log-color: Detect TTY based on stderr, not stdout

Logging goes to stderr, not stdout, so we should base our automated
detection on stderr instead of stdout.

Thanks to Ralf Jung for noticing and reporting the bug!

compiler/rustc_driver/src/lib.rs

@@ -566,6 +566,25 @@ fn stdout_isatty() -> bool {
     }
 }
 
+// FIXME remove these and use winapi 0.3 instead
+#[cfg(unix)]
+fn stderr_isatty() -> bool {
+    unsafe { libc::isatty(libc::STDERR_FILENO) != 0 }
+}
+
+#[cfg(windows)]
+fn stderr_isatty() -> bool {
+    use winapi::um::consoleapi::GetConsoleMode;
+    use winapi::um::processenv::GetStdHandle;
+    use winapi::um::winbase::STD_ERROR_HANDLE;
+
+    unsafe {
+        let handle = GetStdHandle(STD_ERROR_HANDLE);
+        let mut out = 0;
+        GetConsoleMode(handle, &mut out) != 0
+    }
+}
+
 fn handle_explain(registry: Registry, code: &str, output: ErrorOutputType) {
     let normalised =
         if code.starts_with('E') { code.to_string() } else { format!("E{0:0>4}", code) };
@@ -1290,7 +1309,7 @@ pub fn init_env_logger(env: &str) {
         Ok(value) => match value.as_ref() {
             "always" => true,
             "never" => false,
-            "auto" => stdout_isatty(),
+            "auto" => stderr_isatty(),
             _ => early_error(
                 ErrorOutputType::default(),
                 &format!(
@@ -1299,7 +1318,7 @@ pub fn init_env_logger(env: &str) {
                 ),
             ),
         },
-        Err(std::env::VarError::NotPresent) => stdout_isatty(),
+        Err(std::env::VarError::NotPresent) => stderr_isatty(),
         Err(std::env::VarError::NotUnicode(_value)) => early_error(
             ErrorOutputType::default(),
             "non-Unicode log color value: expected one of always, never, or auto",