Commits on Nov 28, 2019

1. Allow any identifier as format arg name …

   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 rust-lang#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.

   

   dtolnay committed Nov 28, 2019
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