Remove the dependency on libm

[brendanzab]

The functions that need to be implemented are:

• cmath::c_float::ceil
• cmath::c_float::trunc
• cmath::c_float::acos
• cmath::c_float::asin
• cmath::c_float::atan
• cmath::c_float::atan2
• cmath::c_float::cbrt
• cmath::c_float::copysign
• cmath::c_float::cosh
• cmath::c_float::exp_m1
• cmath::c_float::abs_sub
• cmath::c_float::next_after
• cmath::c_float::frexp
• cmath::c_float::hypot
• cmath::c_float::ldexp
• cmath::c_float::ln_1p
• cmath::c_float::round
• cmath::c_float::sinh
• cmath::c_float::tan
• cmath::c_float::tanh
• cmath::c_double::ceil
• cmath::c_double::trunc
• cmath::c_double::acos
• cmath::c_double::asin
• cmath::c_double::atan
• cmath::c_double::atan2
• cmath::c_double::cbrt
• cmath::c_double::copysign
• cmath::c_double::cosh
• cmath::c_double::exp_m1
• cmath::c_double::abs_sub
• cmath::c_double::next_after
• cmath::c_double::frexp
• cmath::c_double::hypot
• cmath::c_double::ldexp
• cmath::c_double::ln_1p
• cmath::c_double::round
• cmath::c_double::sinh
• cmath::c_double::tan
• cmath::c_double::tanh

Once this list has been completed we would be free to remove the std::num::cmath module.

[alexcrichton]

I would be wary of defining these functions in libstd. I think that having libnum with a dependence on libm or defining these functions in libnum would certainly be acceptable.

[brendanzab]

We have been discussing having an ieee754 lib. It would be super cool if the base numeric types could be de-magified and moved into a lib (see #11526). But again we would run into the issue of annoying users by not having CTFE.

[brendanzab]

I might have a go at starting an IEEE754 library separate from the main rust repo, based on something like struct F32(u32); struct F64(u64);, and see how I go - then we can decide whether or not to include them in the std.

[brson]

Although I do support the goal of modularizing std, I'm still not sure where I fall on this suggestion. Is there a BSD-licensed libm that is portable to everywhere that we can just link in to remove the external dependency?

[thestinger]

We can provide the ability to use another libm without bringing it into the tree or adding a submodule. If Rust is going to be packaged for Debian and Fedora, it needs to move away from including third party libraries internally.

[thestinger]

The LLVM intrinsics also call the C math symbols, so there's an inherent dependency on the C math library.

[thestinger]

Closing in favour of #10774.