🏗️ Parse into intermediate list of elements

The list of elements idea was inspired by work performed by @elmarco over in dbus2#23 , which brings a lot of benefits that I was trying to provide using explicit low-level readers and writers of XML events: namely that we have access to element sequence (important for "last `<type>` wins" and the correct interpolation of `<include>`). serde offers a [try_from](https://serde.rs/container-attrs.html#try_from) attribute that allows us to abstract this away from the consumer.

src/config.rs

@@ -4,8 +4,6 @@ use anyhow::{Error, Result};
 use quick_xml::{events::Event, Reader};
 use serde::Deserialize;
 
-mod transform;
-
 const EXPECTED_DOCTYPE_PARTS: &[&str] = &[
     "busconfig",
     "PUBLIC",
@@ -19,11 +17,11 @@ const EXPECTED_DOCTYPE_PARTS: &[&str] = &[
 
 /// implements [`dbus-daemon`'s Configuration File](https://dbus.freedesktop.org/doc/dbus-daemon.1.html#configuration_file)
 #[derive(Clone, Debug, Default, Deserialize, PartialEq)]
+#[serde(try_from = "Document")]
 pub struct BusConfig {
     /// If `Some`, connections that authenticated using the ANONYMOUS mechanism will be authorized to connect.
     /// This option has no practical effect unless the ANONYMOUS mechanism has also been enabled using the `auth` option.
-    // TODO: consider finding a way to make this `bool` instead of `Option<()>`
-    pub allow_anonymous: Option<()>,
+    pub allow_anonymous: bool,
 
     /// Lists permitted authorization mechanisms.
     /// If this element doesn't exist, then all known mechanisms are allowed.
@@ -33,18 +31,16 @@ pub struct BusConfig {
     pub auth: HashSet<String>,
 
     /// If `Some`, the bus daemon becomes a real daemon (forks into the background, etc.).
-    // TODO: consider finding a way to make this `bool` instead of `Option<()>`
-    pub fork: Option<()>,
+    pub fork: bool,
 
     /// If `Some`, the bus daemon keeps its original umask when forking.
     /// This may be useful to avoid affecting the behavior of child processes.
-    // TODO: consider finding a way to make this `bool` instead of `Option<()>`
-    pub keep_umask: Option<()>,
+    pub keep_umask: bool,
 
     /// Address(es) that the bus should listen on.
     /// The address is in the standard D-Bus format that contains a transport name plus possible parameters/options.
     #[serde(default)]
-    pub listen: Vec<String>,
+    pub listen: HashSet<String>,
 
     /// The bus daemon will write its pid to the specified file.
     pub pidfile: Option<PathBuf>,
@@ -55,8 +51,7 @@ pub struct BusConfig {
     pub servicedir: Vec<PathBuf>,
 
     /// If `Some`, the bus daemon will log to syslog.
-    // TODO: consider finding a way to make this `bool` instead of `Option<()>`
-    pub syslog: Option<()>,
+    pub syslog: bool,
 
     /// This element only controls which message bus specific environment variables are set in activated clients.
     pub r#type: Option<Type>,
@@ -67,6 +62,37 @@ pub struct BusConfig {
     pub user: Option<String>,
 }
 
+impl TryFrom<Document> for BusConfig {
+    type Error = Error;
+
+    fn try_from(value: Document) -> Result<Self> {
+        let mut bc = BusConfig::default();
+
+        for element in value.busconfig {
+            match element {
+                Element::AllowAnonymous => bc.allow_anonymous = true,
+                Element::Auth(s) => {
+                    bc.auth.insert(s);
+                }
+                Element::Fork => bc.fork = true,
+                Element::KeepUmask => bc.keep_umask = true,
+                Element::Listen(s) => {
+                    bc.listen.insert(s);
+                }
+                Element::Pidfile(p) => bc.pidfile = Some(p),
+                Element::Servicedir(p) => {
+                    bc.servicedir.push(p);
+                }
+                Element::Syslog => bc.syslog = true,
+                Element::Type(TypeElement { r#type: value }) => bc.r#type = Some(value),
+                Element::User(s) => bc.user = Some(s),
+            }
+        }
+
+        Ok(bc)
+    }
+}
+
 impl BusConfig {
     pub fn parse(s: &str) -> Result<Self> {
         let mut reader = Reader::from_reader(s.as_bytes());
@@ -104,10 +130,7 @@ impl BusConfig {
             }
         }
 
-        let reader = transform::resolve_includes(s.as_bytes())?;
-        let reader = transform::last_occurrence_wins(reader.into_inner().as_slice())?;
-
-        quick_xml::de::from_reader(reader.into_inner().as_slice()).map_err(Error::from)
+        quick_xml::de::from_str(s).map_err(Error::from)
     }
 }
 
@@ -118,6 +141,33 @@ pub enum Type {
     System,
 }
 
+#[derive(Clone, Debug, Default, Deserialize, PartialEq)]
+struct Document {
+    #[serde(rename = "$value", default)]
+    busconfig: Vec<Element>,
+}
+
+#[derive(Clone, Debug, Deserialize, PartialEq)]
+#[serde(rename_all = "snake_case")]
+enum Element {
+    AllowAnonymous,
+    Auth(String),
+    Fork,
+    KeepUmask,
+    Listen(String),
+    Pidfile(PathBuf),
+    Servicedir(PathBuf),
+    Syslog,
+    Type(TypeElement),
+    User(String),
+}
+
+#[derive(Clone, Debug, Deserialize, PartialEq)]
+struct TypeElement {
+    #[serde(rename = "$text")]
+    r#type: Type,
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -188,10 +238,10 @@ mod tests {
         assert_eq!(
             busconfig,
             BusConfig {
-                allow_anonymous: Some(()),
-                fork: Some(()),
-                keep_umask: Some(()),
-                syslog: Some(()),
+                allow_anonymous: true,
+                fork: true,
+                keep_umask: true,
+                syslog: true,
                 ..Default::default()
             }
         );
@@ -230,11 +280,11 @@ mod tests {
         assert_eq!(
             busconfig,
             BusConfig {
-                listen: vec![
+                listen: HashSet::from_iter(vec![
                     String::from("unix:path=/tmp/foo"),
                     String::from("tcp:host=localhost,port=1234"),
                     String::from("tcp:host=localhost,port=0,family=ipv4"),
-                ],
+                ]),
                 ..Default::default()
             }
         );
@@ -275,7 +325,7 @@ mod tests {
         assert_eq!(
             busconfig,
             BusConfig {
-                servicedir: vec![PathBuf::from("/example"), PathBuf::from("/anotherexample"),],
+                servicedir: vec![PathBuf::from("/example"), PathBuf::from("/anotherexample")],
                 ..Default::default()
             }
         );