Added support for quickmain to use log crate.

* Added crate feature `quickmain_log`

When `quickmain_log` is enabled, the `quick_main!` entry point writes
the terminating error chain to `log::error!` instead of stdout.

Author: dgrnbrg

Cargo.toml

@@ -20,7 +20,9 @@ travis-ci = { repository = "rust-lang-nursery/error-chain" }
 
 [features]
 default = ["backtrace", "example_generated"]
+quickmain_log = ["log"]
 example_generated = []
 
 [dependencies]
 backtrace = { version = "0.3", optional = true }
+log = {version = "0.3", optional = true }

src/lib.rs

@@ -540,6 +540,10 @@
 #[cfg(feature = "backtrace")]
 extern crate backtrace;
 
+#[cfg(feature = "quickmain_log")]
+#[macro_use]
+extern crate log;
+
 use std::error;
 use std::iter::Iterator;
 #[cfg(feature = "backtrace")]
@@ -559,6 +563,7 @@ mod error_chain;
 #[macro_use]
 mod quick_main;
 pub use quick_main::ExitCode;
+pub use quick_main::print_quickmain_error;
 #[cfg(feature = "example_generated")]
 pub mod example_generated;
 

src/quick_main.rs

@@ -42,13 +42,10 @@
 macro_rules! quick_main {
     ($main:expr) => {
         fn main() {
-            use ::std::io::Write;
-
             ::std::process::exit(match $main() {
                 Ok(ret) => $crate::ExitCode::code(ret),
                 Err(ref e) => {
-                    write!(&mut ::std::io::stderr(), "{}", $crate::ChainedError::display_chain(e))
-                        .expect("Error writing to stderr");
+                    { $crate::print_quickmain_error(e); }
 
                     1
                 }
@@ -75,3 +72,20 @@ impl ExitCode for () {
         0
     }
 }
+
+/// When using `quick_main!`, prints the error if the program doesn't terminate successfully.
+pub fn print_quickmain_error<K,E: ::ChainedError<ErrorKind=K>>(e: &E) {
+    print_error_helper(e);
+}
+
+#[cfg(not(feature = "quickmain_log"))]
+fn print_error_helper<K,E: ::ChainedError<ErrorKind=K>>(e: &E) {
+    use ::std::io::Write;
+    write!(&mut ::std::io::stderr(), "{}", ::ChainedError::display_chain(e))
+        .expect("Error writing to stderr");
+}
+
+#[cfg(feature = "quickmain_log")]
+fn print_error_helper<K,E: ::ChainedError<ErrorKind=K>>(e: &E) {
+    { error!("{}", ::ChainedError::display_chain(e)) }
+}