Lint single-character strings as P: Pattern args

[shssoichiro]

Fixes #650

[mcarton]

That one should be arg.span.

[shssoichiro]

New commit pushed with @mcarton's nits fixed.

[llogiq]

Is there a bug open for that ICE?

[shssoichiro]

I couldn't find one, although there may be (there are a lot of issues on rustc to filter through). It seems to happen here, on outputting the hint for x.split("💣");:

tests/compile-fail/methods.rs:377:13: 377:16 help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#single_char_pattern
tests/compile-fail/methods.rs:378:13: 378:16 error: single-character string constant used as pattern
tests/compile-fail/methods.rs:378     x.split("💣");
                                              ^~~
tests/compile-fail/methods.rs:4:9: 4:15 note: lint level defined here
tests/compile-fail/methods.rs:4 #![deny(clippy, clippy_pedantic)]
                                        ^~~~~~
tests/compile-fail/methods.rs:378:5: 378:17 help: try using a char instead:
error: internal compiler error: Error constructed but not emitted
thread panicked while panicking. aborting.

Probably related, the formatting on the hint output gets broken on multibyte chars, e.g.:

tests/compile-fail/methods.rs:377:13: 377:16 error: single-character string constant used as pattern
tests/compile-fail/methods.rs:377     x.split("ℝ");
                                              ^~~
tests/compile-fail/methods.rs:4:9: 4:15 note: lint level defined here
tests/compile-fail/methods.rs:4 #![deny(clippy, clippy_pedantic)]
                                        ^~~~~~
tests/compile-fail/methods.rs:377:5: 377:17 help: try using a char instead:
tests/compile-fail/methods.rs:        x.split('ℝ')");