[ClangImporter] <rdar://39801675> Treat fake-buffer locs as invalid, avoid nulls in diags.

Author: graydon

lib/ClangImporter/ClangDiagnosticConsumer.cpp

@@ -66,8 +66,18 @@ namespace {
                                StringRef Message,
                                ArrayRef<clang::CharSourceRange> Ranges,
                                clang::DiagOrStoredDiag Info) override {
-      if (shouldSuppressDiagInSwiftBuffers(Info) && isInSwiftBuffers(Loc))
-        return;
+      if (isInSwiftBuffers(Loc)) {
+        // FIXME: Ideally, we'd report non-suppressed diagnostics on synthetic
+        // buffers, printing their names (eg. <swift-imported-modules>:...) but
+        // this risks printing _excerpts_ of those buffers to stderr too; at
+        // present the synthetic buffers are "large blocks of null bytes" which
+        // we definitely don't want to print out. So until we have some clever
+        // way to print the name but suppress printing excerpts, we just replace
+        // the Loc with an invalid one here, which suppresses both.
+        Loc = clang::FullSourceLoc();
+        if (shouldSuppressDiagInSwiftBuffers(Info))
+          return;
+      }
       callback(Loc, Level, Message);
     }
 

test/ClangImporter/Inputs/custom-modules/ConditionallyFoo.h

@@ -0,0 +1,3 @@
+#ifdef WANT_FOO
+int foo();
+#endif

test/ClangImporter/Inputs/custom-modules/module.map

@@ -216,3 +216,7 @@ module Warnings9 { header "Warnings9.h" }
 module ForwardDeclarationsHelper {
   header "ForwardDeclarationsHelper.h"
 }
+module ConditionallyFoo {
+  header "ConditionallyFoo.h"
+  config_macros [exhaustive] WANT_FOO
+}

test/ClangImporter/Inputs/no-fake-source-buffer-excerpts.h

@@ -0,0 +1,2 @@
+#define WANT_FOO
+@import ConditionallyFoo;

test/ClangImporter/no-fake-source-buffer-excerpts.swift

@@ -0,0 +1,11 @@
+// REQUIRES: OS=macosx
+//
+// This triggers a warning about ignored configuration macros; the warning then
+// attempts to emit an excerpt from one of the clang importer's fake buffers
+// (<swift-imported-modules>) which is full of 250kb of nulls. We want to check
+// that we do not emit a gigantic block of nulls to stderr.
+//
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -typecheck -I %S/Inputs/custom-modules -import-objc-header %S/Inputs/no-fake-source-buffer-excerpts.h %s 2>%t/errors
+// RUN: od -a < %t/errors | %FileCheck %s
+// CHECK-NOT: nul nul nul nul