Documentation of what does for each type

src/librustc/session/config.rs

@@ -84,6 +84,7 @@ pub enum ErrorOutputType {
 }
 
 impl Default for ErrorOutputType {
+    /// Creates an `HumanReadble`, initialised with `ColorConfig` enum type `Auto`.
     fn default() -> ErrorOutputType {
         ErrorOutputType::HumanReadable(ColorConfig::Auto)
     }

src/librustc/ty/layout.rs

@@ -45,6 +45,7 @@ pub struct TargetDataLayout {
 }
 
 impl Default for TargetDataLayout {
+    /// Creates an instance of `TargetDataLayout`.
     fn default() -> TargetDataLayout {
         TargetDataLayout {
             endian: Endian::Big,