proposal: add `Must[T]` function

[septemhill]

I would like to propose add func Must[T any](v T, err error) T function in Go. The implementation would be like:

func Must[T any](v T, err error) T {
  if err != nil {
    panic(err)
  }
  return v
}

Motivation

Currently, we have some Must prefix functions in standard libraries. e.g. regexp.MustCompile, template.Must, netip.MustParseAddr, netip.MustParseAddrPort, etc.

Also, I make a github search to list the functions which with Must prefix. And not surprisingly, most of Must/must functions have similar implementation.

There're two styles Must function: (Option 1 is what I propose)

1. func Must(v Type, err error) Type

and

2. func Must(err error)

Most of them just check if err != nil then panic directly.

Before Go 1.18, we didn't have generic, so we need to use MustSomething for each of Something which we want to panic immediately if we got error. Now, we have generic capability, maybe we could make these functions more clear.

[Jorropo]

@septemhill in which package would it being added, builtin ?

[septemhill]

@septemhill in which package would it being added, builtin ?

Actually, I have no idea.

After Go 1.18 introduced generic capability, we could simplify so many exist functions.
But these simplification functions (includes this proposal) just like utility.
If we accept the proposal, that means we might have more proposals like this one.
So, before we figure out how to classify these functions, I'll suggest put it under golang.org/x/somewhere

[seankhliao]

Duplicate of #54297