unused variable in rust codegen

[katef]

Despite my efforts, we still have this occasionally:

[OK    ] line 2648: pcre regexp /\w+(.)(.)?def/s matched "abc\r\ndef"
[OK    ] line 2652: pcre regexp /^\w+=.*(\\\n.*)*/ matched "abc=xyz\\\npqr"
[OK    ] line 2656: pcre regexp /^(a()*)*/ matched "aaaa"
warning: unused variable: `c`
  --> /tmp/fsmcompile_src-sQw3dT.rs:16:22
   |
16 |                 let &c = bytes.next()?;
   |                      ^ help: if this is intentional, prefix it with an underscore: `_c`
   |
   = note: `#[warn(unused_variables)]` on by default
warning: 1 warning emitted
build/test/retest/testinput6.tst: no errors
[OK    ] line 2660: pcre regexp /^(?:a(?:(?:))*)*/ matched "aaaa"
[OK    ] line 2664: pcre regexp /^(a()+)+/ matched "aaaa"
[OK    ] line 2668: pcre regexp /^(?:a(?:(?:))+)+/ matched "aaaa"