util/must: new package (#5307)

The Do function assists in calling functions that must succeed.
It only interacts well with functions that return (T, err).
Signatures with more return arguments are not supported.

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

util/must/must.go

@@ -0,0 +1,17 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package must assists in calling functions that must succeed.
+//
+// Example usage:
+//	var target = must.Do(url.Parse(...))
+package must
+
+// Do returns v as is. It panics if err is non-nil.
+func Do[T any](v T, err error) T {
+	if err != nil {
+		panic(err)
+	}
+	return v
+}