Bootstrap solution

Author: tyranron

src/config.rs

@@ -129,6 +129,25 @@ impl Config {
         self.refresh()
     }
 
+    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 +211,21 @@ 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");
+}