feat(error): implement Error::source when available

Closes #1768

Cargo.toml

@@ -38,6 +38,9 @@ tokio-threadpool = { version = "0.1.3", optional = true }
 tokio-timer = { version = "0.2", optional = true }
 want = "0.0.6"
 
+[build-dependencies]
+rustc_version = "0.2"
+
 [dev-dependencies]
 futures-timer = "0.1"
 num_cpus = "1.0"

build.rs

@@ -0,0 +1,9 @@
+extern crate rustc_version;
+
+use rustc_version::{version, Version};
+
+fn main() {
+    if version().unwrap() >= Version::parse("1.30.0").unwrap() {
+        println!("cargo:rustc-cfg=error_source");
+    }
+}

src/error.rs

@@ -323,13 +323,23 @@ impl StdError for Error {
         }
     }
 
+    #[cfg(not(error_source))]
     fn cause(&self) -> Option<&StdError> {
         self
             .inner
             .cause
             .as_ref()
             .map(|cause| &**cause as &StdError)
     }
+
+    #[cfg(error_source)]
+    fn source(&self) -> Option<&(StdError + 'static)> {
+        self
+            .inner
+            .cause
+            .as_ref()
+            .map(|cause| &**cause as &(StdError + 'static))
+    }
 }
 
 #[doc(hidden)]