rust中过程宏示例：
准备工作
[lib]
proc-macro = true
[dependencies]
syn = { version = "1.0", features = ["full"] }
quote = "1.0"
proc-macro2 = "1.0"
定义一个过程宏: proc_macro_derive
extern crate proc_macro;
use proc_macro::TokenStream;
use quote::{format_ident, quote};
use syn::{self, parse_macro_input, Data, DataStruct, DeriveInput, Fields, FieldsNamed, Type};
#[proc_macro_derive(Builder, attributes(builder))]
pub fn derive(input: TokenStream) -> TokenStream {
eprintln!("===input: {:#?}", input);
// Parse the input tokens into a syntax tree.
let input = parse_macro_input!(input as DeriveInput);
// eprintln!("{:?}", format!("===ast: {}", input.into()));
eprintln!("===ast input atrrs: {:#?}", input.attrs);
eprintln!("===ast input name: {:#?}", input.ident);
eprintln!("===ast input vis: {:#?}", input.vis);
eprintln!("===ast input generics: {:#?}", input.generics);
eprintln!("===ast input data: {:#?}", input.data);
// Used in the quasi-quotation below as `#name`.
let name = input.ident;
// 类似：CommonBuilder格式
let builder = format_ident!("{}Builder", name);
/// 获取struct/enum/union的内容部分
/// 类似struct的如下内容：DataStruct.data部分
/// {
/// executable: String,
/// args: Vec<String>,
/// env: Vec<String>,
/// current_dir: String,
/// }
/// 经过使用syn库转换为syn.AST 大概格式如下：
/// Struct(
/// DataStruct {
/// struct_token: Struct, # 标识当前是一个struct
/// fields: Named( # 在当前struct中定义的field
/// FieldsNamed { # 字段都是其命名的
/// brace_token: Brace,
/// named: [ # 命名field
/// Field { # 类似: executable: String,
/// attrs: [], # 该field在定义时指定了attributes
/// vis: Inherited, # 该field的可见性
/// ident: Some( # 该field的名称及代码中位置
/// Ident {
/// ident: "executable", # 名称
/// span: #0 bytes(1016..1026), # 位置
/// },
/// ),
/// colon_token: Some( # 冒号:
/// Colon,
/// ),
/// ty: Path( # 该field类型
/// TypePath { # 类型路径: 类似A::B::C
/// qself: None,
/// path: Path {
/// leading_colon: None,
/// segments: [
/// PathSegment {
/// ident: Ident {
/// ident: "String",
/// span: #0 bytes(1028..1034),
/// },
/// arguments: None,
/// },
/// ],
/// },
/// },
/// ),
/// },
/// Comma,
/// ],
/// },
/// ),
/// semi_token: None,
/// },
/// )
// 获取当前struct中所有fields: DataStruct::fields::FieldsNamed部分
let data: FieldsNamed = match input.data {
Data::Struct(DataStruct {
fields: Fields::Named(n),
..
}) => n,
other => unimplemented!("{:?}", other),
};
// 遍历每个命名的field：名称、类型; 构建(名称, 类型Option<真实类型>)
let fields = data.named.iter().filter_map(|field| {
let ty = &field.ty;
match &field.ident {
Some(ident) => Some((ident, ty, inner_for_option(ty))),
_ => None,
}
});
// 遍历每个field：名称、类型; 构建(名称, Option<真实类型>)
let names = data.named.iter().filter_map(|field| match &field.ident {
None => None,
Some(ident) => Some((ident, inner_for_option(&field.ty))),
});
// 构建每个字段的初始化值:None; 类似 字段name: None
let initialize = names.clone().map(|(name, _)| quote! { #name: None });
//
let extract = names.clone().map(|(name, option)| match option {
None => quote! { #name: self.#name.clone()? },
Some(_) => quote! { #name: self.#name.clone() },
});
// 构建所有字段的模版： 类似 字段name： 类型Option<#ty>
let quoted_fields = fields.clone().map(|(name, ty, option)| match option {
None => quote! { #name: Option<#ty> },
Some(ty) => quote! { #name: Option<#ty> },
});
// 构建每个字段对应的setter方法
// 格式类似：
// pub fn 字段名称(&mut self, 值value: 类型#ty) {
// self.#name = Some(value);
// self
// }
let methods = fields.clone().map(|(name, ty, option)| match option {
None => quote! {
pub fn #name(&mut self, value: #ty) -> &mut Self {
self.#name = Some(value);
self
}
},
Some(ty) => quote! {
pub fn #name(&mut self, value: #ty) -> &mut Self {
self.#name = Some(value);
self
}
},
});
// 构建最终的Builder模式的对应的模版
let expanded = quote! {
// 生成Builder，并初始化struct不同字段的内容
impl #name {
fn builder() -> #builder {
#builder {
#(
#initialize,
)*
}
}
}
// 定义Builder
struct #builder {
#(
#quoted_fields,
)*
}
// 实现Builder的build及不同字段赋值的方法
impl #builder {
pub fn build(&self) -> Option<#name> {
Some(#name {
#(
#extract,
)*
})
}
#(
#methods
)*
}
};
eprintln!("===expanded: {:#?}", expanded);
// 最终输出proc_macro::TokenStream,并入被编译器rustc输出AST
TokenStream::from(expanded)
}
fn inner_for_option(ty: &Type) -> Option<Type> {
match ty {
Type::Path(syn::TypePath {
path: syn::Path { segments, .. },
..
}) if segments[0].ident == "Option" => {
let segment = &segments[0];
match &segment.arguments {
syn::PathArguments::AngleBracketed(generic) => {
match generic.args.first().unwrap() {
syn::GenericArgument::Type(ty) => Some(ty.clone()),
_ => None,
}
}
_ => None,
}
}
_ => None,
}
}
测试
use derive_builder::Builder;
#[derive(Builder)]
pub struct Command {
executable: std::string::String,
args: Vec<String>,
env: Vec<String>,
current_dir: String,
}
fn main() {}
use derive_builder::Builder;
#[derive(Builder)]
pub struct Command {
executable: String,
args: Vec<String>,
env: Vec<String>,
current_dir: String,
}
fn main() {
let builder = Command::builder();
let _ = builder;
}
补充
派生式过程宏解析
