Fix defaults serialization and 'invalid type: unit value' deserialization error (#60)

src/config.rs

@@ -10,7 +10,7 @@ use ser::ConfigSerializer;
 use source::Source;
 
 use path;
-use value::{Value, ValueKind, ValueWithKey};
+use value::{Table, Value, ValueKind, ValueWithKey};
 
 #[derive(Clone, Debug)]
 enum ConfigKind {
@@ -49,7 +49,12 @@ pub struct Config {
 
 impl Config {
     pub fn new() -> Self {
-        Config::default()
+        Self {
+            kind: ConfigKind::default(),
+            // Config root should be instantiated as an empty table
+            // to avoid deserialization errors.
+            cache: Value::new(None, Table::new()),
+        }
     }
 
     /// Merge in a configuration property source.
@@ -129,6 +134,26 @@ impl Config {
         self.refresh()
     }
 
+    /// Set the configuration defaults by serializing them from given value.
+    pub fn set_defaults<T>(&mut self, value: &T) -> Result<&mut Config>
+    where
+        T: Serialize,
+    {
+        match self.kind {
+            ConfigKind::Mutable {
+                ref mut defaults, ..
+            } => {
+                for (key, val) in Self::try_from(&value)?.collect()? {
+                    defaults.insert(key.parse()?, val);
+                }
+            }
+
+            ConfigKind::Frozen => return Err(ConfigError::Frozen),
+        }
+
+        self.refresh()
+    }
+
     pub fn set<T>(&mut self, key: &str, value: T) -> Result<&mut Config>
     where
         T: Into<Value>,
@@ -192,13 +217,21 @@ impl Config {
         T::deserialize(self)
     }
 
-    /// Attempt to deserialize the entire configuration into the requested type.
+    /// Attempt to serialize the entire configuration from the given type.
     pub fn try_from<T: Serialize>(from: &T) -> Result<Self> {
         let mut serializer = ConfigSerializer::default();
         from.serialize(&mut serializer)?;
         Ok(serializer.output)
     }
 
+    /// Attempt to serialize the entire configuration from the given type
+    /// as default values.
+    pub fn try_defaults_from<T: Serialize>(from: &T) -> Result<Self> {
+        let mut c = Self::new();
+        c.set_defaults(from)?;
+        Ok(c)
+    }
+
     #[deprecated(since = "0.7.0", note = "please use 'try_into' instead")]
     pub fn deserialize<'de, T: Deserialize<'de>>(self) -> Result<T> {
         self.try_into()

tests/defaults.rs

@@ -0,0 +1,36 @@
+extern crate config;
+
+#[macro_use]
+extern crate serde_derive;
+
+use config::*;
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Settings {
+    pub db_host: String,
+}
+
+impl Default for Settings {
+    fn default() -> Self {
+        Settings {
+            db_host: String::from("default"),
+        }
+    }
+}
+
+#[test]
+fn set_defaults() {
+    let mut c = Config::new();
+    c.set_defaults(&Settings::default())
+        .expect("Setting defaults failed");
+    let s: Settings = c.try_into().expect("Deserialization failed");
+
+    assert_eq!(s.db_host, "default");
+}
+
+#[test]
+fn try_from_defaults() {
+    let c = Config::try_from(&Settings::default()).expect("Serialization failed");
+    let s: Settings = c.try_into().expect("Deserialization failed");
+    assert_eq!(s.db_host, "default");
+}

tests/empty.rs

@@ -0,0 +1,21 @@
+extern crate config;
+
+#[macro_use]
+extern crate serde_derive;
+
+use config::*;
+
+#[derive(Debug, Serialize, Deserialize)]
+struct Settings {
+    #[serde(skip)]
+    foo: isize,
+    #[serde(skip)]
+    bar: u8,
+}
+
+#[test]
+fn empty_deserializes() {
+    let s: Settings = Config::new().try_into().expect("Deserialization failed");
+    assert_eq!(s.foo, 0);
+    assert_eq!(s.bar, 0);
+}