为什么我必须使用仅由我的依赖项使用的宏 [英] Why must I use macros only used by my dependencies
问题描述
为什么我必须明确macro_use 宏不直接由我的代码使用，而只能由我的代码依赖项使用?
Why must I explicitly macro_use macros not directly used by my code, but only used by my codes dependencies?
下图有两种情况:
- 宏仅由我的依赖项使用
call、do_parse、map、take、error_if
parse_der_defined(一个函数)、fold_parsers(一个函数)、DerObject(一个结构体)、DerObjectContent(一个结构体)
parse_der_defined(a function),fold_parsers(a function),DerObject(a struct),DerObjectContent(a struct)
额外问题
在编码时处理这个问题的最佳工作流程是什么?只是编译器错误，添加名称，冲洗，重复?
What's the best workflow to deal with this while coding? Just compiler error, add the name, rinse, repeat?
// Why is this necessary? My code does not directly use macros from this scope.
#[macro_use(call, do_parse, map, take)]
extern crate nom;
// Why is this necessary? My code does not directly use macros from this scope.
#[macro_use(error_if)]
extern crate rusticata_macros;
// Why is this necessary? My code does not directly use macros from this scope.
#[macro_use(parse_der_sequence_defined, parse_der_defined, fold_parsers)]
extern crate der_parser;
// My code does not directly use these names. Why do I need to `use` them?
use der_parser::{der_read_element_header, DerObjectContent};
// Why is this necessary? My code does not directly use these names.
use nom::{Err, ErrorKind};
// I actually use these
use nom::IResult;
use der_parser::DerObject;
fn seq_of_integers(input: &[u8]) -> IResult<&[u8], DerObject> {
parse_der_sequence_defined!(input, der_parser::parse_der_integer)
}
fn main() {
let input = [
0x30, // ASN.1 sequence
0x03, // length 3 bytes
0x02, // ASN.1 Integer
0x01, // length 1 byte
0x07, // 7
];
let der_result = seq_of_integers(&input);
match der_result {
IResult::Done(_unparsed_suffix, der) => {
assert_eq!(_unparsed_suffix.len(), 0);
let der_objects = der.as_sequence().unwrap();
for (index, der_object) in der_objects.iter().enumerate() {
println!("{}: {}", index, der_object.content.as_u32().unwrap());
}
}
IResult::Error(error) => {
eprintln!("{}", error);
}
IResult::Incomplete(_needed) => {
eprintln!("{:?}", _needed);
}
};
}
推荐答案
宏是卫生的，但它们不会从定义它们的范围内引入"事物.
Macros are hygenic, but they don't "bring in" things from the scope they are defined in.
如果您在一个 crate 中定义宏，并使用相对名称而不是绝对名称(如果宏使用 der_read_element_name 而不是 ::der_parser::der_read_element_name)，那么您需要使用 use 将这些方法引入范围.
If you define a macro in one crate, and use relative names rather than absolute ones (if the macro produces code using der_read_element_name rather than ::der_parser::der_read_element_name), then you are required to use use to bring those methods into scope.
对此的解决方案是在定义宏时始终使用绝对名称，或者在宏范围内使用"它们.例如，如果您有一个打开文件的宏，您可以执行以下两项操作之一.要么导入:
The solution to this is to always use absolute names when defining macros, or to 'use' them inside the macro scope. For instance, if you have a macro which opened a file, you do one of two things. Either import:
macro_rules! open {
($file:expr) => ({
// note: this has to be inside the macro expansion
// `::std` means this works anywhere, not just in
// the crate root where `std` is in scope.
use ::std::fs::File;
File::open($file)
})
}
或者直接使用绝对路径:
or use absolute paths directly:
macro_rules! open {
($file:expr) => ({
::std:fs::File::open($file)
})
}
<小时>
使用其他宏的宏也会发生类似的事情！如果你有两个板条箱，比如 cratea 和:
macro_rules! say_hello {
() => (println!("hi"))
}
和 crateb 与:
#[macro_use]
extern crate cratea;
macro_rules! conversation {
() => ({
say_hello!();
println!("goodbye");
})
}
然后当有人使用 crateb 和 conversation!() 时，它实际上扩展为 say_hello!();println!("goodbye");，如果 say_hello 在目标 crate 中不存在，则会出错.
then when someone uses crateb with conversation!(), it literally expands to say_hello!(); println!("goodbye");, and this will error if say_hello doesn't exist in the target crate.
对此的解决方案是将所有宏从 cratea 重新导出到 crateb.您可以通过以下方式执行此操作:
The solution to this is to re-export all macros from cratea to crateb. You can do this with the following:
extern crate cratea;
pub use cratea::*;
这意味着任何依赖于在 crateb 上使用 #[macro_use] 的人也可以访问所有 cratea 的宏.因此，当您在 crateb 中的宏扩展为引用 cratea 中的宏时，它将起作用.
This will mean anyone who depends using #[macro_use] on crateb will have access to all of cratea's macros too. So, when your macro in crateb expands to reference a macro in cratea, it will work.
关于工作流程，个人轶事:
On workflow, personal anecdote:
我发现 cargo check 与 cargo-watch成为我所知道的最好的工作流程.我将在终端中启动 cargo watch，每当保存文件时，它都会开始检查并报告语法错误.
I've found cargo check with cargo-watch to be the best workflow I know of. I'll start cargo watch in a terminal, and whenever a file is saved it will start a check and just report syntax errors.
一旦我感到非常自信并且没有错误，我将根据项目实际运行 cargo run 和/或 cargo test.
Once I feel pretty confident and there are no errors, I'll actually run cargo run and/or cargo test depending on the project.
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