[nightly] unnecessary parentheses warning

[SWW13]

Recent nightly versions give warnings when using named!() and do_parse!():

named!(pub parse_string <String>, do_parse!(
    value: map_res!(take_until!("\x00"), |value: &[u8]| String::from_utf8(value.to_vec())) >>
    tag!(b"\x00") >>
    (value)
));

warning: unnecessary parentheses around function argument
   |
13 | / named!(pub parse_string <String>, do_parse!(
14 | |     value: map_res!(take_until!("\x00"), |value: &[u8]| String::from_utf8(value.to_vec())) >>
15 | |     tag!(b"\x00") >>
16 | |     (value)
17 | | ));
   | |___^ help: remove these parentheses
   |
   = note: #[warn(unused_parens)] on by default
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

rustc --version --verbose:

rustc 1.25.0-nightly (4e3901d35 2018-01-23)
binary: rustc
commit-hash: 4e3901d35f6a8652f67111e7272263c9e62ab3e1
commit-date: 2018-01-23
host: x86_64-unknown-linux-gnu
release: 1.25.0-nightly
LLVM version: 4.0

[Geal]

Hi!

apparently, this is fixed in recent rust nightlies, but just in case, I removed the warning case in 98eb710

[kamarkiewicz]

I'm still encountering these warnings using nom from current master (dd33867)
Tested on rustc 1.25.0-nightly (3d292b793 2018-02-03)
Examples: https://travis-ci.org/kamarkiewicz/parsip/jobs/337208273#L837