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Document why not use concat! in dbg! macro #81233

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Jan 22, 2021
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Document why not use concat! in dbg! macro #81233

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4 changes: 4 additions & 0 deletions library/std/src/macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -282,6 +282,10 @@ macro_rules! eprintln {
#[macro_export]
#[stable(feature = "dbg_macro", since = "1.32.0")]
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!());
};
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