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Rollup merge of #81233 - lzutao:dbg, r=KodrAus

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Document why not use concat! in dbg! macro

Original title: Reduce code generated by `dbg!` macro
The expanded code before/after: <https://rust.godbolt.org/z/hE3j95>.

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We cannot use `concat!` since `file!` could contains `{` or the expression is a block (`{ .. }`).
Using it will generated malformed format strings.
So let's document this reason why we don't use `concat!` macro at all.
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Mara Bos 2021-01-22 14:30:17 +00:00 committed by GitHub
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library/std/src/macros.rs
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@ -282,6 +282,10 @@ macro_rules! eprintln {
#[macro_export] #[macro_export]
#[stable(feature = "dbg_macro", since = "1.32.0")] #[stable(feature = "dbg_macro", since = "1.32.0")]
macro_rules! dbg { macro_rules! dbg {
// NOTE: We cannot use `concat!` to make a static string as a format argument
// of `eprintln!` because `file!` could contain a `{` or
// `$val` expression could be a block (`{ .. }`), in which case the `eprintln!`
// will be malformed.
() => { () => {
$crate::eprintln!("[{}:{}]", $crate::file!(), $crate::line!()); $crate::eprintln!("[{}:{}]", $crate::file!(), $crate::line!());
}; };