From 6c74f6d2580ce7c532fdf8866cd0a50c22165d7e Mon Sep 17 00:00:00 2001
From: paq <89paku@gmail.com>
Date: Tue, 12 May 2026 09:26:13 +0900
Subject: [PATCH] fix: support signed repr c_enum derive

---
 zerompk/tests/derive.rs   | 18 ++++++++++
 zerompk_derive/src/lib.rs | 72 +++++++++++++++++++++++++++++++++------
 2 files changed, 79 insertions(+), 11 deletions(-)

diff --git a/zerompk/tests/derive.rs b/zerompk/tests/derive.rs
index bb8b0f9..e8e5926 100644
--- a/zerompk/tests/derive.rs
+++ b/zerompk/tests/derive.rs
@@ -124,6 +124,14 @@ enum BasicLevel {
     High,
 }
 
+#[derive(ToMessagePack, FromMessagePack, Debug, PartialEq)]
+#[msgpack(c_enum)]
+#[repr(i8)]
+enum ReprI8Enum {
+    MinusOne = -1,
+    Zero = 0,
+}
+
 #[derive(ToMessagePack, FromMessagePack, Debug, PartialEq)]
 struct RecursiveNode {
     next: Option<Box<RecursiveNode>>,
@@ -595,6 +603,16 @@ fn derive_c_enum_with_implicit_discriminant() {
     assert_eq!(decoded, value);
 }
 
+#[test]
+fn derive_c_enum_with_signed_discriminant() {
+    let value = ReprI8Enum::MinusOne;
+    let data = zerompk::to_msgpack_vec(&value).unwrap();
+    assert_eq!(data, vec![0xff]);
+
+    let decoded: ReprI8Enum = zerompk::from_msgpack(&data).unwrap();
+    assert_eq!(decoded, value);
+}
+
 #[test]
 fn derive_c_enum_unknown_value_is_error() {
     let err = zerompk::from_msgpack::<HttpStatus>(&[0x03]).unwrap_err();
diff --git a/zerompk_derive/src/lib.rs b/zerompk_derive/src/lib.rs
index f0d355d..9a63c98 100644
--- a/zerompk_derive/src/lib.rs
+++ b/zerompk_derive/src/lib.rs
@@ -43,12 +43,20 @@ struct TypeConfig {
     repr: Option<Repr>,
     c_enum: bool,
     allow_unknown_fields: bool,
+    c_enum_repr: CEnumRepr,
+}
+
+#[derive(Clone, Copy, PartialEq, Eq)]
+enum CEnumRepr {
+    Signed,
+    Unsigned,
 }
 
 fn parse_type_config_from_attrs(attrs: &[syn::Attribute]) -> Result<TypeConfig> {
     let mut repr = None;
     let mut c_enum = false;
     let mut allow_unknown_fields = false;
+    let mut c_enum_repr = CEnumRepr::Unsigned;
 
     for attr in attrs {
         if !attr.path().is_ident("msgpack") {
@@ -91,10 +99,31 @@ fn parse_type_config_from_attrs(attrs: &[syn::Attribute]) -> Result<TypeConfig>
         })?;
     }
 
+    if c_enum {
+        for attr in attrs {
+            if !attr.path().is_ident("repr") {
+                continue;
+            }
+
+            attr.parse_nested_meta(|meta| {
+                if meta.path.is_ident("i8")
+                    || meta.path.is_ident("i16")
+                    || meta.path.is_ident("i32")
+                    || meta.path.is_ident("i64")
+                    || meta.path.is_ident("isize")
+                {
+                    c_enum_repr = CEnumRepr::Signed;
+                }
+                Ok(())
+            })?;
+        }
+    }
+
     Ok(TypeConfig {
         repr,
         c_enum,
         allow_unknown_fields,
+        c_enum_repr,
     })
 }
 
@@ -851,7 +880,7 @@ fn expand(input: DeriveInput, kind: DeriveKind) -> Result<proc_macro2::TokenStre
             }
 
             if type_cfg.c_enum {
-                expand_c_enum(&data)?
+                expand_c_enum(&data, type_cfg.c_enum_repr)?
             } else {
                 expand_enum(&data)?
             }
@@ -1341,7 +1370,7 @@ fn expand_map_struct(data: &DataStruct, allow_unknown_fields: bool) -> Result<Im
     Ok(ImplBody { write, read })
 }
 
-fn expand_c_enum(data: &DataEnum) -> Result<ImplBody> {
+fn expand_c_enum(data: &DataEnum, repr: CEnumRepr) -> Result<ImplBody> {
     let mut write_arms = Vec::new();
     let mut read_arms = Vec::new();
 
@@ -1355,16 +1384,32 @@ fn expand_c_enum(data: &DataEnum) -> Result<ImplBody> {
             ));
         }
 
-        write_arms.push(quote! {
-            Self::#v_ident => {
-                writer.write_u64(Self::#v_ident as u64)?;
-                Ok(())
+        match repr {
+            CEnumRepr::Signed => {
+                write_arms.push(quote! {
+                    Self::#v_ident => {
+                        writer.write_i64(Self::#v_ident as i64)?;
+                        Ok(())
+                    }
+                });
+
+                read_arms.push(quote! {
+                    __value if __value == (Self::#v_ident as i64) => Ok(Self::#v_ident)
+                });
             }
-        });
+            CEnumRepr::Unsigned => {
+                write_arms.push(quote! {
+                    Self::#v_ident => {
+                        writer.write_u64(Self::#v_ident as u64)?;
+                        Ok(())
+                    }
+                });
 
-        read_arms.push(quote! {
-            __value if __value == (Self::#v_ident as u64) => Ok(Self::#v_ident)
-        });
+                read_arms.push(quote! {
+                    __value if __value == (Self::#v_ident as u64) => Ok(Self::#v_ident)
+                });
+            }
+        }
     }
 
     let write = quote! {
@@ -1373,8 +1418,13 @@ fn expand_c_enum(data: &DataEnum) -> Result<ImplBody> {
         }
     };
 
+    let read_value = match repr {
+        CEnumRepr::Signed => quote! { reader.read_i64()? },
+        CEnumRepr::Unsigned => quote! { reader.read_u64()? },
+    };
+
     let read = quote! {
-        let __value = reader.read_u64()?;
+        let __value = #read_value;
         match __value {
             #( #read_arms, )*
             _ => Err(::zerompk::Error::InvalidMarker(0)),