Previously:
error: invalid format string: invalid argument name `_x`
--> src/main.rs:2:16
|
2 | println!("{_x}", a=0);
| ^^ invalid argument name in format string
|
= note: argument names cannot start with an underscore
Not supporting identifiers starting with underscore appears to have been
an arbitrary limitation from 2013 in code that was most likely never
reviewed:
https://github.com/rust-lang/rust/pull/8245/files#diff-0347868ef389c805e97636623e4a4ea6R277
The error message was dutifully improved in #50610 but is there any
reason that leading underscore would be a special case?
This commit updates the format_args parser to accept identifiers with
leading underscores.
