math/big: Rat.Float64 not safe for concurrent use #34919
Description
In math/big, Rat.Float64 modifies x.b.abs by calling set(natNone), which makes it unsafe for concurrent use.
But you would really expect that a reading method like Float64 would be safe.
In fact go/constant's Float64Val assumes this, unless you also want to say that constant.Float64Val is not safe for concurrent use either.
But then you'd have to track all the calls to that, like go/types.roundFloat64, called from the types.Checker, and so on.
It seems like the solution has to be to make Rat.Float64 safe for concurrent use.
It would be fine to just not modify x.b.abs there and do 'b = natOne', no?
This is the root cause of the original go/packages race reported in #31749.
It may also be a mistake to have fixed #33792 by declaring Denom to be unsafe for concurrent use,
for many of the same reasons.
That could have returned a fixed 1 int as well.