feat(lsp)!: change handler signature

Previously, the handler signature was:

  function(err, method, params, client_id, bufnr, config)

In order to better support external plugins that wish to extend the
protocol, there is other information which would be advantageous to
forward to the client, such as the original params of the request that
generated the callback.

In order to do this, we would need to break symmetry of the handlers, to
add an additional "params" as the 7th argument.

Instead, this PR changes the signature of the handlers to:

  function(err, result, ctx, config)

where ctx (the context) includes params, client_id, and bufnr. This also leaves
flexibility for future use-cases.

BREAKING_CHANGE: changes the signature of the built-in client handlers, requiring
updating handler calls

runtime/lua/vim/lsp.lua

@@ -678,7 +678,7 @@ function lsp.start_client(config)
     local handler = resolve_handler(method)
     if handler then
       -- Method name is provided here for convenience.
-      handler(nil, method, params, client_id)
+      handler(nil, params, {method=method, client_id=client_id})
     end
   end
 
@@ -692,7 +692,7 @@ function lsp.start_client(config)
     local handler = resolve_handler(method)
     if handler then
       local _ = log.debug() and log.debug("server_request: found handler for", method)
-      return handler(nil, method, params, client_id)
+      return handler(nil, params, {method=method, client_id=client_id})
     end
     local _ = log.debug() and log.debug("server_request: no handler found for", method)
     return nil, lsp.rpc_response_error(protocol.ErrorCodes.MethodNotFound)
@@ -896,7 +896,7 @@ function lsp.start_client(config)
 
     local _ = log.debug() and log.debug(log_prefix, "client.request", client_id, method, params, handler, bufnr)
     return rpc.request(method, params, function(err, result)
-      handler(err, method, result, client_id, bufnr)
+      handler(err, result, {method=method, client_id=client_id, bufnr=bufnr})
     end)
   end
 
@@ -917,7 +917,7 @@ function lsp.start_client(config)
   ---@see |vim.lsp.buf_request_sync()|
   function client.request_sync(method, params, timeout_ms, bufnr)
     local request_result = nil
-    local function _sync_handler(err, _, result)
+    local function _sync_handler(err, result)
       request_result = { err = err, result = result }
     end
 
@@ -1276,7 +1276,7 @@ function lsp.buf_request(bufnr, method, params, handler)
     local unsupported_err = lsp._unsupported_method(method)
     handler = handler or lsp.handlers[method]
     if handler then
-      handler(unsupported_err, method, bufnr)
+      handler(unsupported_err, nil, {method=method, bufnr=bufnr})
     end
     return
   end
@@ -1316,8 +1316,8 @@ function lsp.buf_request_all(bufnr, method, params, callback)
     end
   end)
 
-  local function _sync_handler(err, _, result, client_id)
-    request_results[client_id] = { error = err, result = result }
+  local function _sync_handler(err, result, ctx)
+    request_results[ctx.client_id] = { error = err, result = result }
     result_count = result_count + 1
     set_expected_result_count()
 
@@ -1423,7 +1423,7 @@ function lsp.omnifunc(findstart, base)
   local params = util.make_position_params()
 
   local items = {}
-  lsp.buf_request(bufnr, 'textDocument/completion', params, function(err, _, result)
+  lsp.buf_request(bufnr, 'textDocument/completion', params, function(err, result)
     if err or not result or vim.fn.mode() ~= "i" then return end
     local matches = util.text_document_completion_list_to_complete_items(result, prefix)
     -- TODO(ashkan): is this the best way to do this?
@@ -1498,8 +1498,8 @@ end
 ---@param handler (function) See |lsp-handler|
 ---@param override_config (table) Table containing the keys to override behavior of the {handler}
 function lsp.with(handler, override_config)
-  return function(err, method, params, client_id, bufnr, config)
-    return handler(err, method, params, client_id, bufnr, vim.tbl_deep_extend("force", config or {}, override_config))
+  return function(err, result, ctx, config)
+    return handler(err, result, ctx, vim.tbl_deep_extend("force", config or {}, override_config))
   end
 end