A Rust procedural macro attribute parser.
This crate offers attribute parsing closer to the design of attributes in C#. It has an interface similar to serde. Attributes are written as plain Rust structs or enums, and then parsers for them are generated automatically. They can contain arbitrary expressions and can inherit from other attributes using a flattening mechanism.
The parsers in this crate directly parse token streams using
syn. As a result, most built-in Rust types and syn
types can be used directly as fields.
Functionality in this crate is centered around three traits, and their respective derive macros:
-
ExtractAttributesExtracts attributes from an object containing a list of
syn::Attribute, and parses them into a Rust type. Should be implemented for top-level structures that will be parsed directly out of a set of matching attributes. -
ParseAttributesParses a Rust type from any object containing a list of
syn::Attribute. Should be used if the set of matching attributes can potentially be shared between this type and other types. -
ParseMetaItemParses a Rust type from a
syn::parse::ParseStream. Should be implemented for any types that can be nested inside an attribute.
Basic usage of this crate in derive macros requires simply deriving one (or a
few) of these traits, and then calling extract_attributes or
parse_attributes. For more advanced functionality, several #[deluxe(...)]
attributes are supported on structs, enums, variants and fields. See the
examples below, and the documentation for each derive macro for a complete
description of the supported attributes.
A list of field types supported by default can be seen in the list of provided
ParseMetaItem
implementations.
For more complex usage, manual implementations of these traits can be provided.
See the documentation on the individual traits for more details on how to
manually implement your own parsers.
Deluxe takes inspiration from the darling crate, but
offers a few enhancements over it. Darling is built around pre-parsed
syn::Meta objects, and therefore is restricted to the meta
syntax.
Deluxe parses its types directly from TokenStream objects in the attributes
and so is able to use any syntax that parses as a valid token tree. Deluxe also
does not provide extra traits for parsing special syn objects like
DeriveInput and Field. Instead, Deluxe uses a generic trait to parse from
any type containing a Vec<syn::Attribute>.
To create a derive macro that can add some simple metadata to a Rust type from
an attribute, start by defining a struct that derives ExtractAttributes.
Then, call extract_attributes in your derive macro to create an instance of
the struct:
#[derive(deluxe::ExtractAttributes)]
#[deluxe(attributes(my_desc))] // Match only `my_desc` attributes
struct MyDescription {
name: String,
version: String,
}
#[proc_macro_derive(MyDescription, attributes(my_desc))]
pub fn derive_my_description(item: TokenStream) -> TokenStream {
let mut input = syn::parse::<syn::DeriveInput>(item).unwrap();
// Extract a description, modifying `input.attrs` to remove the matched attributes.
let MyDescription { name, version } = match deluxe::extract_attributes(&mut input) {
Ok(desc) => desc,
Err(e) => return e.into_compile_error().into()
};
let ident = &input.ident;
let (impl_generics, type_generics, where_clause) = input.generics.split_for_impl();
let tokens = quote::quote! {
impl #impl_generics #ident #type_generics #where_clause {
fn my_desc() -> &'static str {
concat!("Name: ", #name, ", Version: ", #version)
}
}
};
tokens.into()
}Then, try adding the attribute in some code that uses your macro:
#[derive(MyDescription)]
#[my_desc(name = "hello world", version = "0.2")]
struct Hello(String);
let hello = Hello("Moon".into());
assert_eq!(hello.my_desc(), "Name: hello world, Version: 0.2");The parse and parse2 functions included in this crate can also be used
as simple helpers for attribute macros:
#[derive(deluxe::ParseMetaItem)]
struct MyDescription {
name: String,
version: String,
}
#[proc_macro_attribute]
pub fn my_desc(
attr: proc_macro::TokenStream,
item: proc_macro::TokenStream,
) -> proc_macro::TokenStream {
let MyDescription { name, version } = match deluxe::parse::<MyDescription>(attr) {
Ok(desc) => desc,
Err(e) => return e.into_compile_error().into()
};
let tokens = quote::quote! {
fn my_desc() -> &'static str {
concat!("Name: ", #name, ", Version: ", #version)
}
#item
};
tokens.into()
}// In your normal code
#[my_desc(name = "hello world", version = "0.2")]
fn nothing() {}
assert_eq!(my_desc(), "Name: hello world, Version: 0.2");The attributes alias, default, rename, and skip are supported, and
behave the same as in Serde. The append attribute can be used
on Vec fields to aggregate all duplicates of a key. The rest attribute can
be used to do custom processing on any unknown keys.
#[derive(deluxe::ExtractAttributes)]
#[deluxe(attributes(my_object))]
struct MyObject {
// Can be specified with key `id` or `object_id`
#[deluxe(alias = object_id)]
id: u64,
// Field is optional, defaults to `Default::default` if not present
#[deluxe(default)]
count: u64,
// Defaults to "Empty" if not present
#[deluxe(default = String::from("Empty"))]
contents: String,
// Can be specified only with key `name`
#[deluxe(rename = name)]
s: String,
// Skipped during parsing entirely
#[deluxe(skip)]
internal_flag: bool,
// Appends any extra fields with the key `expr` to the Vec
#[deluxe(append, rename = expr)]
exprs: Vec<syn::Expr>,
// Adds any unknown keys to the hash map
#[deluxe(rest)]
rest: std::collections::HashMap<syn::Path, syn::Expr>,
}// Omitted fields will be set to defaults
#[derive(MyObject)]
#[my_object(id = 1, name = "First", expr = 1 + 2, count = 3)]
struct FirstObject;
// `expr` can be specified multiple times because of the `append` attribute
#[derive(MyObject)]
#[my_object(object_id = 2, name = "Second", expr = 1 + 2, expr = 3 + 4)]
struct SecondObject;
// `unknown` and `extra` will be stored in the `rest` hashmap
#[derive(MyObject)]
#[my_object(id = 3, name = "Third", unknown = 1 + 2, extra = 3 + 4)]
struct ThirdObject;The flatten attribute can be used to parse keys from one structure inside another:
#[derive(deluxe::ParseMetaItem)]
struct A {
id: u64,
}
#[derive(deluxe::ExtractAttributes)]
#[deluxe(attributes(b))]
struct B {
#[deluxe(flatten)]
a: A,
name: String,
}Then, fields from both A and B can be used when deriving B:
#[derive(B)]
#[b(id = 123, name = "object")]
struct Object;Extra attributes can be taken from within the code block attached to a macro. When used in an attribute macro, the attributes should be consumed so as not to produce an "unknown attribute" error when outputting tokens.
#[derive(Default, deluxe::ParseMetaItem, deluxe::ExtractAttributes)]
struct MyDescription {
name: String,
version: String,
}
#[derive(deluxe::ExtractAttributes)]
#[deluxe(attributes(author))]
struct Authors(#[deluxe(flatten)] Vec<String>);
#[proc_macro_derive(MyDescription, attributes(my_desc))]
pub fn derive_my_description(item: TokenStream) -> TokenStream {
let mut input = syn::parse::<syn::DeriveInput>(item).unwrap();
// Parsing functions suffixed with `_optional` can be used to continue
// parsing after an error. Any errors will get accumulated into an `Errors`
// structure, which can then be manually included in the token output to
// produce compile errors.
let errors = deluxe::Errors::new();
let MyDescription { name, version } = deluxe::extract_attributes_optional(&mut input, &errors);
let mut authors = Vec::new();
if let syn::Data::Struct(s) = &mut input.data {
// Look through all fields in the struct for `author` attributes
for field in s.fields.iter_mut() {
// Aggregate any errors to avoid exiting the loop early
match deluxe::extract_attributes(field) {
Ok(Authors(a)) => authors.extend(a),
Err(e) => errors.push_syn(e),
}
}
}
let ident = &input.ident;
let (impl_generics, type_generics, where_clause) = input.generics.split_for_impl();
// Make sure to include the errors in the output
let tokens = quote::quote! {
#errors
impl #impl_generics #ident #type_generics #where_clause {
fn my_desc() -> &'static str {
concat!("Name: ", #name, ", Version: ", #version #(, ", Author: ", #authors)*)
}
}
};
tokens.into()
}
#[proc_macro_attribute]
pub fn my_desc_mod(
attr: proc_macro::TokenStream,
item: proc_macro::TokenStream,
) -> proc_macro::TokenStream {
let mut module = syn::parse::<syn::ItemMod>(item) {
Ok(module) => module,
Err(e) => return e.into_compile_error().into()
};
let errors = deluxe::Errors::new();
let MyDescription { name, version } = deluxe::parse_optional(attr, &errors);
let (_, items) = module.content.as_mut().unwrap();
let mut authors = Vec::new();
// Look through all items in the module for `author` attributes
for i in items.iter_mut() {
// Extract the attributes to remove them from the final output
match deluxe::extract_attributes(i) {
Ok(Authors(a)) => authors.extend(a),
Err(e) => errors.push_syn(e),
}
}
// Place a new function inside the module
items.push(syn::parse_quote! {
fn my_desc() -> &'static str {
concat!("Name: ", #name, ", Version: ", #version #(, ", Author: ", #authors)*)
}
});
// Make sure to include the errors in the output
let tokens = quote::quote! { #module #errors };
tokens.into()
}// In your normal code
#[derive(MyDescription, Default)]
#[my_desc(name = "hello world", version = "0.2")]
struct Hello {
#[author("Alice")]
a: i32,
#[author("Bob")]
b: String
}
let hello: Hello = Default::default();
assert_eq!(hello.my_desc(), "Name: hello world, Version: 0.2, Author: Alice, Author: Bob");
#[my_desc_mod(name = "hello world", version = "0.2")]
mod abc {
#[author("Alice", "Bob")]
fn func1() {}
#[author("Carol")]
#[author("Dave")]
fn func2() {}
}
assert_eq!(
abc::my_desc(),
"Name: hello world, Version: 0.2, Author: Alice, Author: Bob, Author: Carol, Author: Dave"
);Deluxe also supports parsing into data structures with unnamed fields.
#[derive(deluxe::ExtractAttributes)]
#[deluxe(attributes(my_tuple))]
struct MyTuple(u64, String);
#[derive(deluxe::ExtractAttributes)]
#[deluxe(attributes(my_idents))]
struct MyIdents {
id: u64,
names: (String, String),
idents: Vec<syn::Ident>
}The standard attribute syntax with parenthesis can be used when specifying a
Vec type. The alternative syntax key = [...] can also be used to have an
appearance similar to an array literal.
#[derive(MyTuple)]
#[my_tuple(123, "object")]
struct Object;
#[derive(MyIdents)]
#[my_idents(id = 7, names("hello", "world"), idents(a, b, c))]
struct ABC;
// `idents` contains same values as above
#[derive(MyIdents)]
#[my_idents(id = 7, names("hello", "world"), idents = [a, b, c])]
struct ABC2;Attributes in C# can support positional arguments first with the named
arguments afterwards. This style can be emulated by using a tuple struct with a
normal struct flattened at the end. Placing #[deluxe(default)] on the struct
behaves the same as Serde, by filling in all fields with values from Default,
allowing every named argument to be optional.
#[derive(deluxe::ParseMetaItem, Default)]
#[deluxe(default)]
struct Flags {
native: bool,
}
#[derive(deluxe::ExtractAttributes)]
#[deluxe(attributes(a))]
struct A(u64, String, #[deluxe(flatten)] Flags);#[derive(A)]
#[a(123, "object")]
struct Object;
#[derive(A)]
#[a(123, "native-object", native = true)]
struct NativeObject;Enums are supported by using the variant name as a single key, in snake-case. Variants can be renamed, aliased and skipped in the same way as fields.
#[derive(deluxe::ExtractAttributes)]
#[deluxe(attributes(my_enum))]
enum MyEnum {
A,
B,
C,
#[deluxe(alias = d)]
AnotherOne,
#[deluxe(rename = e)]
AnotherTwo,
#[deluxe(skip)]
SkipMe
}#[derive(MyEnum)]
#[my_enum(b)]
struct ObjectB;
#[derive(MyEnum)]
#[my_enum(another_one)]
struct ObjectD;Enums with struct and tuple variants are also supported. The data inside is used as arguments to the attribute. All field attributes from structs are also supported inside variants.
Additionally, enum variants with named fields can be flattened. The behavior of
a flattened variant is similar to Serde's untagged mode. In a flattened
variant, the name of the variant will be ignored. Instead, Deluxe will attempt
to use the unique keys in each variant to determine if that variant was
specified. A compile error will be thrown if it is not possible to determine a
unique, unambiguous key between two variants.
#[derive(deluxe::ExtractAttributes)]
#[deluxe(attributes(my_enum))]
enum MyEnum {
A,
B(u64, String),
C { id: u64, name: String },
#[deluxe(flatten)]
D { d: u64, name: String },
}#[derive(MyEnum)]
#[my_enum(a)]
struct ObjectA;
#[derive(MyEnum)]
#[my_enum(b(1, "hello"))]
struct ObjectB;
#[derive(MyEnum)]
#[my_enum(c(id = 2, name = "world"))]
struct ObjectC;
// No inner parenthesis needed here due to flattening
#[derive(MyEnum)]
#[my_enum(d = 3, name = "moon")]
struct ObjectD;During parsing, Deluxe can store references to the container type holding the attributes for easier access. Container fields are skipped during attribute parsing.
#[derive(deluxe::ParseAttributes)]
#[deluxe(attributes(my_object))]
struct MyObject<'t> {
id: u64,
// Fill `container` in using the parsed type. Note this restricts the
// derived `ParseAttributes` impl so it can only be used on `DeriveInput`.
#[deluxe(container)]
container: &'t syn::DeriveInput,
}
#[proc_macro_derive(MyObject, attributes(my_desc))]
pub fn derive_my_object(item: TokenStream) -> TokenStream {
let input = syn::parse::<syn::DeriveInput>(item).unwrap();
// `obj.container` now holds a reference to `input`
let obj: MyObject = match deluxe::parse_attributes(&input) {
Ok(obj) => obj,
Err(e) => return e.into_compile_error().into()
};
let tokens = quote::quote! { /* ... generate some code here ... */ };
tokens.into()
}To support both extracting and parsing, a container field can also be a value type. In that case, the container will be cloned into the structure.