feat: optional extension to early-return rule (#1133)

[mdelah]

This PR implements an extension to the early-return proposed in #1133. Specifically, it suggest a rewrite of the form

if x {
  y
}

to

if !x {
   return|break|continue
}
y

In cases where

• the if statement is at the end of the surrounding function body, for loop, or switch / select branch
• it has no else clause attached
• the if block body consists of multiple statements
• the other common prerequisites for early-return are met
• the new behaviour has been opted into via a configuration argument

Closes #1133

[chavacava]

nit: I'm not comfortable with "" as indicator of no failure. CheckIfElse could return a failure (or an error?)

[mdelah]

Sorry, I'm a little unclear on what you're proposing. Per doc string

// CheckIfElse evaluates the rule against an ifelse.Chain and returns a failure message if applicable.

The term "failure" may be a bit distracting (it could better be termed "suggestion"), but there is no actual possibility of failure/error here, either there is a message, or there isn't.

Would you prefer (string, bool)?

[chavacava]

a bool return for Check-something function sounds good

[mdelah]

Done, I think my reservation with it is it forces us to add bools at a lot of return points within the implementation that end up being equivalent to testing the string against empty anyway. But see what you think. Possibly I've misunderstood what you were getting at.

I also changed the ifelse.Rule interface to a func since it was a bit unnecessary with a single method, and lets the implementations be unexported.

[chavacava]

Thanks for the PR @mdelah

thank you @alexandear and @ccoVeille for the reviews