xload: Detect collisions while working with reused nested structs

[ajatprabha]

Overview

xload, at the moment, does not detect any collisions when working with nested structs that are reused.

Example

type A struct {
	B D `env:",prefix=D_"`
	C D `env:",prefix=D_"`
}

type D struct {
	S string `env:"S"`
}

When a loader returns "D_S: "value", both A.B.S and A.C.S are assigned value "value", without any warning, potentially introducing uncaught behaviour exceptions.

Proposal

Add some mechanism to detect collisions in the overall state of a Load call on the root struct.