thought: with verbose option for listing why a method isn't a candidate for pure/mutation-free, etc.
Psalm now does this!
This was already added in August, >= v3.15?
E.g. MissingPureAnnotation & MissingImmutableAnnotation: https://github.com/vimeo/psalm/commit/94cec1559891ef32bf6d443f9e796a674994b6af
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Psalm now does this!