bug#29619: Fwd: [xref.el] Add `xref-find-references` to `xref-prompt-for-identifier`

[Fangrui Song]

On 2017-12-10, Dmitry Gutov wrote:
> On 12/9/17 7:52 PM, Ray wrote:
> > Thanks for being open to change the default if more people feel the same.
> >
> > With the xref system people use drifting from tag based
> > (ctags,etags,GNU GLOBAL,cscope,...) tools to Language Server Protocol
> > (cquery, rls, ...),
> > a single identifier without position has become insufficient to
> > describe the one the user wants to look up. For example, a local
> > variable/struct/lambda `foo` may exist
> > in different functions.
>
> That's fine, actually, and as designed. As long as the different 
> global identifiers can be represented uniquely as strings (but using 
> text properties for e.g. a local variable at point is good too).

> > I'm using a C++ language server called cquery. This is what I get (for
> > the argument `identifier`) when I hit the key bound to
> > `xref-find-definitions`:
> >
> > #("QueryDatabase" 0 13 (fontified t ref-params (:textDocument (:uri
> > "file:///home/maskray/Dev/Util/cquery/src/query_utils.h") :position
> > (:line 11 :character 54) :context (:includeDeclaration :json-false))
> > def-params (:textDocument (:uri
> > "file:///home/maskray/Dev/Util/cquery/src/query_utils.h") :position
> > (:line 11 :character 54))))
> >
> > Here the text properties are more useful than the identifier itself,
> > because LSP uses position instead of identifier to sending requests to
> > the language server.
> > https://github.com/Microsoft/language-server-protocol/blob/master/protocol.md#textdocumentpositionparams
>
> So it doesn't actually let you choose? Only supports the identifier at 
> point?

The identifier (if chosen from xref prompt) is ignored by Language
Server Protocol and only the position information is what matters.

According to 
https://github.com/Microsoft/language-server-protocol/blob/master/protocol.md#textDocument_references
When the user wants to find a reference of an identifier,
information of the following interface is sent to language servers.

   interface ReferenceParams extends TextDocumentPositionParams {
        context: ReferenceContext
   }

TextDocumentPositionParams is the interesting one:

   interface TextDocumentPositionParams {
        textDocument: TextDocumentIdentifier;   /// wrapper of filename
        position: Position;  /// line, column; see, no identifier is used
   }

lsp-mode provides a backend of xref generic functions.
Here is how xref-find-references is implemented in lsp-mode:
https://github.com/emacs-lsp/lsp-mode/blob/master/lsp-methods.el#L1425

   (cl-defmethod xref-backend-references ((_backend (eql xref-lsp)) identifier)
     (let* ((properties (text-properties-at 0 identifier))
            (params (plist-get properties 'ref-params))
            (refs (lsp--send-request (lsp--make-request
                                      "textDocument/references"
                                      (or params 
(lsp--make-reference-params))))))
       (lsp--locations-to-xref-items refs)))

The `identifier` text itself is ignored and only the text properties
(which encode position information) are used.