问题

在Rust中，您可以使用宏（macros）来实现与C中#define类似的纯文本替换。以下是示例宏定义，用于实现您描述的功能：

macro_rules! unwrap_or_default {
    ($expr:expr) => {
        $expr.unwrap_or_default()
    };
}

macro_rules! option_type {
    ($t:ty) => {
        Option<$t>
    };
}

然后，您可以在代码中使用这些宏来进行替换，如下所示：

struct ReturnPath {
    name: unwrap_or_default!(String),
    file_type: unwrap_or_default!(String),
    mtime: unwrap_or_default!(i64),
}

这将使您的代码更简洁，类似于您在问题中描述的所需效果。希望这可以帮助您实现您的目标。

struct ReturnPath {
    name: Option<String>,
    file_type: Option<String>,
    mtime: Option<i64>
}

struct ReturnPath {
    name: String??,
    file_type: String??,
    mtime: i64??
}

答案1

得分: 1

How can I achieve the same plain-text replacement in rust?
如何在Rust中实现相同的纯文本替换？

You can't do this without a DSL macro, essentially.
基本上，你不能在没有DSL宏的情况下实现这个。

C macros are token-replacement and happen before the syntax-checking step -- Rust macros are more akin to Lisp macros, which happen during the parsing step. See: https://softwareengineering.stackexchange.com/a/28387
C宏是令牌替换，发生在语法检查步骤之前 - Rust宏更类似于Lisp宏，在解析步骤期间发生。参见：https://softwareengineering.stackexchange.com/a/28387

replacing (\w+)?? with Option<$1> to simplify optional type syntax.
将 (\w+)?? 替换为 Option<$1> 以简化可选类型语法也不会在更复杂的情况下实现你想要的效果，比如如果你有 Result<T, R>?? 或 Vec<&'static str>?? 等情况。这是因为Rust的语法比正则表达式更强大 - 它在很大程度上是上下文无关的，但也有一些上下文敏感的部分（链接：https://stackoverflow.com/questions/43677722/is-rusts-syntactical-grammar-context-free-or-context-sensitive#:~:text=Answer%20straight%20from%20the%20source%20code%20for%20Rust%3A,string%20literals%20are%20the%20source%20of%20the%20problem.）。

What you're probably looking for is some sort of declarative and syntactical replacement from ($t:ty) \? \? => Option<$t>.
你可能正在寻找的是一种从 ($t:ty) \? \? => Option<$t> 进行声明性和语法替换的方式。

In addition, this regex can also probably match on non-types like a?? if a: Option<Option<T>>.
此外，如果 a: Option<Option<T>>，这个正则表达式可能也能匹配非类型的内容，比如 a??。

If you wanted to create a special syntax for this, you'd probably do something like:
如果你想要为此创建特殊的语法，你可能会这样做：

macro_rules! shorthand_struct_fields {

  /* rows of idents to types and then adjust struct definition but with T?? substituted for Option<T> */

}

struct Foo {
shorthand_struct_fields! {
  x: i32??,
}
}

I'm not sure if this is such a good idea, but if you do want to create this large macro, you would be best suited by writing a declarative macros with sub-macros that use @ rules, like so: http://danielkeep.github.io/tlborm/book/pat-internal-rules.html
我不确定这是否是一个好主意，但如果你确实想创建这个大型宏，最好的方法是编写一个使用 @ 规则的声明性宏，其中包含子宏，就像这样：http://danielkeep.github.io/tlborm/book/pat-internal-rules.html

macro_rules! shorthand_struct_fields {

  /* rows of idents to types and then adjust struct definition but with T?? substituted for Option<T> */

}

struct Foo {
shorthand_struct_fields! {
  x: i32??,
}
}