fix: implement valid AccountId generation from arbitrary data (#9107)

Looks like the former implementation of `Arbitrary` for `AccountId` would assume all strings were well-formed `AccountId`'s. Skipping the validation logic. Which is untrue.

This patch swaps out the derive for a custom implementation that ensures all arbitrary byte slices are validated before an `AccountId` is constructed.

core/account-id/src/lib.rs

@@ -377,15 +377,18 @@ impl<'a> arbitrary::Arbitrary<'a> for AccountId {
     }
 
     fn arbitrary(u: &mut arbitrary::Unstructured<'a>) -> arbitrary::Result<Self> {
-        let mut s = u.arbitrary::<&str>()?;
-        loop {
-            match s.parse::<AccountId>() {
-                Ok(account_id) => break Ok(account_id),
-                Err(ParseAccountError { char: Some((idx, _)), .. }) => {
-                    s = &s[..idx];
-                    continue;
+        let s = u.arbitrary::<&str>()?;
+        match s.parse::<AccountId>() {
+            Ok(account_id) => Ok(account_id),
+            Err(e) => {
+                if let Some((valid_up_to, _)) = e.char {
+                    let valid = &s[..valid_up_to];
+                    debug_assert!(AccountId::validate(valid).is_ok());
+                    #[allow(deprecated)]
+                    Ok(AccountId::new_unvalidated(valid.to_string()))
+                } else {
+                    Err(arbitrary::Error::IncorrectFormat)
                 }
-                _ => break Err(arbitrary::Error::IncorrectFormat),
             }
         }
     }
@@ -719,26 +722,4 @@ mod tests {
             );
         }
     }
-
-    #[test]
-    #[cfg(feature = "arbitrary")]
-    fn test_arbitrary() {
-        let corpus = [
-            ("a|bcd", None),
-            ("ab|cde", Some("ab")),
-            ("a_-b", None),
-            ("ab_-c", Some("ab")),
-            ("a", None),
-            ("miraclx.near", Some("miraclx.near")),
-            ("01234567890123456789012345678901234567890123456789012345678901234", None),
-        ];
-
-        for (input, expected_output) in corpus {
-            assert!(input.len() <= u8::MAX as usize);
-            let data = [input.as_bytes(), &[input.len() as _]].concat();
-            let mut u = arbitrary::Unstructured::new(&data);
-
-            assert_eq!(u.arbitrary::<AccountId>().as_deref().ok(), expected_output);
-        }
-    }
 }