feat(lua): make =expr print result of expr

runtime/doc/lua.txt

@@ -249,13 +249,15 @@ arguments separated by " " (space) instead of "\t" (tab).
                                                         *:lua*
 :[range]lua {chunk}
                         Executes Lua chunk {chunk}.
-
+                        if {chunk} starts with "=" the rest of the chunk is
+                        evaluated as an expression and printed. `:lua =expr`
+                        is equivalent to `:lua print(vim.inspect(expr))`
                         Examples: >
                             :lua vim.api.nvim_command('echo "Hello, Nvim!"')
 <                        To see the Lua version: >
                             :lua print(_VERSION)
 <                        To see the LuaJIT version: >
-                            :lua print(jit.version)
+                            :lua =jit.version
 <
                                                         *:lua-heredoc*
 :[range]lua << [endmarker]

src/nvim/lua/executor.c

@@ -1115,11 +1115,23 @@ void ex_lua(exarg_T *const eap)
   FUNC_ATTR_NONNULL_ALL
 {
   size_t len;
-  char *const code = script_get(eap, &len);
+  char *code = script_get(eap, &len);
   if (eap->skip) {
     xfree(code);
     return;
   }
+  // When =expr is used transform it to print(vim.inspect(expr))
+  if (code[0] == '=') {
+    len += sizeof("print(vim.inspect())") - sizeof("=");
+    // code_buf needs to be 1 char larger then len for null byte in the end.
+    // lua nlua_typval_exec doesn't expect null terminated string so len
+    // needs to end before null byte.
+    char *code_buf = xmallocz(len);
+    vim_snprintf(code_buf, len+1, "print(vim.inspect(%s))", code+1);
+    xfree(code);
+    code = code_buf;
+  }
+
   nlua_typval_exec(code, len, ":lua", NULL, 0, false, NULL);
 
   xfree(code);

test/functional/lua/commands_spec.lua

@@ -141,6 +141,15 @@ describe(':lua command', function()
       {4:Press ENTER or type command to continue}^ |
     ]]}
   end)
+
+  it('Can print results of =expr', function()
+    helpers.exec_lua("x = 5")
+    eq("5", helpers.exec_capture(':lua =x'))
+    helpers.exec_lua("function x() return 'hello' end")
+    eq([["hello"]], helpers.exec_capture(':lua = x()'))
+    helpers.exec_lua("x = {a = 1, b = 2}")
+    eq("{\n  a = 1,\n  b = 2\n}", helpers.exec_capture(':lua  =x'))
+  end)
 end)
 
 describe(':luado command', function()