tr: improve argument validation errors

text/tests/tr/mod.rs

@@ -628,3 +628,16 @@ fn tr_streaming_state() {
 fn tr_minimal_d_s() {
     tr_test(&["-d", "-s", "", "A"], "1AA", "1A");
 }
+
+#[test]
+fn tr_missing_equiv() {
+    tr_bad_arguments_failure_test(
+        &["-d", "[==]"],
+        "tr: missing equivalence class character '[==]'\n",
+    );
+}
+
+#[test]
+fn tr_missing_character_class() {
+    tr_bad_arguments_failure_test(&["-d", "[::]"], "tr: missing character class name '[::]'\n");
+}

text/tr.rs

@@ -388,7 +388,7 @@ mod parsing {
         let char_between_equals_signs = match between_equals_signs.as_slice() {
             &[ch] => ch,
             &[] => {
-                unreachable!();
+                return Err("tr: missing equivalence class character '[==]'".to_owned());
             }
             sl => {
                 const ERROR_MESSAGE_PREFIX: &str = "tr: ";
@@ -756,6 +756,9 @@ mod parsing {
                 .chain('A'..='F')
                 .chain('a'..='f')
                 .collect::<Vec<_>>(),
+            "" => {
+                return Err("tr: missing character class name '[::]'".to_string());
+            }
             st => return Err(format!("tr: invalid character class ‘{st}’")),
         };