For example, the source code like below:

package main

import "fmt"

func main() {
    fmt.Println("hello world")
}

If we use go mod vendor for it, it can't guarantee the code of fmt is consistent.

Pros:

• It will help people to track the changes on stdlib, such as unexpected changes on stdlib when they upgrade Go version.
• Make debugging stdlib on the fly much easier. Such as people may modify the source code of fmt in their local GOROOT and forget to revert it. If the vendor has a copy of fmt, it will be much easier to modify the stdlib.
• Make cross-compile stdlib between different Go runtime much easier. Such as let old stdlib benefit from the improvements from newer GC, etc.

Cons:

• May break things if the newer Go compiler tries to compile old stdlib?
• Hard to implement? The development cost can't justify the gain.

Maybe we can first add a flag to the CLI to make it optional: go mod vendor -s, s means also include the stdlib.