Report an error if you try to link against .dll instead of .lib.

[ruiu]

Summary

It is a usage error to feed a .dll file instead of a .dll to COFF linker.
Previously, lld failed with a mysterious error message. Now we reject
it at the driver.

Fixes https://bugs.llvm.org/show_bug.cgi?id=36440

[smeenai]

"It is a usage error to feed a .dll file instead of a .dll to COFF linker"

Should be

"It is a usage error to feed a .dll file instead of a .lib to COFF linker"

Do you think it's worth printing a nicer error for the specific case of DLLs (something that tells the user they should be linking against the import library instead)? In particular, binutils supports linking against DLLs directly (it synthesizes an import library on the fly), so a nicer error message could be helpful for users coming from that toolchain.

lld/COFF/Driver.cpp

135

Is it standard LLVM or LLD coding style to not have the newlines?

[ruiu]

• show a better error message

[smeenai]

LGTM

lld/COFF/Driver.cpp

160

Nit: I'd prefer "Did you specify a DLL instead of an import library?" (DLL file -> DLL, import file -> import library)

lld/COFF/Driver.cpp

void LinkerDriver::addBuffer(std::unique_ptr<MemoryBuffer> MB,
   MemoryBufferRef MBRef = takeBuffer(std::move(MB));
   FilePaths.push_back(MBRef.getBufferIdentifier());
 
   // File type is detected by contents, not by file extension.
   switch (identify_magic(MBRef.getBuffer())) {
   case file_magic::windows_resource:
     Resources.push_back(MBRef);
     break;
-
   case file_magic::archive:
     if (WholeArchive) {
       std::unique_ptr<Archive> File =
           CHECK(Archive::create(MBRef),
                 MBRef.getBufferIdentifier() + ": failed to parse archive");
 
       for (MemoryBufferRef M : getArchiveMembers(File.get()))
         addArchiveBuffer(M, "<whole-archive>", MBRef.getBufferIdentifier());
       return;
     }
     Symtab->addFile(make<ArchiveFile>(MBRef));
     break;
-
   case file_magic::bitcode:
     Symtab->addFile(make<BitcodeFile>(MBRef));
     break;
-
+  case file_magic::coff_object:
+  case file_magic::coff_import_library:
+    Symtab->addFile(make<ObjFile>(MBRef));
+    break;
   case file_magic::coff_cl_gl_object:
     error(MBRef.getBufferIdentifier() + ": is not a native COFF file. "
           "Recompile without /GL");
     break;
-
   default:
-    Symtab->addFile(make<ObjFile>(MBRef));
+    error(MBRef.getBufferIdentifier() + ": unknown file type");
     break;
   }
 }
 
 void LinkerDriver::enqueuePath(StringRef Path, bool WholeArchive) {
   auto Future =
       std::make_shared<std::future<MBErrPair>>(createFutureForFile(Path));
   std::string PathStr = Path;
   enqueueTask([=]() {

lld/test/COFF/driver.test

 # RUN: not lld-link nosuchfile.obj >& %t.log
 # RUN: FileCheck -check-prefix=MISSING %s < %t.log
 MISSING: nosuchfile.obj: {{[Nn]}}o such file or directory
 
 # RUN: lld-link --version | FileCheck -check-prefix=VERSION %s
 VERSION: {{LLD [0-9]+\.[0-9]+}}
+
+# RUN: yaml2obj < %p/Inputs/export.yaml > %t.obj
+# RUN: lld-link /out:%t.dll /dll %t.obj
+# RUN: not lld-link /out:%t.exe %t.dll 2>&1 | FileCheck -check-prefix=BADFILE %s
+BADFILE: unknown file type