I’d also like an example of that. It seems wrong. I know that casts can cause UB, but transmute just copies the bits verbatim. All bit strings of length 32 are valid binary32 floats (though about 2^23 are NaNs) and all bit strings of length 64 are valid binary64 values (though about 2^52 are NaNs). There are no invalid representations that a transmute could produce, unless some layer between Rust and the silicon is actively breaking NaN with weird bit patterns (I say “actively” because I can’t imagine anything to be gained from such a restriction).