commit 353d0c07d4fee96992ecce109259f420f8bb014f
Author: Aymeric Beringer <aymeric.beringer@epita.fr>
Date:   Fri Oct 11 16:32:09 2019 +0200

    Implement serialization for unit variants and named variants

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..6936990
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,3 @@
+/target
+**/*.rs.bk
+Cargo.lock
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..85a4534
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,18 @@
+[package]
+name = "miniserde-enum"
+version = "0.1.0"
+authors = ["Aymeric Beringer <aymeric.beringer@epita.fr>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+[lib]
+proc-macro = true
+
+[dev-dependencies]
+miniserde = "0.1"
+
+[dependencies]
+syn = { version = "1.0", features = ["derive", "extra-traits"] }
+quote = "1.0"
+proc-macro2 = "1.0"
+miniserde = "0.1" # TODO: remove
diff --git a/src/attr.rs b/src/attr.rs
new file mode 100644
index 0000000..d7ee3a6
--- /dev/null
+++ b/src/attr.rs
@@ -0,0 +1,97 @@
+use syn::{Attribute, Error, Field, Fields, Lit, Meta, NestedMeta, Result, Variant, DataEnum};
+use crate::TagType;
+
+pub(crate) fn tag_type(attrs: &[Attribute], enumeration: &DataEnum) -> Result<TagType> {
+    let mut tag_type = None;
+
+    for attr in attrs {
+        if !attr.path.is_ident("serde") {
+            continue;
+        }
+
+        let list = match attr.parse_meta()? {
+            Meta::List(list) => list,
+            other => return Err(Error::new_spanned(other, "unsupported attribute")),
+        };
+
+        for meta in &list.nested {
+            match meta {
+                NestedMeta::Meta(Meta::NameValue(value)) => {
+                    if value.path.is_ident("tag") {
+                        if let Lit::Str(s) = &value.lit {
+                            if tag_type.is_some() {
+                                return Err(Error::new_spanned(meta, "duplicate tag attribute"));
+                            }
+                            for fields in enumeration.variants.iter().map(|v| &v.fields) {
+                                if let Fields::Unnamed(_) = fields {
+                                    return Err(Error::new_spanned(meta,
+                                        "enums containing tuple variants cannot be internally tagged"));
+                                }
+                            }
+                            tag_type = Some(TagType::Internal(s.value()));
+                            continue;
+                        }
+                    }
+                }
+                NestedMeta::Meta(Meta::Path(path)) => {
+                    if path.is_ident("untagged") {
+                        if tag_type.is_some() {
+                            return Err(Error::new_spanned(meta, "duplicate tag attribute"));
+                        }
+                        tag_type = Some(TagType::Untagged);
+                        continue;
+                    }
+                }
+                _ => (),
+            }
+            return Err(Error::new_spanned(meta, "unsupported attribute"));
+        }
+    }
+
+    Ok(tag_type.unwrap_or(TagType::External))
+}
+
+/// Find the value of a #[serde(rename = "...")] attribute.
+fn attr_rename(attrs: &[Attribute]) -> Result<Option<String>> {
+    let mut rename = None;
+
+    for attr in attrs {
+        if !attr.path.is_ident("serde") {
+            continue;
+        }
+
+        let list = match attr.parse_meta()? {
+            Meta::List(list) => list,
+            other => return Err(Error::new_spanned(other, "unsupported attribute")),
+        };
+
+        for meta in &list.nested {
+            if let NestedMeta::Meta(Meta::NameValue(value)) = meta {
+                if value.path.is_ident("rename") {
+                    if let Lit::Str(s) = &value.lit {
+                        if rename.is_some() {
+                            return Err(Error::new_spanned(meta, "duplicate rename attribute"));
+                        }
+                        rename = Some(s.value());
+                        continue;
+                    }
+                }
+            }
+            return Err(Error::new_spanned(meta, "unsupported attribute"));
+        }
+    }
+
+    Ok(rename)
+}
+
+/// Determine the name of a field, respecting a rename attribute.
+pub fn name_of_field(field: &Field) -> Result<String> {
+    let rename = attr_rename(&field.attrs)?;
+    Ok(rename.unwrap_or_else(|| field.ident.as_ref().unwrap().to_string()))
+}
+
+/// Determine the name of a variant, respecting a rename attribute.
+pub fn name_of_variant(var: &Variant) -> Result<String> {
+    let rename = attr_rename(&var.attrs)?;
+    Ok(rename.unwrap_or_else(|| var.ident.to_string()))
+}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..84f12b0
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,51 @@
+extern crate proc_macro;
+
+mod ser;
+pub(crate) mod attr;
+
+use syn::{
+    parse_macro_input, DeriveInput, Error, Data, Ident, Result,
+};
+use std::convert::From;
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn it_works() {
+        assert_eq!(2 + 2, 4);
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct Variant {
+    ident: Ident,
+    name: Option<String>,
+}
+
+impl From<&syn::Variant> for Variant {
+    fn from(v: &syn::Variant) -> Self {
+        let ident = v.ident.clone();
+        Variant {
+            ident,
+            name: None,
+        }
+    }
+}
+
+#[derive(Debug)]
+enum TagType {
+    External,
+    Internal(String),
+    Untagged,
+}
+
+#[proc_macro_derive(Serialize_enum, attributes(serde))]
+pub fn derive_serialize(tokens: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    let input = parse_macro_input!(tokens as DeriveInput);
+    let en = match &input.data {
+        Data::Enum(en) => en,
+        _ => return Error::new_spanned(input, "Serialize_enum can only be applied to enums")
+            .to_compile_error().into()
+    };
+    ser::derive(&input, &en).unwrap_or_else(|err| err.to_compile_error()).into()
+}
diff --git a/src/ser.rs b/src/ser.rs
new file mode 100644
index 0000000..62a1ad8
--- /dev/null
+++ b/src/ser.rs
@@ -0,0 +1,168 @@
+use proc_macro2::{TokenStream, Span};
+use syn::{
+    DeriveInput, Result, DataEnum, Error, Fields, FieldsNamed,
+};
+use quote::quote;
+use crate::{Variant, TagType};
+use crate::attr;
+
+pub fn derive(input: &DeriveInput, enumeration: &DataEnum) -> Result<TokenStream> {
+    if input.generics.lt_token.is_some() || input.generics.where_clause.is_some() {
+        return Err(Error::new(
+            Span::call_site(),
+            "Enums with generics are not supported",
+        ));
+    }
+    let ident = &input.ident;
+    let variants: Vec<Variant> = enumeration.variants.iter().map(Into::into).collect();
+    let tag_type = attr::tag_type(&input.attrs, &enumeration)?;
+    let names = enumeration.variants
+        .iter()
+        .map(attr::name_of_variant)
+        .collect::<Result<Vec<_>>>()?;
+    let begin = enumeration.variants
+        .iter()
+        .zip(names.iter())
+        .map(|(variant, name)| {
+            let var_ident = &variant.ident;
+            Ok(match &variant.fields {
+                Fields::Unit => {
+                    let implementation = serialize_unit(name, &tag_type)?;
+                    quote!{
+                        #ident::#var_ident => {#implementation}
+                    }
+                }
+                Fields::Named(fields) => {
+                    let implementation = serialize_named(&fields, name, &tag_type)?;
+                    let field_ident = fields.named.iter().map(|field| &field.ident).collect::<Vec<_>>();
+
+                    quote!{
+                        #ident::#var_ident{#(#field_ident),*} => {
+                            #implementation
+                        }
+                    }
+                },
+                Fields::Unnamed(fields) => quote!{_ => unimplemented!(),},
+            })
+        })
+        .collect::<Result<Vec<_>>>()?;
+
+    Ok(quote!{
+        const _: () = {
+            impl miniserde::Serialize for #ident {
+                fn begin(&self) -> miniserde::ser::Fragment {
+                    match self {
+                        #(#begin)*
+                    }
+                }
+            }
+        };
+    })
+}
+
+fn serialize_unit(variant_name: &str, tag_type: &TagType) -> Result<TokenStream> {
+    Ok(if let TagType::Internal(tag) = &tag_type {
+        quote!{
+            struct __Map {
+                state: miniserde::export::usize,
+            }
+
+            impl miniserde::ser::Map for __Map {
+                fn next(&mut self) -> miniserde::export::Option<(miniserde::export::Cow<miniserde::export::str>, &dyn miniserde::Serialize)> {
+                    let __state = self.state;
+                    self.state = __state + 1;
+                    match __state {
+                        0 => miniserde::export::Some((
+                                miniserde::export::Cow::Borrowed(#tag),
+                                &#variant_name,
+                                )),
+                        _ => miniserde::export::None,
+                    }
+                }
+            }
+
+            miniserde::ser::Fragment::Map(miniserde::export::Box::new(__Map {state: 0}))
+        }
+    } else {
+        quote!{miniserde::ser::Fragment::Str(miniserde::export::Cow::Borrowed(#variant_name))}
+    })
+}
+
+fn serialize_named(fields: &FieldsNamed, variant_name: &str, tag_type: &TagType) -> Result<TokenStream> {
+    let field_ident = fields.named.iter().map(|field| &field.ident).collect::<Vec<_>>();
+    let field_name = fields.named.iter().map(attr::name_of_field).collect::<Result<Vec<_>>>()?;
+    let field_type = fields.named.iter().map(|field| &field.ty).collect::<Vec<_>>();
+    Ok(if let TagType::External = tag_type {
+        quote!{
+            use miniserde::Serialize;
+            #[derive(Serialize)]
+            struct __AsStruct<'__b> {
+                #(#field_ident: &'__b #field_type),*,
+            }
+
+            struct __SuperMap<'__b> {
+                data: __AsStruct<'__b>,
+                state: miniserde::export::usize,
+            }
+
+            impl<'__a> miniserde::ser::Map for __SuperMap<'__a> {
+                fn next(&mut self) -> miniserde::export::Option<(miniserde::export::Cow<miniserde::export::str>, &dyn miniserde::Serialize)> {
+                    let __state = self.state;
+                    self.state = __state + 1;
+                    match __state {
+                        0 => miniserde::export::Some((
+                            miniserde::export::Cow::Borrowed(#variant_name),
+                            &self.data,
+                        )),
+                        _ => miniserde::export::None,
+                    }
+                }
+            }
+
+            miniserde::ser::Fragment::Map(miniserde::export::Box::new(__SuperMap {
+                data: __AsStruct { #(#field_ident),* },
+                state: 0,
+            }))
+        }
+    } else {
+        let (start, tag_arm) = if let TagType::Internal(ref tag) = &tag_type {
+            (0usize, quote!{
+                0 => miniserde::export::Some((
+                        miniserde::export::Cow::Borrowed(#tag),
+                        &#variant_name,
+                        )),
+            })
+        } else {
+            (1, quote!())
+        };
+        let index = 1usize..;
+        quote!{
+            struct __Map<'__a> {
+                #(#field_ident: &'__a #field_type),*,
+                state: miniserde::export::usize,
+            }
+
+            impl<'__a> miniserde::ser::Map for __Map<'__a> {
+                fn next(&mut self) -> miniserde::export::Option<(miniserde::export::Cow<miniserde::export::str>, &dyn miniserde::Serialize)> {
+                    let __state = self.state;
+                    self.state = __state + 1;
+                    match __state {
+                        #tag_arm
+                        #(#index => {
+                            miniserde::export::Some((
+                                    miniserde::export::Cow::Borrowed(#field_name),
+                                    self.#field_ident,
+                                    ))
+                        })*,
+                        _ => miniserde::export::None,
+                    }
+                }
+            }
+
+            miniserde::ser::Fragment::Map(miniserde::export::Box::new(__Map {
+                #(#field_ident),*,
+                state: #start,
+            }))
+        }
+    })
+}
diff --git a/tests/serialize.rs b/tests/serialize.rs
new file mode 100644
index 0000000..27f7fe8
--- /dev/null
+++ b/tests/serialize.rs
@@ -0,0 +1,52 @@
+use miniserde::{json, Serialize};
+use miniserde_enum::Serialize_enum;
+
+#[test]
+fn test_internal() {
+    #[serde(tag = "type")]
+    #[derive(Serialize_enum)]
+    enum Internal {
+        A,
+        #[serde(rename = "renamedB")]
+        B,
+        C{x: i32},
+    }
+    use Internal::*;
+    let example = [A, B, C{x: 2}];
+    let actual = json::to_string(&example[..]);
+    let expected = r#"[{"type":"A"},{"type":"renamedB"},{"type":"C","x":2}]"#;
+    assert_eq!(actual, expected);
+}
+
+#[test]
+fn test_external() {
+    #[derive(Serialize_enum)]
+    enum External {
+        A,
+        #[serde(rename = "renamedB")]
+        B,
+        C{x: i32},
+    }
+    use External::*;
+    let example = [A, B, C{x: 2}];
+    let actual = json::to_string(&example[..]);
+    let expected = r#"["A","renamedB",{"C":{"x":2}}]"#;
+    assert_eq!(actual, expected);
+}
+
+#[test]
+fn test_untagged() {
+    #[serde(untagged)]
+    #[derive(Serialize_enum)]
+    enum Untagged {
+        A,
+        #[serde(rename = "renamedB")]
+        B,
+        C{x: i32},
+    }
+    use Untagged::*;
+    let example = [A, B, C{x: 2}];
+    let actual = json::to_string(&example[..]);
+    let expected = r#"["A","renamedB",{"x":2}]"#;
+    assert_eq!(actual, expected);
+}