Auto merge of #22466 - Kimundi:str_pattern_ai_safe, r=aturon

This is not a complete implementation of the RFC:

- only existing methods got updated, no new ones added
- doc comments are not extensive enough yet
- optimizations got lost and need to be reimplemented

See rust-lang/rfcs#528

Technically a

[breaking-change]

Author: bors

lib.rs

@@ -463,7 +463,7 @@ impl<'a> LabelText<'a> {
     fn pre_escaped_content(self) -> Cow<'a, str> {
         match self {
             EscStr(s) => s,
-            LabelStr(s) => if s.contains_char('\\') {
+            LabelStr(s) => if s.contains('\\') {
                 (&*s).escape_default().into_cow()
             } else {
                 s