Detect the Lua project root using .luarc.json

autoload/ale/lua.vim

@@ -4,6 +4,7 @@
 " Find project root for a Lua language server.
 function! ale#lua#FindProjectRoot(buffer) abort
     let l:possible_project_roots = [
+    \   '.luarc.json',
     \   '.git',
     \   bufname(a:buffer),
     \]

test/linter/test_lua_language_server.vader

@@ -14,3 +14,8 @@ Execute(lua-language-server should be configurable):
 
   AssertLinter 'billy', ale#Escape('billy')
   AssertLSPConfig {'x': 'y'}
+
+Execute(lua-language-server should detect the project root using .luarc.json):
+  call ale#test#SetFilename('../lua/dummy.lua')
+
+  AssertLSPProject ale#path#Simplify(g:dir . '/../lua')

test/lua/.luarc.json

@@ -0,0 +1,19 @@
+{
+  "$schema": "https://raw.githubusercontent.com/LuaLS/vscode-lua/master/setting/schema.json",
+  "diagnostics.globals": [
+    "vim",
+    "describe",
+    "it",
+    "before_each",
+    "after_each",
+    "setup",
+    "teardown",
+    "pending",
+    "assert"
+  ],
+  "workspace.library": [
+    "../../lua"
+  ],
+  "runtime.version": "LuaJIT",
+  "hint.enable": false
+}