That would require Rust to convert all NaNs to a single "canonical" NaN. I don't think it does that, in fact you don't want Rust to do that, since it would make it impossible to do these NaN-boxing tricks by hand.
On the other hand, a CanonicalNaN<T> could be created in the future to allow these tricks, similar to the NonNull<T>/NonZeroU8/etc mentioned above.
(As an interesting aside, for the RISC-V ISA, all floating-point operations generate a single canonical NaN: "Except when otherwise stated, if the result of a floating-point operation is NaN, it is the canonical NaN. The canonical NaN has a positive sign and all significand bits clear except the MSB, a.k.a. the quiet bit. For single-precision floating-point, this corresponds to the pattern 0x7fc00000." So this scheme would work even better on RISC-V, since after any floating-point operation, it's guaranteed that any NaN is the canonical NaN.)
On the other hand, a CanonicalNaN<T> could be created in the future to allow these tricks, similar to the NonNull<T>/NonZeroU8/etc mentioned above.
(As an interesting aside, for the RISC-V ISA, all floating-point operations generate a single canonical NaN: "Except when otherwise stated, if the result of a floating-point operation is NaN, it is the canonical NaN. The canonical NaN has a positive sign and all significand bits clear except the MSB, a.k.a. the quiet bit. For single-precision floating-point, this corresponds to the pattern 0x7fc00000." So this scheme would work even better on RISC-V, since after any floating-point operation, it's guaranteed that any NaN is the canonical NaN.)