如何避免使用可选消息参数的函数出现重复代码

Question

我想编写自己的断言函数，该函数可以有一条消息：

fn foo(msg: Option<&str>) {
if let Some(m) = msg {
panic!("{} - Something went wrong: {}", m, "additional info");
} else {
panic!("Something went wrong: {}", "additional info")
}
}

如何避免类似的重复代码？如何避免对消息字符串进行多次(重新(分配？

Answer 1

我经常使用这样的小包装器：

struct OrEmptyS<T>(Option<T>);
impl<'a, T: std::fmt::Display> std::fmt::Display for OrEmptyS<T> {
fn fmt(&self, f: &mut std::fmt::Formatter) -> Result<(), std::fmt::Error> {
if let Some(v) = &self.0 {
v.fmt(f)
} else {
Ok(())
}
}
}
// Extension trait to add methods on `Option`
trait OrEmpty<T> {
fn or_empty(&self) -> OrEmptyS<&T>;
fn empty_or<'a, U: ?Sized>(&'a self, alt: &'a U) -> OrEmptyS<&'a U>;
}
impl<T> OrEmpty<T> for Option<T> {
fn or_empty(&self) -> OrEmptyS<&T> {
OrEmptyS(self.as_ref())
}
fn empty_or<'a, U: ?Sized>(&'a self, alt: &'a U) -> OrEmptyS<&'a U> {
OrEmptyS(self.as_ref().map(|_| alt))
}
}

可以用作

fn foo(msg: Option<&str>) {
panic!(
"{}{}Something went wrong: {}",
msg.or_empty(),
msg.empty_or(" - "),
"additional info"
);
}

(永久连接到操场(

Answer 2

在不打印时使用空字符串。

fn foo(msg: Option<&str>) {
panic!("{}{}Something went wrong: {}", 
{ if let Some(m) = msg { m } else { "" } },
{ if let Some(_m) = msg { " - " } else { "" } },
"additional info");
}

在rustfmt之后看起来是这样的：

fn foo(msg: Option<&str>) {
panic!(
"{}{}Something went wrong: {}",
{
if let Some(m) = msg {
m
} else {
""
}
},
{
if let Some(_m) = msg {
" - "
} else {
""
}
},
"additional info"
);
}