From c5b8fe315162440c1d808bc0a684a412d78bfa76 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date: Thu, 5 Dec 2013 14:31:57 -0500
Subject: Print a trace message if a build fails due to the platform being
 unknown

---
 src/libstore/build.cc | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 63e34d256..0a8bdbaf5 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -1623,10 +1623,13 @@ void DerivationGoal::startBuilder()
     startNest(nest, lvlInfo, format(repair ? "repairing path(s) %1%" : "building path(s) %1%") % showPaths(missingPaths));
 
     /* Right platform? */
-    if (!canBuildLocally(drv.platform))
+    if (!canBuildLocally(drv.platform)) {
+        if (settings.printBuildTrace)
+            printMsg(lvlError, format("@ unsupported-platform %1% %2%") % drvPath % drv.platform);
         throw Error(
             format("a `%1%' is required to build `%3%', but I am a `%2%'")
             % drv.platform % settings.thisSystem % drvPath);
+    }
 
     /* Construct the environment passed to the builder. */
 
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