Enforcing laziness or strictness annotations on data fields

Motivation

When defining a new data type it's easy to forget to add strictness annotations to its fields. StrictData is a suboptimal remedy as it changes the default: an unsuspecting reader may believe a field to be lazy when it is in fact strict.

Proposal

Add a new language extension (ExplicitlyStrictData?). When enabled, each field of a data declaration must be annotated with either a ~ or a !.

This declaration would be rejected:

data D a b = D {strict :: a, lazy :: b}

This one would be accepted:

data D a b = D {strict :: !a, lazy :: ~b}

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The idea to enforce strictness annotations was brought by @rae over lunch at ZuriHac. I'm simply recording it here. The proper process would surely involve a (rather simple) GHC proposal but I'm unlikely to submit it anytime soon. So I'm putting it up for grabs here.