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

[Fangrui Song <emacsray@gmail.com>]

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.