[idna] Add unit tests for punycode prefix edge cases

Helps with answering question asked in
servo#373

The test invariants here are based on and checked against:

* Processing Step 4 of UTS#46
  (<http://www.unicode.org/reports/tr46/#ProcessingStepPunycode>)

* 6.2 Decoding procedure of Punycode RFC.
  (<https://ietf.org/rfc/rfc3492.txt>)

idna/src/uts46.rs

@@ -357,7 +357,7 @@ enum Error {
 ///
 /// This is opaque for now, only indicating the presence of at least one error.
 /// More details may be exposed in the future.
-#[derive(Debug)]
+#[derive(Debug, Eq, PartialEq)]
 pub struct Errors(Vec<Error>);
 
 /// http://www.unicode.org/reports/tr46/#ToASCII

idna/tests/unit.rs

@@ -5,16 +5,69 @@ use idna::uts46;
 use unicode_normalization::char::is_combining_mark;
 
 
-fn _to_ascii(domain: &str) -> Result<String, uts46::Errors> {
-    uts46::to_ascii(domain, uts46::Flags {
-        transitional_processing: false,
-        use_std3_ascii_rules: true,
-        verify_dns_length: true,
-    })
+/// https://github.com/servo/rust-url/issues/373
+#[test]
+fn test_punycode_prefix_with_length_check() {
+    fn _to_ascii(domain: &str) -> Result<String, uts46::Errors> {
+        uts46::to_ascii(
+            domain,
+            uts46::Flags {
+                transitional_processing: false,
+                use_std3_ascii_rules: true,
+                verify_dns_length: true,
+            },
+        )
+    }
+
+    assert!(_to_ascii("xn--").is_err());
+    assert!(_to_ascii("xn---").is_err());
+    assert!(_to_ascii("xn-----").is_err());
+    assert!(_to_ascii("xn--.").is_err());
+    assert!(_to_ascii("xn--...").is_err());
+    assert!(_to_ascii(".xn--").is_err());
+    assert!(_to_ascii("...xn--").is_err());
+    assert!(_to_ascii("xn--.xn--").is_err());
+    assert!(_to_ascii("xn--.example.org").is_err());
+}
+
+/// https://github.com/servo/rust-url/issues/373
+#[test]
+fn test_punycode_prefix_without_length_check() {
+    fn _to_ascii(domain: &str) -> Result<String, uts46::Errors> {
+        uts46::to_ascii(
+            domain,
+            uts46::Flags {
+                transitional_processing: false,
+                use_std3_ascii_rules: true,
+                verify_dns_length: false,
+            },
+        )
+    }
+
+    assert_eq!(_to_ascii("xn--"), Ok("".to_owned()));
+    assert!(_to_ascii("xn---").is_err());
+    assert!(_to_ascii("xn-----").is_err());
+    assert_eq!(_to_ascii("xn--."), Ok(".".to_owned()));
+    assert_eq!(_to_ascii("xn--..."), Ok("...".to_owned()));
+    assert_eq!(_to_ascii(".xn--"), Ok(".".to_owned()));
+    assert_eq!(_to_ascii("...xn--"), Ok("...".to_owned()));
+    assert_eq!(_to_ascii("xn--.xn--"), Ok(".".to_owned()));
+    assert_eq!(_to_ascii("xn--.example.org"), Ok(".example.org".to_owned()));
 }
 
 #[test]
 fn test_v5() {
+    fn _to_ascii(domain: &str) -> Result<String, uts46::Errors> {
+        uts46::to_ascii(
+            domain,
+            uts46::Flags {
+                transitional_processing: false,
+                use_std3_ascii_rules: true,
+                verify_dns_length: true,
+            },
+        )
+    }
+
     // IdnaTest:784 蔏｡𑰺
     assert!(is_combining_mark('\u{11C3A}'));
     assert!(_to_ascii("\u{11C3A}").is_err());
@@ -24,12 +77,26 @@ fn test_v5() {
 
 #[test]
 fn test_v8_bidi_rules() {
-    assert_eq!(_to_ascii("abc").unwrap(), "abc");
-    assert_eq!(_to_ascii("123").unwrap(), "123");
-    assert_eq!(_to_ascii("אבּג").unwrap(), "xn--kdb3bdf");
-    assert_eq!(_to_ascii("ابج").unwrap(), "xn--mgbcm");
-    assert_eq!(_to_ascii("abc.ابج").unwrap(), "abc.xn--mgbcm");
-    assert_eq!(_to_ascii("אבּג.ابج").unwrap(), "xn--kdb3bdf.xn--mgbcm");
+    fn _to_ascii(domain: &str) -> Result<String, uts46::Errors> {
+        uts46::to_ascii(
+            domain,
+            uts46::Flags {
+                transitional_processing: false,
+                use_std3_ascii_rules: true,
+                verify_dns_length: true,
+            },
+        )
+    }
+
+    assert_eq!(_to_ascii("abc"), Ok("abc".to_owned()));
+    assert_eq!(_to_ascii("123"), Ok("123".to_owned()));
+    assert_eq!(_to_ascii("אבּג"), Ok("xn--kdb3bdf".to_owned()));
+    assert_eq!(_to_ascii("ابج"), Ok("xn--mgbcm".to_owned()));
+    assert_eq!(_to_ascii("abc.ابج"), Ok("abc.xn--mgbcm".to_owned()));
+    assert_eq!(
+        _to_ascii("אבּג.ابج"),
+        Ok("xn--kdb3bdf.xn--mgbcm".to_owned())
+    );
 
     // Bidi domain names cannot start with digits
     assert!(_to_ascii("0a.\u{05D0}").is_err());