lsp: make the command error message more detailed (#11633)

* lsp.lua: make the error message more detailed

* test: add lsp._cmd_part test

runtime/lua/vim/lsp.lua

@@ -122,20 +122,20 @@ local function validate_encoding(encoding)
 end
 
 function lsp._cmd_parts(input)
-  local cmd, cmd_args
-  if vim.tbl_islist(input) then
-    cmd = input[1]
-    cmd_args = {}
+  vim.validate{cmd={
+    input,
+    function() return vim.tbl_islist(input) end,
+    "list"}}
+
+  local cmd = input[1]
+  local cmd_args = {}
   -- Don't mutate our input.
   for i, v in ipairs(input) do
-      assert(type(v) == 'string', "input arguments must be strings")
+    vim.validate{["cmd argument"]={v, "s"}}
     if i > 1 then
       table.insert(cmd_args, v)
     end
   end
-  else
-    error("cmd type must be list.")
-  end
   return cmd, cmd_args
 end
 
@@ -524,7 +524,7 @@ function lsp.start_client(config)
   function client.request(method, params, callback, bufnr)
     if not callback then
       callback = resolve_callback(method)
-        or error("not found: request callback for client "..client.name)
+        or error(string.format("not found: %q request callback for client %q.", method, client.name))
     end
     local _ = log.debug() and log.debug(log_prefix, "client.request", client_id, method, params, callback, bufnr)
     -- TODO keep these checks or just let it go anyway?

test/functional/plugin/lsp_spec.lua

@@ -6,6 +6,7 @@ local buf_lines = helpers.buf_lines
 local dedent = helpers.dedent
 local exec_lua = helpers.exec_lua
 local eq = helpers.eq
+local pcall_err = helpers.pcall_err
 local pesc = helpers.pesc
 local insert = helpers.insert
 local retry = helpers.retry
@@ -705,7 +706,6 @@ describe('LSP', function()
         end;
       }
     end)
-
   end)
 
   describe("parsing tests", function()
@@ -733,7 +733,23 @@ describe('LSP', function()
         end;
       }
     end)
+  end)
+  describe('lsp._cmd_parts test', function()
+    local function _cmd_parts(input)
+      return exec_lua([[
+        lsp = require('vim.lsp')
+        return lsp._cmd_parts(...)
+      ]], input)
+    end
+    it('should valid cmd argument', function()
+      eq(true, pcall(_cmd_parts, {"nvim"}))
+      eq(true, pcall(_cmd_parts, {"nvim", "--head"}))
+    end)
 
+    it('should invalid cmd argument', function()
+      eq('Error executing lua: .../shared.lua: cmd: expected list, got nvim', pcall_err(_cmd_parts, "nvim"))
+      eq('Error executing lua: .../shared.lua: cmd argument: expected string, got number', pcall_err(_cmd_parts, {"nvim", 1}))
+    end)
   end)
 end)