[clangd] Fix conversion from Windows UNC paths to file URI format.

[ilya-golovenko]

Summary

The fix improves handling of Windows UNC paths to align with Appendix E. Nonstandard Syntax Variations of RFC 8089.

Before this fix it was difficult to use Windows UNC paths in compile_commands.json database as such paths were converted to file URIs using 'file:////auth/share/file.cpp' notation instead of recommended 'file://auth/share/file.cpp'.

As an example, VS.Code cannot understand file URIs with 4 starting slashes, thus such features as go-to-definition, jump-to-file, hover tooltip, etc. stop working. This also applicable to files which reside on Windows network-mapped drives because clangd internally resolve file paths to real paths in some cases and such path get resolved to UNC paths.

[ilya-golovenko]

This is an attempt to improve handling of Windows UNC paths in clangd. Any feedback is appreciated.

[ilya-golovenko]

Consistent naming: Name => Path

[ilya-golovenko]

Consistent naming: hasDriveLetter => isWindowsPath

clang-tools-extra/clangd/URI.cpp

 namespace clangd {
 namespace {
 
 inline llvm::Error make_string_error(const llvm::Twine &Message) {
   return llvm::make_error<llvm::StringError>(Message,
                                              llvm::inconvertibleErrorCode());
 }
 
-bool hasDriveLetter(llvm::StringRef Path) {
+bool isWindowsPath(llvm::StringRef Path) {
   return Path.size() > 1 && llvm::isAlpha(Path[0]) && Path[1] == ':';
 }
 
-bool isNetworkPath(llvm::StringRef Name) {
+bool isNetworkPath(llvm::StringRef Path) {
   llvm::StringRef Sep = llvm::sys::path::get_separator();
-  return Name.consume_front(Sep) && Name.consume_front(Sep) && !Name.empty();
+  return Path.consume_front(Sep) && Path.consume_front(Sep) && !Path.empty();
 }
 
 /// This manages file paths in the file system. All paths in the scheme
 /// are absolute (with leading '/').
 /// Note that this scheme is hardcoded into the library and not registered in
 /// registry.
 class FileSystemScheme : public URIScheme {
 public:
   llvm::Expected<std::string>
   getAbsolutePath(llvm::StringRef Authority, llvm::StringRef Body,
                   llvm::StringRef /*HintPath*/) const override {
     if (!Body.startswith("/"))
       return make_string_error("File scheme: expect body to be an absolute "
                                "path starting with '/': " +
                                Body);
     llvm::SmallString<128> Path;
     // For Windows UNC paths e.g. \\server\share
     if (!Authority.empty())
       ("//" + Authority).toVector(Path);
     // For Windows paths e.g. /X:
-    if (hasDriveLetter(Body.substr(1)))
+    if (isWindowsPath(Body.substr(1)))
       Body.consume_front("/");
     Path.append(Body);
     llvm::sys::path::native(Path);
     return std::string(Path);
   }
 
   llvm::Expected<URI>
   uriFromAbsolutePath(llvm::StringRef AbsolutePath) const override {
     std::string Body;
     llvm::StringRef Authority;
     llvm::StringRef Root = llvm::sys::path::root_name(AbsolutePath);
     // For Windows UNC paths e.g. \\server\share
     if (isNetworkPath(Root)) {
       Authority = Root.drop_front(2);
       AbsolutePath.consume_front(Root);
     }
     // For Windows paths e.g. X:
-    if (hasDriveLetter(Root))
+    if (isWindowsPath(Root))
       Body = "/";
     Body += llvm::sys::path::convert_to_slash(AbsolutePath);
     return URI("file", Authority, Body);
   }
 };
 
 llvm::Expected<std::unique_ptr<URIScheme>>
 findSchemeByName(llvm::StringRef Scheme) {