Add more detailed Domain errors

Also, a bug in `Domain.try_from` was found while working on this.
The check for empty domain was incorrect, as the `split` will always
return at least one element (the original element when the separator is
never found) and so `labels.is_empty()` would never return `false`.

lib/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "dns_lib"
-version = "0.6.0"
+version = "0.7.0"
 description = "An implementation of the DNS protocol from scratch based on the many DNS RFCs."
 
 rust-version.workspace = true

lib/src/domain.rs

@@ -18,7 +18,19 @@ const MAX_LABEL_LENGTH: usize = 63;
 const LABEL_SEPARATOR: char = '.';
 
 #[derive(Debug)]
-pub struct InvalidDomainError;
+pub enum DomainError {
+    EmptyDomain,
+    InvalidLabelFormat,
+}
+
+impl fmt::Display for DomainError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Self::EmptyDomain => write!(f, "domain is empty"),
+            Self::InvalidLabelFormat => write!(f, "label is invalid"),
+        }
+    }
+}
 
 /// Representation of a DNS domain name.
 ///
@@ -39,7 +51,7 @@ pub struct Domain {
 }
 
 impl TryFrom<String> for Domain {
-    type Error = InvalidDomainError;
+    type Error = DomainError;
 
     /// Tries to convert a [`String`] into a `Domain`.
     ///
@@ -67,7 +79,7 @@ impl TryFrom<String> for Domain {
 }
 
 impl TryFrom<&[u8]> for Domain {
-    type Error = InvalidDomainError;
+    type Error = DomainError;
 
     /// Tries to convert a slice `&[u8]` into a `Domain`.
     ///
@@ -90,15 +102,15 @@ impl TryFrom<&[u8]> for Domain {
     ///
     /// [RFC 1034, Section 3.5]: https://datatracker.ietf.org/doc/html/rfc1034#section-3.5
     fn try_from(value: &[u8]) -> Result<Self, Self::Error> {
-        let labels: Vec<&[u8]> = value.split(|&byte| byte == LABEL_SEPARATOR as u8).collect();
-
-        if labels.is_empty() {
-            return Err(InvalidDomainError);
+        if value.is_empty() {
+            return Err(DomainError::EmptyDomain);
         }
 
+        let labels: Vec<&[u8]> = value.split(|&byte| byte == LABEL_SEPARATOR as u8).collect();
+
         for label in &labels {
             if !bytes_are_label(label) {
-                return Err(InvalidDomainError);
+                return Err(DomainError::InvalidLabelFormat);
             }
         }
 
@@ -226,6 +238,16 @@ mod tests {
         assert_eq!(result.unwrap(), ok);
     }
 
+    #[rstest]
+    #[case("-.com", "label is invalid".to_string())]
+    #[case("sübway.com", "label is invalid".to_string())]
+    #[case("", "domain is empty".to_string())]
+    fn domain_try_from_string_fails(#[case] input: String, #[case] error_msg: String) {
+        let result = Domain::try_from(input);
+        assert!(result.is_err());
+        assert_eq!(result.unwrap_err().to_string(), error_msg);
+    }
+
     #[rstest]
     #[case("example")]
     #[case("mercedes-benz.de")]