librustc: Don't ICE with type when name only contain underscores.

src/librustc/lint/builtin.rs

@@ -754,7 +754,7 @@ impl LintPass for NonCamelCaseTypes {
 
             // start with a non-lowercase letter rather than non-uppercase
             // ones (some scripts don't have a concept of upper/lowercase)
-            !ident.char_at(0).is_lowercase() && !ident.contains_char('_')
+            ident.len() > 0 && !ident.char_at(0).is_lowercase() && !ident.contains_char('_')
         }
 
         fn to_camel_case(s: &str) -> String {

src/test/compile-fail/lint-non-camel-case-types.rs

@@ -37,4 +37,6 @@ struct foo7 {
     bar: int,
 }
 
+type __ = int; //~ ERROR type `__` should have a camel case name such as ``
+
 fn main() { }