Improve build failure error messages

They're now laid out in a more readable way, and they shows the output
paths (if known).

src/libstore/build/derivation-goal.cc

@@ -354,6 +354,22 @@ struct value_comparison
 };
 
 
+std::string showKnownOutputs(Store & store, const Derivation & drv)
+{
+    std::string msg;
+    StorePathSet expectedOutputPaths;
+    for (auto & i : drv.outputsAndOptPaths(store))
+        if (i.second.second)
+            expectedOutputPaths.insert(*i.second.second);
+    if (!expectedOutputPaths.empty()) {
+        msg += "\nOutput paths:";
+        for (auto & p : expectedOutputPaths)
+            msg += fmt("\n  %s", Magenta(store.printStorePath(p)));
+    }
+    return msg;
+}
+
+
 /* At least one of the output paths could not be
    produced using a substitute.  So we have to build instead. */
 Goal::Co DerivationGoal::gaveUpOnSubstitution()
@@ -430,9 +446,14 @@ Goal::Co DerivationGoal::gaveUpOnSubstitution()
     if (nrFailed != 0) {
         if (!useDerivation)
             throw Error("some dependencies of '%s' are missing", worker.store.printStorePath(drvPath));
-        co_return done(BuildResult::DependencyFailed, {}, Error(
-                "%s dependencies of derivation '%s' failed to build",
-                nrFailed, worker.store.printStorePath(drvPath)));
+        auto msg = fmt(
+            "Cannot build '%s'.\n"
+            "Reason: " ANSI_RED "%d %s failed" ANSI_NORMAL ".",
+            Magenta(worker.store.printStorePath(drvPath)),
+            nrFailed,
+            nrFailed == 1 ? "dependency" : "dependencies");
+        msg += showKnownOutputs(worker.store, *drv);
+        co_return done(BuildResult::DependencyFailed, {}, Error(msg));
     }
 
     if (retrySubstitution == RetrySubstitution::YesNeed) {
@@ -1033,7 +1054,7 @@ void runPostBuildHook(
 void DerivationGoal::appendLogTailErrorMsg(std::string & msg)
 {
     if (!logger->isVerbose() && !logTail.empty()) {
-        msg += fmt(";\nlast %d log lines:\n", logTail.size());
+        msg += fmt("\nLast %d log lines:\n", logTail.size());
         for (auto & line : logTail) {
             msg += "> ";
             msg += line;
@@ -1090,10 +1111,14 @@ Goal::Co DerivationGoal::hookDone()
 
     /* Check the exit status. */
     if (!statusOk(status)) {
-        auto msg = fmt("builder for '%s' %s",
+        auto msg = fmt(
+            "Cannot build '%s'.\n"
+            "Reason: " ANSI_RED "builder %s" ANSI_NORMAL ".",
             Magenta(worker.store.printStorePath(drvPath)),
             statusToString(status));
 
+        msg += showKnownOutputs(worker.store, *drv);
+
         appendLogTailErrorMsg(msg);
 
         outputLocks.unlock();

src/libstore/include/nix/store/build/derivation-building-misc.hh

@@ -9,6 +9,7 @@
 namespace nix {
 
 class Store;
+struct Derivation;
 
 /**
  * Unless we are repairing, we don't both to test validity and just assume it,
@@ -49,4 +50,9 @@ struct InitialOutput
 
 void runPostBuildHook(Store & store, Logger & logger, const StorePath & drvPath, const StorePathSet & outputPaths);
 
+/**
+ * Format the known outputs of a derivation for use in error messages.
+ */
+std::string showKnownOutputs(Store & store, const Derivation & drv);
+
 }

src/libstore/unix/build/derivation-builder.cc

@@ -585,10 +585,14 @@ std::variant<std::pair<BuildResult::Status, Error>, SingleDrvOutputs> Derivation
 
             diskFull |= cleanupDecideWhetherDiskFull();
 
-            auto msg = fmt("builder for '%s' %s",
+            auto msg = fmt(
+                "Cannot build '%s'.\n"
+                "Reason: " ANSI_RED "builder %s" ANSI_NORMAL ".",
                 Magenta(store.printStorePath(drvPath)),
                 statusToString(status));
 
+            msg += showKnownOutputs(store, drv);
+
             miscMethods->appendLogTailErrorMsg(msg);
 
             if (diskFull)

tests/functional/build.sh

@@ -179,12 +179,23 @@ test "$(<<<"$out" grep -cE '^error:')" = 4
 out="$(nix build -f fod-failing.nix -L x4 2>&1)" && status=0 || status=$?
 test "$status" = 1
 test "$(<<<"$out" grep -cE '^error:')" = 2
+
+if isDaemonNewer "2.29pre"; then
+    <<<"$out" grepQuiet -E "error: Cannot build '.*-x4\\.drv'"
+    <<<"$out" grepQuiet -E "Reason: 1 dependency failed."
+else
     <<<"$out" grepQuiet -E "error: 1 dependencies of derivation '.*-x4\\.drv' failed to build"
+fi
 <<<"$out" grepQuiet -E "hash mismatch in fixed-output derivation '.*-x2\\.drv'"
 
 out="$(nix build -f fod-failing.nix -L x4 --keep-going 2>&1)" && status=0 || status=$?
 test "$status" = 1
 test "$(<<<"$out" grep -cE '^error:')" = 3
+if isDaemonNewer "2.29pre"; then
+    <<<"$out" grepQuiet -E "error: Cannot build '.*-x4\\.drv'"
+    <<<"$out" grepQuiet -E "Reason: 2 dependencies failed."
+else
     <<<"$out" grepQuiet -E "error: 2 dependencies of derivation '.*-x4\\.drv' failed to build"
+fi
 <<<"$out" grepQuiet -vE "hash mismatch in fixed-output derivation '.*-x3\\.drv'"
 <<<"$out" grepQuiet -vE "hash mismatch in fixed-output derivation '.*-x2\\.drv'"