Reject invalid raw identifiers in the fallback

src/fallback.rs

@@ -640,13 +640,24 @@ impl Ident {
     fn _new(string: &str, raw: bool, span: Span) -> Self {
         validate_ident(string);
 
+        if raw && !Self::can_be_raw(string) {
+            panic!("`{}` cannot be a raw identifier", string);
+        }
+
         Ident {
             sym: string.to_owned(),
             span,
             raw,
         }
     }
 
+    fn can_be_raw(string: &str) -> bool {
+        match string {
+            "" | "_" | "super" | "self" | "Self" | "crate" | "$crate" | "{{root}}" => false,
+            _ => true,
+        }
+    }
+
     pub fn new(string: &str, span: Span) -> Self {
         Ident::_new(string, false, span)
     }

tests/test.rs

@@ -21,7 +21,16 @@ fn raw_idents() {
         "r#String"
     );
     assert_eq!(Ident::new_raw("fn", Span::call_site()).to_string(), "r#fn");
-    assert_eq!(Ident::new_raw("_", Span::call_site()).to_string(), "r#_");
+}
+
+#[test]
+#[should_panic(expected = "`_` cannot be a raw identifier")]
+fn ident_raw_blocked() {
+    Ident::new_raw("_", Span::call_site());
+    Ident::new_raw("super", Span::call_site());
+    Ident::new_raw("self", Span::call_site());
+    Ident::new_raw("Self", Span::call_site());
+    Ident::new_raw("crate", Span::call_site());
 }
 
 #[test]