Don't crash when asking for comparison but not incremental

Author: David Ungar

include/swift/Driver/Compilation.h

@@ -79,6 +79,7 @@ using CommandSet = llvm::SmallPtrSet<const Job *, 16>;
 class Compilation {
 public:
   class IncrementalSchemeComparator {
+    const bool EnableIncrementalBuildWhenConstructed;
     const bool &EnableIncrementalBuild;
     const bool EnableSourceRangeDependencies;
     const bool &UseSourceRangeDependencies;
@@ -108,7 +109,8 @@ class Compilation {
                                 const StringRef CompareIncrementalSchemesPath,
                                 unsigned SwiftInputCount,
                                 DiagnosticEngine &Diags)
-        : EnableIncrementalBuild(EnableIncrementalBuild),
+        : EnableIncrementalBuildWhenConstructed(EnableIncrementalBuild),
+          EnableIncrementalBuild(EnableIncrementalBuild),
           EnableSourceRangeDependencies(EnableSourceRangeDependencies),
           UseSourceRangeDependencies(UseSourceRangeDependencies),
           CompareIncrementalSchemesPath(CompareIncrementalSchemesPath),

lib/Driver/Compilation.cpp

@@ -2003,6 +2003,10 @@ void Compilation::IncrementalSchemeComparator::outputComparison() const {
 
 void Compilation::IncrementalSchemeComparator::outputComparison(
     llvm::raw_ostream &out) const {
+  if (!EnableIncrementalBuildWhenConstructed) {
+    out << "*** Incremental build was not enabled in the command line ***\n";
+    return;
+  }
   if (!EnableIncrementalBuild) {
     // No stats will have been gathered
     assert(!WhyIncrementalWasDisabled.empty() && "Must be a reason");