Refactor is_snake_case.

Author: remexre

compiler/rustc_lint/src/nonstandard_style.rs

@@ -274,18 +274,13 @@ impl NonSnakeCase {
 let ident = ident.trim_start_matches('\'');
 let ident = ident.trim_matches('_');
 
-letmut allow_underscore = true;
- ident.chars().all(|c| {
- allow_underscore = match c {
-'_'if !allow_underscore => returnfalse,
-'_' => false,
-// It would be more obvious to use `c.is_lowercase()`,
-// but some characters do not have a lowercase form
- c if !c.is_uppercase() => true,
- _ => returnfalse,
-};
-true
-})
+if ident.contains("__"){
+returnfalse;
+}
+
+// This correctly handles letters in languages with and without
+// cases, as well as numbers and underscores.
+ !ident.chars().any(char::is_uppercase)
 }
 
 let name = ident.name.as_str();