Lint single-character strings as P: Pattern args

Fixes #650

tests/compile-fail/methods.rs

@@ -2,7 +2,7 @@
 #![plugin(clippy)]
 
 #![deny(clippy, clippy_pedantic)]
-#![allow(unused, print_stdout)]
+#![allow(unused, print_stdout, non_ascii_literal)]
 
 use std::collections::BTreeMap;
 use std::collections::HashMap;
@@ -355,3 +355,92 @@ fn clone_on_double_ref() {
                                 //~^^^ERROR using `clone` on a `Copy` type
     println!("{:p} {:p}",*y, z);
 }
+
+fn single_char_pattern() {
+    let x = "foo";
+    x.split("x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.split('x');
+
+    x.split("xx");
+
+    x.split('x');
+
+    let y = "x";
+    x.split(y);
+
+    // Not yet testing for multi-byte characters
+    // Changing `r.len() == 1` to `r.chars().count() == 1` in `lint_single_char_pattern`
+    // should have done this but produced an ICE
+    x.split("ß");
+    x.split("ℝ");
+    x.split("💣");
+    // Can't use this lint for unicode code points which don't fit in a char
+    x.split("❤️");
+
+    x.contains("x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.contains('x');
+    x.starts_with("x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.starts_with('x');
+    x.ends_with("x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.ends_with('x');
+    x.find("x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.find('x');
+    x.rfind("x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.rfind('x');
+    x.rsplit("x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.rsplit('x');
+    x.split_terminator("x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.split_terminator('x');
+    x.rsplit_terminator("x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.rsplit_terminator('x');
+    x.splitn(0, "x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.splitn(0, 'x');
+    x.rsplitn(0, "x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.rsplitn(0, 'x');
+    x.matches("x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.matches('x');
+    x.rmatches("x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.rmatches('x');
+    x.match_indices("x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.match_indices('x');
+    x.rmatch_indices("x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.rmatch_indices('x');
+    x.trim_left_matches("x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.trim_left_matches('x');
+    x.trim_right_matches("x");
+    //~^ ERROR single-character string constant used as pattern
+    //~| HELP try using a char instead:
+    //~| SUGGESTION x.trim_right_matches('x');
+}