Crate macroex

source ·
Expand description

An extractor based low level macro parsing crate that provides high level parsing support through derive macros.

FromMacro and Extractor

FromMacro is the bread and butter of this crate. FromMacro provides a from_one and a from_many function that parses a TokenTree and a TokenStream respectively.

We can mostly assume from_many will contain two or more TokenTrees. If not that is considered a bug in an extractor.

All implementors of FromMacro are Extractors of one TokenTree. When used on a TokenStream iterator directly, they will consume a single TokenTree and try to parse it using from_one.

let mut iter = quote!{45, Hello; true false}.into_iter();
let a: i32 = iter.extract()?;
let b: Punct = iter.extract()?;
// Extracts a string value from an ident
let IdentString(c) = iter.extract()?;
// Validates a semicolon
let d: PunctOf<';'> = iter.extract()?;
let e: bool = iter.extract()?;
// Validates a false literal
let f: LitFalse = iter.extract()?;

This is pretty great! As most things can be represented as a single TokenTree.

// This is a single TokenTree::Group
    name: "Tom".
    age: 45,
    children: [
        "Tim", "Tam"

However there are other things one TokenTree cannot account for.

// This fails because -45 is two tokens
let a: i32 = quote!{-45}.into_iter().extract().unwrap();

Wrapping FromMacro implementers in other Extractors allow FromMacro implementors to parse additional TokenTrees and potentially utilize the from_many method if more than one TokenTree is matched.

// Note -45 is two tokens
let mut iter = quote!{-45}.into_iter();
// All extracts everything from a stream
let All(a) = iter.extract()?;
assert_eq!(a, -45i32);
let mut iter = quote!{-45, 21, 9.5,}.into_iter();
// CommaExtractor extracts until a comma or end of stream is found.
let CommaExtractor(a) = iter.extract()?;
let CommaExtractor(b) = iter.extract()?;
let CommaExtractor(c) = iter.extract()?;
// EndOfStream is a unit struct extractor and this asserts iter is empty
let EndOfStream = iter.extract()?;
assert_eq!(a, -45);
assert_eq!(b, 21);
assert_eq!(c, 9.5);


We provide derive macro FromMacro and FromAttrs that functions similarly to to serde::Deserialize. This enables ergonomic parsing for structs and enums following a specific data format.

FromMacro parses syntax similar to native rust, while FromAttrs parses syntax commonly used in macro attributes.

Why not serde_tokenstream?

Since we do not use the serde data model. Our data model is much more powerful in the macro context. We are allowed to extract all FromMacro implementors, including TokenStream, Ident, Group, etc.


FromMacro parses syntax similar to native rust.

Unit StructStructName
Tuple Struct(tuple, ..)StructName (tuple, ..)
Named Struct{field: value, ..}StructName {field: value, ..}
Unit Enum VariantVariantName
Tuple Enum VariantVariantName (tuple, ..)
Named Enum VariantVariantName {field: value, ..}


TypeRust Typefrom_onefrom_many
Unit Structstruct Red;Red
Tuple Structstruct Vec2 (i32, i32)(4, 5)Vec2 (4, 5)
Named Structstruct Vec2 {x: i32, y: i32}{x: 4, y: 5}Vec2 {x: 4, y: 5}
Unit Variantenum Color {Black, White}Black
Tuple Variantenum Animals {Dog(String), Sheep(usize)}Dog ("Rex")
Named Variantenum Shapes {Square {x: f32}, Rect {x: f32, y: f32}}Rect {x: 4, y: 5}

Use Case

Since we are likely to be parsing configurations in macros, we supply a Default::default() value if a field is not found.

You are required to opt out of this with #[macroex(required)] if your type does not implement Default.

pub struct Person {
    pub name: String,
    pub age: i32,
    pub height: f32,
    // This works as long as Gender implements `FromMacro` and `Default`
    pub gender: Gender,
    // Using Option is idiomatic to handle the default case.
    pub hair_color: Option<NumberList<[f32;4]>>,
    // We can extract macro based things
    pub do_something: Option<TokenStream>,

Example macro input:

person! {
    name: "Lina",
    age: 23,
    gender: Female,
    hair_color: [0.7, 0.4, 0],
    do_something: {
        let w = "world";
        println!("Hello, {}!", w)


The FromMacro macro supports the following attributes:

// We use the same casing names as serde.
pub struct Animal {
    // Errors if not specified.
    pub name: String,
    // Evaluate an expression instead of `Default::default()`
    pub height: f32,
    pub species: String,
    // Take strings as inputs, and collects them into a vec.
    // and rename "nicknames" to "nickname" during parsing.
    pub nicknames: Vec<String>,


FromAttrs Generates a simple FromMacro implementation for syntax commonly associated with macro attributes.

This macro is only allowed on named structs and supports 3 basic syntax:

  • .., name, .. parses to name: true, which matches a boolean value.
  • .., name = expr, .. parses to name: expr
  • .., name(..), .. parses to name: T{ .. }

Other types like fieldless enums can potentially use FromMacro to generated compatible FromMacro implementations to use with this macro.


We use the same set of attributes as FromMacro

pub struct Animal {
    pub name: String,
    pub height: f32,
    #[macroex(default=r#"Ident::new("Dog", Span::call_site())"#)]
    pub species: Ident,
    #[macroex(repeat, rename="mascot")]
    pub mascot_infos: Vec<MascotInfo>,

Example attribute:

#[animal(name = "Ferris", species = Crab, mascot(language = "Rust"))]

We can parse either

(name = "Ferris", species = Crab, mascot(language = "Rust"))

with from_one, or

name = "Ferris", species = Crab, mascot(language = "Rust")

with from_many, commonly extracted with syn.

Macro Chaining and Hygeine

We treat our input as string-like and we will try to flatten all None delimited groups encountered during parsing.







  • Format a fixed sized array into a function call.
  • Format a fixed sized array into a function call.
  • Flattens all none delimited groups and returns the length.

Type Aliases

Derive Macros

  • Generates a simple FromMacro implementation for macro attributes.
  • Generates a FromMacro implementation