I've recently asked if it's time to revisit the decision to make structs in maps in Go mutable, and was politely declined, with a reference to this being a refusted proposal since 2012:
https://github.com/golang/go/issues/53857
Since arrays work fine, if a is a slice or an array:
a[10].field = 42
it seems bogus that maps don't; this doesn't compile in Go if m is a map:
m[10].field = 42
In both cases, the address of the struct is calculated at runtime, as it must be known to access the field.
Instead of leaving this as a rant, let's make it a question: what's your "what is this insanity?" peeve with your favourite language?