This has come up many times; here's one I found from a couple months ago: Suggestion: FFastMath intrinsics in stable - #3 by scottmcm
I strongly believe that crate-level is inappropriate for this. Even function-level, because that composes poorly with things like .sqrt() which is a method, not an operator, so there's no nice way to get the fast math flags to apply to it.
That said, there's certainly interest in the area, and I'd like to have something for it. I think it needs someone to charter a lang/libs-supervized initiative to figure out the best way to expose it, though. See the portable simd work for potential inspiration.