The question is how to enable all this in Rust... Right now there's no simple way to just tell Rust to be fast & loose with floats.
but better support dies in endless bikeshed of:
• People imagine enabling fast float by "scope", but there's no coherent way to specify that when it can mix with closures (even across crates) and math operators expand to std functions defined in a different scope.
• Type-based float config could work, but any proposal of just "fastfloat32" grows into a HomerMobile of "Float<NaN=false, Inf=Maybe, NegZeroEqualsZero=OnFullMoonOnly, etc.>"
• Rust doesn't want to allow UB in safe code, and LLVM will cry UB if it sees Inf or Nan, but nobody wants compiler inserting div != 0 checks.
• You could wrap existing fast intrinsics in a newtype, except newtypes don't support literals, and <insert user-defined literals bikeshed here>
Blanket flags for the whole program do not fit very well with Rust, while point use of these flags is inconvenient or needs new syntax.. but there are discussions about these topics.
> just a flag to enable all this and be fine with consequences
Is probably the opposite of what the language is about.
At least it doesn't need the errno ones since rust don't use errno.
Besides, algorithms that benefit from -ffast-math can be implemented in C and with Rust bindings automatically generated.
This solution isn't exactly "simple", but it could help projects keep track of the expectations of correctness between different algorithm implementations.