Show inputs mismatch in -driver-show-incremental

SR-2855 suggests `-driver-show-incremental` not only print information
about why certain files are included in incremental compilation, but
also print out why incremental compilation may be disabled altogether.

Add a message for one such reason: when the inputs passed to the
Swift compiler don't match the ones used previously.

Author: modocache

lib/Driver/Driver.cpp

@@ -356,9 +356,34 @@ static bool populateOutOfDateMap(InputInfoMap &map, StringRef argsHashStr,
     map[inputPair.second] = iter->getValue();
   }
 
-  // If a file was removed, we've lost its dependency info. Rebuild everything.
-  // FIXME: Can we do better?
-  return numInputsFromPrevious != previousInputs.size();
+  if (numInputsFromPrevious == previousInputs.size()) {
+    return false;
+  } else {
+    // If a file was removed, we've lost its dependency info. Rebuild everything.
+    // FIXME: Can we do better?
+    if (ShowIncrementalBuildDecisions) {
+      llvm::StringSet<> inputArgs;
+      for (auto &inputPair : inputs) {
+        inputArgs.insert(inputPair.second->getValue());
+      }
+
+      SmallVector<StringRef, 8> missingInputs;
+      for (auto &previousInput : previousInputs) {
+        auto previousInputArg = previousInput.getKey();
+        if (inputArgs.find(previousInputArg) == inputArgs.end()) {
+          missingInputs.push_back(previousInputArg);
+        }
+      }
+
+      llvm::outs() << "Incremental compilation has been disabled, because "
+                   << "the following inputs were used in the previous "
+                   << "compilation, but not in the current compilation:\n";
+      for (auto &missing : missingInputs) {
+        llvm::outs() << "\t" << missing << "\n";
+      }
+    }
+    return true;
+  }
 }
 
 std::unique_ptr<Compilation> Driver::buildCompilation(

test/Driver/Dependencies/driver-show-incremental-inputs.swift

@@ -0,0 +1,23 @@
+// Test that when:
+//
+// 1. Using -incremental -v -driver-show-incremental, and...
+// 2. ...the inputs passed to the Swift compiler version differ from the ones
+//    used in the original compilation...
+//
+// ...then the driver prints a message indicating that incremental compilation
+// is disabled.
+
+
+// RUN: rm -rf %t && cp -r %S/Inputs/one-way/ %t
+// RUN: %S/Inputs/touch.py 443865900 %t/*
+// RUN: echo '{version: "'$(%swiftc_driver_plain -version | head -n1)'", inputs: {"./main.swift": [443865900, 0], "./other.swift": [443865900, 0]}}' > %t/main~buildrecord.swiftdeps
+
+// RUN: cd %t && %swiftc_driver -driver-use-frontend-path %S/Inputs/update-dependencies.py -c ./main.swift ./other.swift -module-name main -incremental -v -driver-show-incremental -output-file-map %t/output.json | %FileCheck --check-prefix CHECK-INCREMENTAL %s
+// CHECK-INCREMENTAL-NOT: Incremental compilation has been disabled
+// CHECK-INCREMENTAL: Queuing main.swift (initial)
+
+// RUN: cd %t && %swiftc_driver -driver-use-frontend-path %S/Inputs/update-dependencies.py -c ./main.swift -module-name main -incremental -v -driver-show-incremental -output-file-map %t/output.json | %FileCheck --check-prefix CHECK-INPUTS-MISMATCH %s
+// CHECK-INPUTS-MISMATCH: Incremental compilation has been disabled{{.*}}inputs
+// CHECK-INPUTS-MISMATCH: ./other.swift
+// CHECK-INPUTS-MISMATCH-NOT: Queuing main.swift (initial)
+

test/Driver/Dependencies/driver-show-incremental-swift-version.swift

@@ -18,7 +18,7 @@
 
 // RUN: echo '{version: "bogus", inputs: {"./main.swift": [443865900, 0], "./other.swift": [443865900, 0]}}' > %t/main~buildrecord.swiftdeps
 // RUN: cd %t && %swiftc_driver -driver-use-frontend-path %S/Inputs/update-dependencies.py -c ./main.swift ./other.swift -module-name main -incremental -v -driver-show-incremental -output-file-map %t/output.json | %FileCheck --check-prefix CHECK-VERSION-MISMATCH %s
-// CHECK-VERSION-MISMATCH: Incremental compilation has been disabled
+// CHECK-VERSION-MISMATCH: Incremental compilation has been disabled{{.*}}compiler version mismatch
 // CHECK-VERSION-MISMATCH: Compiling with:
 // CHECK-VERSION-MISMATCH: Previously compiled with: bogus
 // CHECK-VERSION-MISMATCH-NOT: Queuing main.swift (initial)