[ASTImporter] Also import overwritten file buffers

[teemperor]

Summary

Overwritten file buffers are at the moment ignored when importing and
instead only the underlying file buffer is imported.

This patch fixes this by not going to the underlying file entry if the file has
an overwritten buffer.

[shafik]

LGTM

[martong]

Okay, no objections.
Just out of curiosity, do you guys rewrite the content of a buffer in D78095 or in a later patch?

[teemperor]

In D78086#1983933, @martong wrote:

Okay, no objections.
Just out of curiosity, do you guys rewrite the content of a buffer in D78095 or in a later patch?

D78095 is already rewriting it. In the future we want to load the real source code in LLDB and D78095 is only the fallback code path that will be used when a file can't be found. In that case we just take the real file path with our own made-up file buffer. A follow-up patch actually implements the real file loading. But until that follow-up patch is done, D78095 is essentially what we use in LLDB to always set for every file a overwritten file buffer.

clang/lib/AST/ASTImporter.cpp

if (FromEx.isMacroArgExpansion()) {
                                        FromEx.isExpansionTokenRange());
       else
         return ToExLocE.takeError();
     }
     ToID = ToSM.getFileID(MLoc);
   } else {
     const SrcMgr::ContentCache *Cache = FromSLoc.getFile().getContentCache();
 
-    if (!IsBuiltin) {
+    if (!IsBuiltin && !Cache->BufferOverridden) {
       // Include location of this file.
       ExpectedSLoc ToIncludeLoc = Import(FromSLoc.getFile().getIncludeLoc());
       if (!ToIncludeLoc)
         return ToIncludeLoc.takeError();
 
       // Every FileID that is not the main FileID needs to have a valid include
       // location so that the include chain points to the main FileID. When
       // importing the main FileID (which has no include location), we need to

clang/unittests/AST/ASTImporterTest.cpp

 //===- unittest/AST/ASTImporterTest.cpp - AST node import test ------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // Tests for the correct import of AST nodes from one AST context to another.
 //
 //===----------------------------------------------------------------------===//
 
 #include "clang/ASTMatchers/ASTMatchers.h"
 #include "llvm/ADT/StringMap.h"
+#include "llvm/Support/SmallVectorMemoryBuffer.h"
 
 #include "clang/AST/DeclContextInternals.h"
 #include "gtest/gtest.h"
 
 #include "ASTImporterFixtures.h"
 #include "MatchVerifier.h"
 
 namespace clang {
TEST_P(ImportSourceLocations, PreserveFileIDTreeStructure) {
   EXPECT_EQ(ToSM.getIncludeLoc(FileID2), FileStart);
 
   // Let the SourceManager check the order of the locations. The order should
   // be the order in which the declarations are imported.
   EXPECT_TRUE(ToSM.isBeforeInTranslationUnit(Location1, Location2));
   EXPECT_FALSE(ToSM.isBeforeInTranslationUnit(Location2, Location1));
 }
 
+TEST_P(ImportSourceLocations, NormalFileBuffer) {
+  // Test importing normal file buffers.
+
+  std::string Path = "input0.c";
+  std::string Source = "int X;";
+  TranslationUnitDecl *FromTU = getTuDecl(Source, Lang_C, Path);
+
+  SourceLocation ImportedLoc;
+  {
+    // Import the VarDecl to trigger the importing of the FileID.
+    auto Pattern = varDecl(hasName("X"));
+    VarDecl *FromD = FirstDeclMatcher<VarDecl>().match(FromTU, Pattern);
+    ImportedLoc = Import(FromD, Lang_C)->getLocation();
+  }
+
+  // Make sure the imported buffer has the original contents.
+  SourceManager &ToSM = ToAST->getSourceManager();
+  FileID ImportedID = ToSM.getFileID(ImportedLoc);
+  EXPECT_EQ(Source, ToSM.getBuffer(ImportedID, SourceLocation())->getBuffer());
+}
+
+TEST_P(ImportSourceLocations, OverwrittenFileBuffer) {
+  // Test importing overwritten file buffers.
+
+  std::string Path = "input0.c";
+  TranslationUnitDecl *FromTU = getTuDecl("int X;", Lang_C, Path);
+
+  // Overwrite the file buffer for our input file with new content.
+  const std::string Contents = "overwritten contents";
+  SourceLocation ImportedLoc;
+  {
+    SourceManager &FromSM = FromTU->getASTContext().getSourceManager();
+    clang::FileManager &FM = FromSM.getFileManager();
+    const clang::FileEntry &FE =
+        *FM.getVirtualFile(Path, static_cast<off_t>(Contents.size()), 0);
+
+    llvm::SmallVector<char, 64> Buffer;
+    Buffer.append(Contents.begin(), Contents.end());
+    auto FileContents =
+        std::make_unique<llvm::SmallVectorMemoryBuffer>(std::move(Buffer), Path);
+    FromSM.overrideFileContents(&FE, std::move(FileContents));
+
+    // Import the VarDecl to trigger the importing of the FileID.
+    auto Pattern = varDecl(hasName("X"));
+    VarDecl *FromD = FirstDeclMatcher<VarDecl>().match(FromTU, Pattern);
+    ImportedLoc = Import(FromD, Lang_C)->getLocation();
+  }
+
+  // Make sure the imported buffer has the overwritten contents.
+  SourceManager &ToSM = ToAST->getSourceManager();
+  FileID ImportedID = ToSM.getFileID(ImportedLoc);
+  EXPECT_EQ(Contents,
+            ToSM.getBuffer(ImportedID, SourceLocation())->getBuffer());
+}
+
 TEST_P(ASTImporterOptionSpecificTestBase, ImportExprOfAlignmentAttr) {
   // Test if import of these packed and aligned attributes does not trigger an
   // error situation where source location from 'From' context is referenced in
   // 'To' context through evaluation of the alignof attribute.
   // This happens if the 'alignof(A)' expression is not imported correctly.
   Decl *FromTU = getTuDecl(
       R"(
       struct __attribute__((packed)) A { int __attribute__((aligned(8))) X; };