Float to double conversion is safe. The thing that's not safe in your example is to write `0.3f` (or `0.3d`) in the first place. The conversions from decimal to float or double is unsafe (inexact) and gives different results from floats than for doubles. But the conversion of float to double, in itself, is always exact.
The float representation of 0.3 (e.g.) does not, when cast to double, represent 0.3 - in contrast the i32 representation of any number when cast to i64 represents the same number.
