refactor(eval): treat v:lua call as simple function

runtime/doc/options.txt

@@ -406,7 +406,14 @@ The value of a few options, such as 'foldexpr', is an expression that is
 evaluated to get a value.  The evaluation can have quite a bit of overhead.
 One way to minimize the overhead, and also to keep the option value very
 simple, is to define a function and set the option to call it without
-arguments.
+arguments.  A |v:lua-call| can also be used.  Example: >vim
+	lua << EOF
+	  function _G.MyFoldFunc()
+	    -- ... compute fold level for line v:lnum
+	    return level
+	  end
+	EOF
+	set foldexpr=v:lua.MyFoldFunc()
 
 
 Setting the filetype

src/nvim/eval.c

@@ -1466,14 +1466,14 @@ Object eval_foldtext(win_T *wp)
 /// When "use_namespace" is true recognize "b:", "s:", etc.
 /// Return a pointer to just after the name.  Equal to "arg" if there is no
 /// valid name.
-static char *to_name_end(char *arg, bool use_namespace)
+static const char *to_name_end(const char *arg, bool use_namespace)
 {
   // Quick check for valid starting character.
   if (!eval_isnamec1(*arg)) {
     return arg;
   }
 
-  char *p;
+  const char *p;
   for (p = arg + 1; *p != NUL && eval_isnamec(*p); MB_PTR_ADV(p)) {
     // Include a namespace such as "s:var" and "v:var".  But "n:" is not
     // and can be used in slice "[n:]".
@@ -2621,18 +2621,25 @@ int eval0(char *arg, typval_T *rettv, exarg_T *eap, evalarg_T *const evalarg)
 
 /// If "arg" is a simple function call without arguments then call it and return
 /// the result.  Otherwise return NOTDONE.
-static int may_call_simple_func(char *arg, typval_T *rettv)
+static int may_call_simple_func(const char *arg, typval_T *rettv)
 {
-  char *parens = strstr(arg, "()");
+  const char *parens = strstr(arg, "()");
   int r = NOTDONE;
 
   // If the expression is "FuncName()" then we can skip a lot of overhead.
   if (parens != NULL && *skipwhite(parens + 2) == NUL) {
-    char *p = strncmp(arg, "<SNR>", 5) == 0 ? skipdigits(arg + 5) : arg;
+    if (strnequal(arg, "v:lua.", 6)) {
+      const char *p = arg + 6;
+      if (skip_luafunc_name(p) == parens) {
+        r = call_simple_luafunc(p, (size_t)(parens - p), rettv);
+      }
+    } else {
+      const char *p = strncmp(arg, "<SNR>", 5) == 0 ? skipdigits(arg + 5) : arg;
       if (to_name_end(p, true) == parens) {
         r = call_simple_func(arg, (size_t)(parens - arg), rettv);
       }
     }
+  }
   return r;
 }
 

src/nvim/eval/userfunc.c

@@ -1776,6 +1776,15 @@ theend:
   return ret;
 }
 
+int call_simple_luafunc(const char *funcname, size_t len, typval_T *rettv)
+  FUNC_ATTR_NONNULL_ALL
+{
+  typval_T argvars[1];
+  argvars[0].v_type = VAR_UNKNOWN;
+  nlua_typval_call(funcname, len, argvars, 0, rettv);
+  return OK;
+}
+
 /// Call a function without arguments, partial or dict.
 /// This is like call_func() when the call is only "FuncName()".
 /// To be used by "expr" options.