Add typed support for `SIZE` parameter (RFC 1870)

This also makes `Parameter` type an enum so future type support for
other parameters can be more easily added. Note that the enum is
non-exhaustive so that future variant addtions would not cause API
breaks.

src/parse/command.rs

@@ -88,10 +88,18 @@ pub fn Mail_parameters(input: &[u8]) -> IResult<&[u8], Vec<Parameter>> {
 
 /// esmtp-param = esmtp-keyword ["=" esmtp-value]
 pub fn esmtp_param(input: &[u8]) -> IResult<&[u8], Parameter> {
+    alt((
+        map_res(tuple((tag_no_case(b"SIZE="), esmtp_value)), |(_, value)| {
+            value.parse().map(Parameter::Size)
+        }),
         map(
             tuple((esmtp_keyword, opt(preceded(tag(b"="), esmtp_value)))),
-        |(keyword, value)| Parameter::new(keyword, value),
-    )(input)
+            |(keyword, value)| Parameter::Other {
+                keyword: keyword.to_owned(),
+                value: value.map(String::from),
+            },
+        ),
+    ))(input)
 }
 
 /// esmtp-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
@@ -353,7 +361,7 @@ pub fn Dot_string(input: &[u8]) -> IResult<&[u8], &str> {
 
 #[cfg(test)]
 mod test {
-    use super::{ehlo, helo, mail};
+    use super::{ehlo, helo, mail, Parameter};
     use crate::types::{Command, DomainOrAddress};
 
     #[test]
@@ -382,12 +390,12 @@ mod test {
 
     #[test]
     fn test_mail() {
-        let (rem, parsed) = mail(b"MAIL FROM:<userx@y.foo.org>\r\n???").unwrap();
+        let (rem, parsed) = mail(b"MAIL FROM:<userx@y.foo.org> size=12345\r\n???").unwrap();
         assert_eq!(
             parsed,
             Command::Mail {
                 reverse_path: "userx@y.foo.org".into(),
-                parameters: Vec::default(),
+                parameters: vec![Parameter::Size(12345)],
             }
         );
         assert_eq!(rem, b"???");

src/types.rs

@@ -110,9 +110,14 @@ impl DomainOrAddress {
 }
 
 #[derive(Clone, Debug, PartialEq, Eq)]
-pub struct Parameter {
+#[non_exhaustive]
+pub enum Parameter {
+    /// Message size declaration [RFC1870]
+    Size(u32),
+    Other {
         keyword: String,
         value: Option<String>,
+    },
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -244,20 +249,20 @@ impl Command {
 }
 
 impl Parameter {
-    pub fn new<K: Into<String>, V: Into<String>>(keyword: K, value: Option<V>) -> Parameter {
-        Parameter {
-            keyword: keyword.into(),
-            value: value.map(Into::into),
-        }
-    }
-
     pub fn serialize(&self, writer: &mut impl Write) -> std::io::Result<()> {
-        writer.write_all(self.keyword.as_bytes())?;
+        match self {
+            Parameter::Size(size) => {
+                write!(writer, "SIZE={}", size)?;
+            }
+            Parameter::Other { keyword, value } => {
+                writer.write_all(keyword.as_bytes())?;
 
-        if let Some(ref value) = self.value {
+                if let Some(ref value) = value {
                     writer.write_all(b"=")?;
                     writer.write_all(value.as_bytes())?;
                 }
+            }
+        };
 
         Ok(())
     }