Support for `errors.Join`

[sebastianst]

nilaway detects false positives when errors.Join is used together with errors that are guaranteed to not be nil, like errors.Join(errors.New("an error")).

nilaway could have the rule: if any of the argument to errors.Join are guaranteed to not be nil, the joined error is also guaranteed to not be nil.

The full working example:

package main

import "errors"

func main() {
	printFn(true)
	printFn(false)
}

func printFn(ferr bool) {
	i, err := fn(ferr)
	if err != nil {
		println(err.Error())
	} else {
		println(*i)
	}
}

func fn(err bool) (*int, error) {
	if err {
		return nil, errors.Join(errors.New("an error"), errors.New("another error"))
	} else {
		i := 42
		return &i, nil
	}
}

runs fine, but produces the nilaway error output

main.go:15:12: error: Potential nil panic detected. Observed nil flow from source to dereference point:
	-> nilaway-errors-join/main.go:21:10: literal `nil` returned from `fn()` in position 0 when the error return in position 1 is not guaranteed to be non-nil through all paths
	-> nilaway-errors-join/main.go:15:12: result 0 of `fn()` dereferenced