vlib: add toml module + tests (#11964)

vlib/toml/input/input.v

@@ -0,0 +1,22 @@
+// Copyright (c) 2021 Lars Pontoppidan. All rights reserved.
+// Use of this source code is governed by an MIT license
+// that can be found in the LICENSE file.
+module input
+
+// Config is used to configure input to the toml module.
+// Only one of the fields `text` and `file_path` is allowed to be set at time of configuration.
+pub struct Config {
+pub:
+	text      string // TOML text
+	file_path string // '/path/to/file.toml'
+}
+
+pub fn (c Config) validate() ? {
+	if c.file_path != '' && c.text != '' {
+		error(@MOD + '.' + @FN +
+			' ${typeof(c).name} should contain only one of the fields `file_path` OR `text` filled out')
+	} else if c.file_path == '' && c.text == '' {
+		error(@MOD + '.' + @FN +
+			' ${typeof(c).name} must either contain a valid `file_path` OR a non-empty `text` field')
+	}
+}