我有一个可以工作的宏:
#[macro_export]
macro_rules! btreemap {
($( $key: expr => $val: expr ),*) => {{
let mut map = ::std::collections::BTreeMap::new();
$( map.insert($key, $val); )*
map
}}
}
但编译器警告:
warning: variable does not need to be mutable
--> src/util.rs:16:11
|
16 | let mut map = ::std::collections::BTreeMap::new();
| ----^^^
| |
| help: remove this `mut`
|
::: src/foo.rs:49:13
|
79 | foo: btreemap!{},
| ----------- in this macro invocation
|
= note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
如果我删除mut
,我会得到一个错误:
error[E0596]: cannot borrow `map` as mutable, as it is not declared as mutable
--> src/util.rs:17:10
|
16 | let map = ::std::collections::BTreeMap::new();
| --- help: consider changing this to be mutable: `mut map`
17 | $( map.insert($key, $val); )*
| ^^^ cannot borrow as mutable
|
::: src/bar.rs:110:18
|
116 | let bar = btreemap!{quux => wibble};
| -------------------------- in this macro invocation
|
= note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
即,如果mut
,它在空调用上发出警告,但如果不是,则在非空调用上出现错误。
我该怎么解决这个问题?
如何修复此问题?
您可以通过将#[allow(unused_mut)]
添加到宏生成的代码中来抑制警告:
macro_rules! btreemap {
($( $key: expr => $val: expr ),*) => {{
#[allow(unused_mut)]
let mut map = ::std::collections::BTreeMap::new();
$( map.insert($key, $val); )*
map
}}
}
该警告与警告所能得到的一样温和,这种警告抑制通常出现在宏定义中。
另一种可能性是你已经发现的,对特殊情况下的空扩展。但我更喜欢明确的警告抑制,因为它消除了(虚假的(警告,而不会使宏复杂化。您介绍的特殊情况实际上并不是所必需的-如果没有它,将生成完全相同的代码。
通过为空调用添加特殊情况修复:
#[macro_export]
macro_rules! btreemap {
() => {{
::std::collections::BTreeMap::new()
}};
($( $key: expr => $val: expr ),*) => {{
let mut map = ::std::collections::BTreeMap::new();
$( map.insert($key, $val); )*
map
}};
}