Suggestions for additional floating-point types

I noticed that, like other languages, the only floating-point types built-in are f32 and f64. However, I often have limitations with just these. I propose the following: fsize, freal, and f128

fsize would be like isize but for floats. Basically, use the version that's most efficient for your processor. On modern 64-bit processors with wide FPUs and/or 256-bit SIMD this would become f64.

Sometimes I want to be able to have a variable for real numbers, or I don't know what precision I want yet. In C++ I can do the following to have an abstract precision that I control via compiler flags:

#ifdef REAL_T_IS_DOUBLE
typedef double real_t;
#else
typedef float real_t;
#endif

I propose something similar in Rust, where you can just write freal or something and be able to change the precision later with compiler flags. The default would probably be f32.

Finally, it would be nice to have 128-bit floats (f128) in the language. These are not normally needed, but there are use cases for them, and it'd be nice to have it as a language built-in type. Some newer processors have 512-bit SIMD chipsets that can process these efficiently, though most don't.

If you only implement some of these proposals, that's fine too. Originally posted at https://github.com/rust-lang/rust/issues/57928