_into();
+    assert_eq!(zero, Ok(FirstNumber::Zero));
+
+    let three: Result<FirstNumber, _> = 3u8.try_into();
+    assert_eq!(three.unwrap_err(), CustomError { bad_value: 3u8 });
+
+    let three: Result<SecondNumber, _> = 3u8.try_into();
+    assert_eq!(three.unwrap_err(), CustomError { bad_value: 3u8 });
+}
+
 // #[derive(FromPrimitive)] generates implementations for the following traits:
 //
 // - `FromPrimitive<T>`

num_enum_derive/Cargo.toml

@@ -36,3 +36,6 @@ proc-macro2 = "1.0.60"
 proc-macro-crate = { version = "1", optional = true }
 quote = "1"
 syn = { version = "2", features = ["parsing"] }
+
+[dev-dependencies]
+syn = { version = "2", features = ["extra-traits", "parsing"] }