Support deriving serde traits

Cargo.toml

@@ -11,13 +11,20 @@ categories = ["no-std"]
 edition = "2021"
 exclude = ["/tests", "/.github"]
 
+[features]
+serde = ["dep:serde", "bitflags-derive-macros/serde"]
+
 [dependencies.bitflags-derive-macros]
 path = "macros"
 version = "0.0.3"
 
 [dependencies.bitflags]
 version = "2"
 
+[dependencies.serde]
+version = "1"
+optional = true
+
 # Adding new library support to `bitflags-derive`:
 #
 # 1. Add an optional dependency here

macros/Cargo.toml

@@ -13,6 +13,9 @@ edition = "2021"
 [lib]
 proc-macro = true
 
+[features]
+serde = []
+
 [dependencies.proc-macro2]
 version = "1"
 

macros/src/lib.rs

@@ -15,6 +15,9 @@ mod debug;
 mod display;
 mod from_str;
 
+#[cfg(feature = "serde")]
+mod serde;
+
 /**
 Derive [`Debug`](https://doc.rust-lang.org/std/fmt/trait.Debug.html).
 
@@ -48,6 +51,26 @@ pub fn derive_bitflags_from_str(item: proc_macro::TokenStream) -> proc_macro::To
     from_str::expand(syn::parse_macro_input!(item as syn::DeriveInput)).unwrap_or_compile_error()
 }
 
+/**
+Derive [`Serialize`](https://docs.rs/serde/latest/serde/trait.Serialize.html).
+*/
+#[cfg(feature = "serde")]
+#[proc_macro_derive(FlagsSerialize)]
+pub fn derive_bitflags_serialize(item: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    serde::serialize::expand(syn::parse_macro_input!(item as syn::DeriveInput))
+        .unwrap_or_compile_error()
+}
+
+/**
+Derive [`Deserialize`](https://docs.rs/serde/latest/serde/trait.Deserialize.html).
+*/
+#[cfg(feature = "serde")]
+#[proc_macro_derive(FlagsDeserialize)]
+pub fn derive_bitflags_deserialize(item: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    serde::deserialize::expand(syn::parse_macro_input!(item as syn::DeriveInput))
+        .unwrap_or_compile_error()
+}
+
 trait ResultExt {
     fn unwrap_or_compile_error(self) -> proc_macro::TokenStream;
 }

macros/src/serde.rs

@@ -0,0 +1,44 @@
+pub(crate) mod serialize {
+    use proc_macro2::TokenStream;
+
+    pub(crate) fn expand(item: syn::DeriveInput) -> Result<TokenStream, syn::Error> {
+        let ident = item.ident;
+        let (impl_generics, ty_generics, where_clause) = item.generics.split_for_impl();
+
+        Ok(
+            quote!(impl #impl_generics bitflags_derive::__private::serde::Serialize for #ident #ty_generics #where_clause {
+                fn serialize<S: bitflags_derive::__private::serde::Serializer>(&self, serializer: S) -> bitflags_derive::__private::core::result::Result<S::Ok, S::Error> {
+                    bitflags_derive::__private::serde::serialize(self, serializer)
+                }
+            }),
+        )
+    }
+}
+
+pub(crate) mod deserialize {
+    use proc_macro2::{Span, TokenStream};
+
+    pub(crate) fn expand(item: syn::DeriveInput) -> Result<TokenStream, syn::Error> {
+        let ident = item.ident;
+
+        let de_lt = syn::Lifetime::new("'bitflags_derive_de", Span::call_site());
+
+        let mut de_generics = item.generics.clone();
+        de_generics
+            .params
+            .push_value(syn::GenericParam::Lifetime(syn::LifetimeParam::new(
+                de_lt.clone(),
+            )));
+
+        let (impl_generics, _, where_clause) = de_generics.split_for_impl();
+        let (_, ty_generics, _) = item.generics.split_for_impl();
+
+        Ok(
+            quote!(impl #impl_generics bitflags_derive::__private::serde::Deserialize<#de_lt> for #ident #ty_generics #where_clause {
+                fn deserialize<D: bitflags_derive::__private::serde::Deserializer<#de_lt>>(deserializer: D) -> bitflags_derive::__private::core::result::Result<Self, D::Error> {
+                    bitflags_derive::__private::serde::deserialize(deserializer)
+                }
+            }),
+        )
+    }
+}

src/__private/serde.rs

@@ -0,0 +1,80 @@
+extern crate serde;
+
+use bitflags::{
+    parser::{self, ParseHex, WriteHex},
+    Flags,
+};
+
+use core::{fmt, str};
+
+pub use serde::{
+    de::{Error, Visitor},
+    Deserialize, Deserializer, Serialize, Serializer,
+};
+
+struct AsDisplay<'a, B>(pub(crate) &'a B);
+
+impl<'a, B: Flags> fmt::Display for AsDisplay<'a, B>
+where
+    B::Bits: WriteHex,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        parser::to_writer(self.0, f)
+    }
+}
+
+/**
+Serialize a set of flags as a human-readable string or their underlying bits.
+
+Any unknown bits will be retained.
+*/
+pub fn serialize<B: Flags, S: Serializer>(flags: &B, serializer: S) -> Result<S::Ok, S::Error>
+where
+    B::Bits: WriteHex + Serialize,
+{
+    // Serialize human-readable flags as a string like `"A | B"`
+    if serializer.is_human_readable() {
+        serializer.collect_str(&AsDisplay(flags))
+    }
+    // Serialize non-human-readable flags directly as the underlying bits
+    else {
+        flags.bits().serialize(serializer)
+    }
+}
+
+/**
+Deserialize a set of flags from a human-readable string or their underlying bits.
+
+Any unknown bits will be retained.
+*/
+pub fn deserialize<'de, B: Flags, D: Deserializer<'de>>(deserializer: D) -> Result<B, D::Error>
+where
+    B::Bits: ParseHex + Deserialize<'de>,
+{
+    if deserializer.is_human_readable() {
+        // Deserialize human-readable flags by parsing them from strings like `"A | B"`
+        struct FlagsVisitor<B>(core::marker::PhantomData<B>);
+
+        impl<'de, B: Flags> Visitor<'de> for FlagsVisitor<B>
+        where
+            B::Bits: ParseHex,
+        {
+            type Value = B;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+                formatter.write_str("a string value of `|` separated flags")
+            }
+
+            fn visit_str<E: Error>(self, flags: &str) -> Result<Self::Value, E> {
+                parser::from_str(flags).map_err(|e| E::custom(e))
+            }
+        }
+
+        deserializer.deserialize_str(FlagsVisitor(Default::default()))
+    } else {
+        // Deserialize non-human-readable flags directly from the underlying bits
+        let bits = B::Bits::deserialize(deserializer)?;
+
+        Ok(B::from_bits_retain(bits))
+    }
+}

src/lib.rs

@@ -50,6 +50,9 @@ pub use bitflags_derive_macros::*;
 
 #[doc(hidden)]
 pub mod __private {
+    #[cfg(feature = "serde")]
+    pub mod serde;
+
     pub use bitflags;
     pub use core;
 }

tests/ui/Cargo.toml

@@ -4,11 +4,18 @@ version = "0.0.0"
 edition = "2021"
 publish = false
 
+[features]
+serde = ["bitflags-derive/serde", "dep:serde_test"]
+
 [dependencies.bitflags-derive]
 path = "../../"
 
 [dependencies.bitflags]
 version = "2"
 
+[dependencies.serde_test]
+version = "1"
+optional = true
+
 [dependencies.trybuild]
 version = "1"

tests/ui/src/lib.rs

@@ -9,3 +9,6 @@ extern crate bitflags_derive;
 mod debug;
 mod display;
 mod from_str;
+
+#[cfg(feature = "serde")]
+mod serde;

tests/ui/src/serde.rs

@@ -0,0 +1,21 @@
+use serde_test::{assert_tokens, Configure, Token::*};
+
+#[test]
+fn derive_serialize_deserialize() {
+    bitflags! {
+        #[derive(FlagsSerialize, FlagsDeserialize, PartialEq, Eq, Debug)]
+        struct Flags: u8 {
+            const A = 1;
+            const B = 1 << 1;
+            const C = 1 << 2;
+        }
+    }
+
+    assert_tokens(&Flags::empty().readable(), &[Str("")]);
+
+    assert_tokens(&Flags::empty().compact(), &[U8(0)]);
+
+    assert_tokens(&(Flags::A | Flags::B).readable(), &[Str("A | B")]);
+
+    assert_tokens(&(Flags::A | Flags::B).compact(), &[U8(1 | 2)]);
+}