[DebugInfo] Don't record that the standard library imports itself (#17226)

This causes problems for cross-compilation -parse-stdlib tests that
emit debug info. At the moment we have zero of those, but we're
trying to add one.

Also, don't try to load new modules when recording imports. (This
isn't harmful, just inefficient.)

Author: jrose-apple

lib/IRGen/GenDecl.cpp

@@ -1111,17 +1111,25 @@ void IRGenerator::emitGlobalTopLevel(bool emitForParallelEmission) {
 
 void IRGenModule::finishEmitAfterTopLevel() {
   // Emit the implicit import of the swift standard library.
+  // FIXME: We'd get the exact set of implicit imports if we went through the
+  // SourceFile's getImportedModules instead, but then we'd lose location info
+  // for the explicit imports.
   if (DebugInfo) {
-    std::pair<swift::Identifier, swift::SourceLoc> AccessPath[] = {
-      { Context.StdlibModuleName, swift::SourceLoc() }
-    };
+    if (ModuleDecl *TheStdlib = Context.getStdlibModule()) {
+      if (TheStdlib != getSwiftModule()) {
+        std::pair<swift::Identifier, swift::SourceLoc> AccessPath[] = {
+          { Context.StdlibModuleName, swift::SourceLoc() }
+        };
 
-    auto Imp = ImportDecl::create(Context,
-                                  getSwiftModule(),
-                                  SourceLoc(),
-                                  ImportKind::Module, SourceLoc(),
-                                  AccessPath);
-    DebugInfo->emitImport(Imp);
+        auto Imp = ImportDecl::create(Context,
+                                      getSwiftModule(),
+                                      SourceLoc(),
+                                      ImportKind::Module, SourceLoc(),
+                                      AccessPath);
+        Imp->setModule(TheStdlib);
+        DebugInfo->emitImport(Imp);
+      }
+    }
   }
 }
 

lib/IRGen/IRGenDebugInfo.cpp

@@ -1696,15 +1696,8 @@ void IRGenDebugInfoImpl::emitImport(ImportDecl *D) {
   if (Opts.DebugInfoKind <= IRGenDebugInfoKind::LineTables)
     return;
 
-  swift::ModuleDecl *M = IGM.Context.getModule(D->getModulePath());
-  if (!M &&
-      D->getModulePath()[0].first == IGM.Context.TheBuiltinModule->getName())
-    M = IGM.Context.TheBuiltinModule;
-  if (!M) {
-    assert(M && "Could not find module for import decl.");
-    return;
-  }
-  ModuleDecl::ImportedModule Imported = {D->getModulePath(), M};
+  assert(D->getModule() && "compiler-synthesized ImportDecl is incomplete");
+  ModuleDecl::ImportedModule Imported = {D->getModulePath(), D->getModule()};
   auto DIMod = getOrCreateModule(Imported);
   auto L = getDebugLoc(*this, D);
   auto *File = getOrCreateFile(L.Filename);

test/DebugInfo/ImportsStdlib.swift

@@ -0,0 +1,26 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -emit-ir -parse-stdlib -module-name NotTheStdlib %s -I %t -g -o - > %t.ll
+// RUN: %FileCheck %s < %t.ll
+// RUN: %FileCheck -check-prefix=NEGATIVE %s < %t.ll
+
+// RUN: %target-swift-frontend -c -parse-stdlib -module-name NotTheStdlib %s -I %t -g -o %t.o
+// RUN: %llvm-dwarfdump -a %t.o > %t.dump
+// RUN: %FileCheck -check-prefix=DWARF %s < %t.dump
+// RUN: %FileCheck -check-prefix=NEGATIVE-DWARF %s < %t.dump
+
+// CHECK-DAG: ![[MODULE:[0-9]+]] = !DIModule({{.*}}, name: "NotTheStdlib", includePath: "{{.*}}test{{.*}}DebugInfo{{.*}}"
+// CHECK-DAG: !DIImportedEntity(tag: DW_TAG_imported_module, scope: ![[THISFILE:[0-9]+]], entity: ![[MODULE]]
+// CHECK-DAG: ![[THISFILE]] = !DIFile(filename: "ImportsStdlib.swift", directory: "{{.*}}test/DebugInfo")
+
+// NEGATIVE-NOT: !DIFile(filename: "Swift.swiftmodule"
+// NEGATIVE-NOT: !DIModule({{.*}}, name: "Swift"
+
+// DWARF: .debug_info
+// DWARF: DW_TAG_module
+// DWARF:   DW_AT_name ("NotTheStdlib")
+// DWARF:   DW_AT_LLVM_include_path
+
+// DWARF: file_names{{.*}} ImportsStdlib.swift
+
+// NEGATIVE-DWARF-NOT: DW_AT_name ("Swift")