当编写一个声明式的(macro_rules!)宏时，我们自动获得了宏卫生。在这个例子中，我在宏中声明了一个名为f的变量，并传入一个标识符f，它会成为一个局部变量：
macro_rules! decl_example {
($tname:ident, $mname:ident, ($($fstr:tt),*)) => {
impl std::fmt::Display for $tname {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
let Self { $mname } = self;
write!(f, $($fstr),*)
}
}
}
}
struct Foo {
f: String,
}
decl_example!(Foo, f, ("I am a Foo: {}", f));
fn main() {
let f = Foo {
f: "with a member named `f`".into(),
};
println!("{}", f);
}
这段代码编译通过，但如果你查看部分展开后的代码，你会发现存在一个明显的冲突：
impl std::fmt::Display for Foo {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
let Self { f } = self;
write!(f, "I am a Foo: {}", f)
}
}
我正在将这个声明宏写成一个过程宏，但不知道如何避免用户提供的标识符与由我的宏创建的标识符之间可能存在的名称冲突。就我所看到的，生成的代码没有卫生概念，只是一个字符串： src/main.rs
use my_derive::MyDerive;
#[derive(MyDerive)]
#[my_derive(f)]
struct Foo {
f: String,
}
fn main() {
let f = Foo {
f: "with a member named `f`".into(),
};
println!("{}", f);
}
Cargo.toml
[package]
name = "example"
version = "0.1.0"
edition = "2018"
[dependencies]
my_derive = { path = "my_derive" }
my_derive/src/lib.rs
extern crate proc_macro;
use proc_macro::TokenStream;
use quote::quote;
use syn::{parse_macro_input, DeriveInput, Meta, NestedMeta};
#[proc_macro_derive(MyDerive, attributes(my_derive))]
pub fn my_macro(input: TokenStream) -> TokenStream {
let input = parse_macro_input!(input as DeriveInput);
let name = input.ident;
let attr = input.attrs.into_iter().filter(|a| a.path.is_ident("my_derive")).next().expect("No name passed");
let meta = attr.parse_meta().expect("Unknown attribute format");
let meta = match meta {
Meta::List(ml) => ml,
_ => panic!("Invalid attribute format"),
};
let meta = meta.nested.first().expect("Must have one path");
let meta = match meta {
NestedMeta::Meta(Meta::Path(p)) => p,
_ => panic!("Invalid nested attribute format"),
};
let field_name = meta.get_ident().expect("Not an ident");
let expanded = quote! {
impl std::fmt::Display for #name {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
let Self { #field_name } = self;
write!(f, "I am a Foo: {}", #field_name)
}
}
};
TokenStream::from(expanded)
}
my_derive/Cargo.toml
[package]
name = "my_derive"
version = "0.1.0"
edition = "2018"
[lib]
proc-macro = true
[dependencies]
syn = "1.0.13"
quote = "1.0.2"
proc-macro2 = "1.0.7"
在Rust 1.40中，这会产生编译器错误：
error[E0599]: no method named `write_fmt` found for type `&std::string::String` in the current scope
--> src/main.rs:3:10
|
3 | #[derive(MyDerive)]
| ^^^^^^^^ method not found in `&std::string::String`
|
= help: items from traits can only be used if the trait is in scope
= note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
help: the following trait is implemented but not in scope; perhaps add a `use` for it:
|
1 | use std::fmt::Write;
|
有哪些技术可以将我的标识符与我不能控制的标识符隔离开来？