场景
开发过程宏时经常需要处理结构体或枚举体上的属性参数，如下 Command 结构体的 args 字段有属性 each = "arg",
代码语言：rust
复制
| pub struct Command { | |
| executable: String, | |
| args: Vec<String>, | |
| env: Vec<String>, | |
| current_dir: Option<String>, | |
| } |
AST
对应的语法树结构：
代码语言：ini
复制
| // Command 语法树 | |
| DeriveInput { | |
| // Command 结构体属性 当前没有 | |
| attrs: [], | |
| vis: Visibility::Public( | |
| Pub, | |
| ), | |
| // 结构体名称 | |
| ident: Ident { | |
| ident: "Command", | |
| span: #0 bytes(206..213), | |
| }, | |
| generics: Generics { | |
| lt_token: None, | |
| params: [], | |
| gt_token: None, | |
| where_clause: None, | |
| }, | |
| data: Data::Struct { | |
| struct_token: Struct, | |
| // Fields::Named字段集合 | |
| fields: Fields::Named { | |
| brace_token: Brace, | |
| named: [ | |
| // executable 字段 | |
| Field { | |
| attrs: [], | |
| vis: Visibility::Inherited, | |
| mutability: FieldMutability::None, | |
| // 字段名 executable | |
| ident: Some( | |
| Ident { | |
| ident: "executable", | |
| span: #0 bytes(220..230), | |
| }, | |
| ), | |
| colon_token: Some( | |
| Colon, | |
| ), | |
| // 字段类型 String | |
| ty: Type::Path { | |
| qself: None, | |
| path: Path { | |
| leading_colon: None, | |
| segments: [ | |
| PathSegment { | |
| ident: Ident { | |
| ident: "String", | |
| span: #0 bytes(232..238), | |
| }, | |
| arguments: PathArguments::None, | |
| }, | |
| ], | |
| }, | |
| }, | |
| }, | |
| Comma, | |
| // args 字段 | |
| Field { | |
| // 字段指定的属性参数集合 | |
| attrs: [ | |
| Attribute { | |
| pound_token: Pound, | |
| style: AttrStyle::Outer, | |
| bracket_token: Bracket, | |
| meta: Meta::List { | |
| // 属性名称 builder | |
| path: Path { | |
| leading_colon: None, | |
| segments: [ | |
| PathSegment { | |
| ident: Ident { | |
| ident: "builder", | |
| span: #0 bytes(246..253), | |
| }, | |
| arguments: PathArguments::None, | |
| }, | |
| ], | |
| }, | |
| delimiter: MacroDelimiter::Paren( | |
| Paren, | |
| ), | |
| tokens: TokenStream [ | |
| // 第一个属性 key = each | |
| Ident { | |
| ident: "each", | |
| span: #0 bytes(254..258), | |
| }, | |
| Punct { | |
| ch: '=', | |
| spacing: Alone, | |
| span: #0 bytes(259..260), | |
| }, | |
| // 第一个属性 value = arg | |
| Literal { | |
| kind: Str, | |
| symbol: "arg", | |
| suffix: None, | |
| span: #0 bytes(261..266), | |
| }, | |
| ], | |
| }, | |
| }, | |
| ], | |
| vis: Visibility::Inherited, | |
| mutability: FieldMutability::None, | |
| // 字段名 args | |
| ident: Some( | |
| Ident { | |
| ident: "args", | |
| span: #0 bytes(273..277), | |
| }, | |
| ), | |
| colon_token: Some( | |
| Colon, | |
| ), | |
| // 字段类型 Vec<String> | |
| ty: Type::Path { | |
| qself: None, | |
| path: Path { | |
| leading_colon: None, | |
| segments: [ | |
| PathSegment { | |
| ident: Ident { | |
| ident: "Vec", | |
| span: #0 bytes(279..282), | |
| }, | |
| arguments: PathArguments::AngleBracketed { | |
| colon2_token: None, | |
| lt_token: Lt, | |
| args: [ | |
| GenericArgument::Type( | |
| Type::Path { | |
| qself: None, | |
| path: Path { | |
| leading_colon: None, | |
| segments: [ | |
| PathSegment { | |
| ident: Ident { | |
| ident: "String", | |
| span: #0 bytes(283..289), | |
| }, | |
| arguments: PathArguments::None, | |
| }, | |
| ], | |
| }, | |
| }, | |
| ), | |
| ], | |
| gt_token: Gt, | |
| }, | |
| }, | |
| ], | |
| }, | |
| }, | |
| }, | |
| // 其它字段省略 |
宏入口
代码语言：rust
复制
| use proc_macro::TokenStream; | |
| use syn::{parse_macro_input, DeriveInput}; | |
| pub fn builder_derive(input: TokenStream) -> TokenSteam { | |
| let input = parse_macro_input(input as DeriveInput); | |
| // 从 data中获取 fields 字段数据集合 | |
| let fields = ...; | |
| let setter_fns = fields.iter().map(|f| { | |
| // 无属性参数时功能处理 | |
| if f.attrs.is_empty() { return quote!{}; } | |
| // 遍历属性参数 | |
| for attr in f.attrs.iter() { | |
| // parse_builder_attr 解析当前的属性数据 | |
| let arguments = parse_builder_attr(attr); | |
| // 根据属性参数再处理其它处理 | |
| } | |
| }); | |
| } |
属性解析实现
代码语言：rust
复制
| /// builder 属性参数数据类型 | |
| struct BuilderAttribute { | |
| /// 属性Key | |
| pub key : String, | |
| /// 属性值 | |
| pub value: Option<String>, | |
| /// 属性meta path, 用于错误提示位置显示 | |
| /// 类似： | |
| /// --> main.rs:14:7 | |
| /// | | |
| /// 14| #[builder(each = "arg")] | |
| /// | ^^^^^^^^^^^^^^^^^^^^ | |
| pub meta: Meta, | |
| } | |
| /// 属性参数解析 | |
| /// #[builder(each = "env")] | |
| /// 转为 | |
| /// [BuilderAttribute { key: "each", value: Some("env"), meta: syn::Meta }] | |
| fn parse_builder_attr(attr: &syn::Attribute) -> Vec<BuilderAttribute> { | |
| let mut arguments = Vec::new(); | |
| if attr.path().is_ident("builder") { | |
| // parse_args_with 参考: | |
| // https://docs.rs/syn/2.0.48/syn/struct.Attribute.html#alternatives | |
| // nested 类型 Vec<Meta::NameValue> | |
| if let Ok(nested) = attr.parse_args_with(Punctuated::<Meta, Token![,]>::parse_terminated) { | |
| for meta in &nested { | |
| if let Some(ident) = meta.path().get_ident() { | |
| let key = ident.to_string(); | |
| let mut value: Option<String> = None; | |
| if let Meta::NameValue(name_value) = meta { | |
| if let Expr::Lit(ref expr_lit) = name_value.value { | |
| value = match expr_lit.lit { | |
| Lit::Str(ref lit_str) => Some(lit_str.value()), | |
| Lit::Bool(ref lit_bool) => Some(lit_bool.value.to_string()), | |
| Lit::Int(ref lit_int) => Some(lit_int.base10_digits().to_owned()), | |
| _ => None, | |
| } | |
| } | |
| }; | |
| arguments.push(BuilderAttribute { | |
| key, | |
| value, | |
| meta: attr.meta.clone(), | |
| }); | |
| } | |
| } | |
| } | |
| } | |
| arguments | |
| } |
解析结果
代码语言：ini
复制
| [ | |
| BuilderAttribute { | |
| // 属性键 | |
| key: "each", | |
| // 属性值 | |
| value: Some( | |
| "arg", | |
| ), | |
| // Meta | |
| meta: Meta::List { | |
| path: Path { | |
| leading_colon: None, | |
| segments: [ | |
| PathSegment { | |
| ident: Ident { | |
| ident: "builder", | |
| span: #0 bytes(246..253), | |
| }, | |
| arguments: PathArguments::None, | |
| }, | |
| ], | |
| }, | |
| delimiter: MacroDelimiter::Paren( | |
| Paren, | |
| ), | |
| tokens: TokenStream [ | |
| Ident { | |
| ident: "each", | |
| span: #0 bytes(254..258), | |
| }, | |
| Punct { | |
| ch: '=', | |
| spacing: Alone, | |
| span: #0 bytes(259..260), | |
| }, | |
| Literal { | |
| kind: Str, | |
| symbol: "arg", | |
| suffix: None, | |
| span: #0 bytes(261..266), | |
| }, | |
| ], | |
| }, | |
| }, | |
| ] |