Auto merge of rust-lang#8067 - frobiac:8060-backslash_escaped_in_single_char_pattern, r=giraffate

Escape backslash in single_char_pattern.rs

Escape backslash in single_char_pattern.

Previously, the proposed clippy fix for a single backslash in raw strings ```r"\"``` or ```r#"\"#``` was also only an unescaped, *single* ```'\'```:
```shell
warning: single-character string constant used as pattern
2 |     let s = r#"abc\xyz/"#.find(r"\");
  |                                ^^^^ help: try using a `char` instead: `'\'`
  |
  = note: `#[warn(clippy::single_char_pattern)]` on by default
```

This PR corrects this to a properly escaped *double* backslash ```'\\'```.
I haven't come up with any other problematic cases, a single quote was already handled.

Closes: rust-lang#8060

changelog:  Escape backslash in ``[`single_char_pattern`]``

Author: bors

clippy_lints/src/methods/utils.rs

@@ -66,7 +66,13 @@ pub(super) fn get_hint_if_single_char_arg(
                 // for regular string: "a"
                 &snip[1..(snip.len() - 1)]
             };
-            let hint = format!("'{}'", if ch == "'" { "\\'" } else { ch });
+
+            let hint = format!("'{}'", match ch {
+                "'" => "\\'" ,
+                r"\" => "\\\\",
+                _ => ch,
+            });
+
             Some(hint)
         } else {
             None

tests/ui/single_char_pattern.fixed

@@ -56,4 +56,7 @@ fn main() {
     x.split('a');
     x.split('\'');
     x.split('#');
+    // Must escape backslash in raw strings when converting to char #8060
+    x.split('\\');
+    x.split('\\');
 }

tests/ui/single_char_pattern.rs

@@ -56,4 +56,7 @@ fn main() {
     x.split(r###"a"###);
     x.split(r###"'"###);
     x.split(r###"#"###);
+    // Must escape backslash in raw strings when converting to char #8060
+    x.split(r#"\"#);
+    x.split(r"\");
 }

tests/ui/single_char_pattern.stderr

@@ -192,5 +192,17 @@ error: single-character string constant used as pattern
 LL |     x.split(r###"#"###);
    |             ^^^^^^^^^^ help: try using a `char` instead: `'#'`
 
-error: aborting due to 32 previous errors
+error: single-character string constant used as pattern
+  --> $DIR/single_char_pattern.rs:60:13
+   |
+LL |     x.split(r#"/"#);
+   |             ^^^^^^ help: try using a `char` instead: `'/'`
+
+error: single-character string constant used as pattern
+  --> $DIR/single_char_pattern.rs:61:13
+   |
+LL |     x.split(r"/");
+   |             ^^^^ help: try using a `char` instead: `'/'`
+
+error: aborting due to 34 previous errors