diff --git a/clang-tools-extra/clangd/Diagnostics.cpp b/clang-tools-extra/clangd/Diagnostics.cpp
--- a/clang-tools-extra/clangd/Diagnostics.cpp
+++ b/clang-tools-extra/clangd/Diagnostics.cpp
@@ -672,14 +672,14 @@
 
 void StoreDiags::HandleDiagnostic(DiagnosticsEngine::Level DiagLevel,
                                   const clang::Diagnostic &Info) {
-  // If the diagnostic was generated for a different SourceManager, skip it.
+  bool FromModule = false;
+  // If the diagnostic was generated for a different SourceManager.
   // This happens when a module is imported and needs to be implicitly built.
   // The compilation of that module will use the same StoreDiags, but different
-  // SourceManager.
+  // SourceManager. Treat them as coming from command line.
   if (OrigSrcMgr && Info.hasSourceManager() &&
       OrigSrcMgr != &Info.getSourceManager()) {
-    IgnoreDiagnostics::log(DiagLevel, Info);
-    return;
+    FromModule = true;
   }
 
   DiagnosticConsumer::HandleDiagnostic(DiagLevel, Info);
@@ -687,7 +687,7 @@
       Info.getDiags()->getDiagnosticIDs()->isDefaultMappingAsError(
           Info.getID());
 
-  if (Info.getLocation().isInvalid()) {
+  if (Info.getLocation().isInvalid() || FromModule) {
     // Handle diagnostics coming from command-line arguments. The source manager
     // is *not* available at this point, so we cannot use it.
     if (!OriginallyError) {
@@ -702,6 +702,11 @@
     LastDiagOriginallyError = OriginallyError;
     LastDiag->ID = Info.getID();
     fillNonLocationData(DiagLevel, Info, *LastDiag);
+    // Update message to mention original location.
+    const char *Prefix =
+        FromModule ? "in implicitly built module" : "in command line";
+    LastDiag->Message = llvm::formatv("{0}: {1}", Prefix, LastDiag->Message);
+
     LastDiag->InsideMainFile = true;
     // Put it at the start of the main file, for a lack of a better place.
     LastDiag->Range.start = Position{0, 0};
diff --git a/clang-tools-extra/clangd/unittests/ModulesTests.cpp b/clang-tools-extra/clangd/unittests/ModulesTests.cpp
--- a/clang-tools-extra/clangd/unittests/ModulesTests.cpp
+++ b/clang-tools-extra/clangd/unittests/ModulesTests.cpp
@@ -18,6 +18,12 @@
 namespace clangd {
 namespace {
 
+using ::testing::ElementsAre;
+
+MATCHER_P(diag, Desc, "") {
+  return llvm::StringRef(arg.Message).contains(Desc);
+}
+
 TEST(Modules, TextualIncludeInPreamble) {
   TestTU TU = TestTU::withCode(R"cpp(
     #include "Textual.h"
@@ -88,7 +94,13 @@
 )modulemap";
 
   // Test that we do not crash.
-  TU.build();
+  auto AST = TU.build();
+
+  // Test expected diagnostics.
+  EXPECT_THAT(AST.getDiagnostics(),
+              ElementsAre(diag("in implicitly built module: module M does not "
+                               "depend on a module exporting 'non-modular.h'"),
+                          diag("could not build module 'M'")));
 }
 
 // Unknown module formats are a fatal failure for clang. Ensure we don't crash.
diff --git a/clang-tools-extra/clangd/unittests/TUSchedulerTests.cpp b/clang-tools-extra/clangd/unittests/TUSchedulerTests.cpp
--- a/clang-tools-extra/clangd/unittests/TUSchedulerTests.cpp
+++ b/clang-tools-extra/clangd/unittests/TUSchedulerTests.cpp
@@ -1079,7 +1079,8 @@
       ElementsAre(AllOf(
           Field(&Diag::ID, Eq(diag::err_drv_unknown_argument)),
           Field(&Diag::Name, Eq("drv_unknown_argument")),
-          Field(&Diag::Message, "unknown argument: '-fsome-unknown-flag'"))));
+          Field(&Diag::Message,
+                "in command line: unknown argument: '-fsome-unknown-flag'"))));
 }
 
 TEST_F(TUSchedulerTests, CommandLineWarnings) {