[Tooling] When transferring compile commands between files, always use '--'

"driver <flags> -- <input>" is a particularly convenient form of the
compile command to manipulate, with fewer special cases to handle.

Guaranteeing that the output command is of that form is cheap and makes
it easier to consume the result in some cases.

Differential Revision: https://reviews.llvm.org/D116721

clang-tools-extra/clangd/CompileCommands.cpp

@@ -290,16 +290,9 @@ void CommandMangler::adjust(std::vector<std::string> &Cmd,
     TransferCmd.CommandLine = std::move(Cmd);
     TransferCmd = transferCompileCommand(std::move(TransferCmd), File);
     Cmd = std::move(TransferCmd.CommandLine);
-
-    // Restore the canonical "driver --opts -- filename" form we expect.
-    // FIXME: This is ugly and coupled. Make transferCompileCommand ensure it?
-    assert(!Cmd.empty() && Cmd.back() == File);
-    Cmd.pop_back();
-    if (!Cmd.empty() && Cmd.back() == "--")
-      Cmd.pop_back();
-    assert(!llvm::is_contained(Cmd, "--"));
-    Cmd.push_back("--");
-    Cmd.push_back(File.str());
+    assert(Cmd.size() >= 2 && Cmd.back() == File &&
+           Cmd[Cmd.size() - 2] == "--" &&
+           "TransferCommand should produce a command ending in -- filename");
   }
 
   for (auto &Edit : Config::current().CompileFlags.Edits)

clang/include/clang/Tooling/CompilationDatabase.h

@@ -216,6 +216,8 @@ private:
 /// Transforms a compile command so that it applies the same configuration to
 /// a different file. Most args are left intact, but tweaks may be needed
 /// to certain flags (-x, -std etc).
+///
+/// The output command will always end in {"--", Filename}.
 tooling::CompileCommand transferCompileCommand(tooling::CompileCommand,
                                                StringRef Filename);
 

clang/lib/Tooling/InterpolatingCompilationDatabase.cpp

@@ -243,8 +243,7 @@ struct TransferableCommand {
           llvm::Twine(ClangCLMode ? "/std:" : "-std=") +
           LangStandard::getLangStandardForKind(Std).getName()).str());
     }
-    if (Filename.startswith("-") || (ClangCLMode && Filename.startswith("/")))
-      Result.CommandLine.push_back("--");
+    Result.CommandLine.push_back("--");
     Result.CommandLine.push_back(std::string(Filename));
     return Result;
   }

clang/unittests/Tooling/CompilationDatabaseTest.cpp

@@ -739,6 +739,9 @@ protected:
     EXPECT_EQ(Results[0].CommandLine.back(), MakeNative ? path(F) : F)
         << "Last arg should be the file";
     Results[0].CommandLine.pop_back();
+    EXPECT_EQ(Results[0].CommandLine.back(), "--")
+        << "Second-last arg should be --";
+    Results[0].CommandLine.pop_back();
     return llvm::join(Results[0].CommandLine, " ");
   }
 
@@ -826,18 +829,6 @@ TEST_F(InterpolateTest, StripDoubleDash) {
   EXPECT_EQ(getCommand("dir/bar.cpp"), "clang -D dir/foo.cpp -Wall -std=c++14");
 }
 
-TEST_F(InterpolateTest, InsertDoubleDash) {
-  add("dir/foo.cpp", "-o foo.o -std=c++14 -Wall");
-  EXPECT_EQ(getCommand("-dir/bar.cpp", false),
-            "clang -D dir/foo.cpp -Wall -std=c++14 --");
-}
-
-TEST_F(InterpolateTest, InsertDoubleDashForClangCL) {
-  add("dir/foo.cpp", "clang-cl", "/std:c++14 /W4");
-  EXPECT_EQ(getCommand("/dir/bar.cpp", false),
-            "clang-cl -D dir/foo.cpp /W4 /std:c++14 --");
-}
-
 TEST_F(InterpolateTest, Case) {
   add("FOO/BAR/BAZ/SHOUT.cc");
   add("foo/bar/baz/quiet.cc");
@@ -879,7 +870,7 @@ TEST(TransferCompileCommandTest, Smoke) {
   CompileCommand Transferred = transferCompileCommand(std::move(Cmd), "foo.h");
   EXPECT_EQ(Transferred.Filename, "foo.h");
   EXPECT_THAT(Transferred.CommandLine,
-              ElementsAre("clang", "-Wall", "-x", "c++-header", "foo.h"));
+              ElementsAre("clang", "-Wall", "-x", "c++-header", "--", "foo.h"));
   EXPECT_EQ(Transferred.Directory, "dir");
 }