Irrefutable struct patterns

[mattheww]

§5.2:4 (fls_r8rat3qmc4hy) says "A struct pattern is an irrefutable pattern when all of its subpatterns are irrefutable patterns."

But this isn't sufficient: if the deconstructee is an enum variant, for the
pattern to be irrefutable it must also be the case that the enum has only one variant.