Allow unstable_name_collisions

In recent nightlies we are hitting errors like the following:

     error: an associated constant with this name may be added to the standard library in the future
       --> libm/src/math/support/float_traits.rs:248:48
        |
    248 |               const SIGN_MASK: Self::Int = 1 << (Self::BITS - 1);
        |                                                  ^^^^^^^^^^
    ...
    324 | / float_impl!(
    325 | |     f32,
    326 | |     u32,
    327 | |     i32,
    ...   |
    333 | |     fmaf32
    334 | | );
        | |_- in this macro invocation
        |
        = warning: once this associated item is added to the standard library, the ambiguity may cause an error or change in behavior!
        = note: for more information, see issue #48919 <rust-lang/rust#48919>
        = note: `-D unstable-name-collisions` implied by `-D warnings`
        = help: to override `-D warnings` add `#[allow(unstable_name_collisions)]`
        = note: this error originates in the macro `float_impl` (in Nightly builds, run with -Z macro-backtrace for more info)
    help: use the fully qualified path to the associated const
        |
    248 -             const SIGN_MASK: Self::Int = 1 << (Self::BITS - 1);
    248 +             const SIGN_MASK: Self::Int = 1 << (<f32 as float_traits::Float>::BITS - 1);
        |
    help: add `#![feature(float_bits_const)]` to the crate attributes to enable `core::f32::<impl f32>::BITS`
       --> libm/src/lib.rs:26:1
        |
     26 + #![feature(float_bits_const)]
        |

Using fully qualified syntax is verbose and `BITS` only exists since
recently, so allow this lint instead.

Author: tgross35

compiler-builtins/src/lib.rs

@@ -11,11 +11,11 @@
 #![feature(repr_simd)]
 #![feature(macro_metavar_expr_concat)]
 #![feature(rustc_attrs)]
-#![feature(float_bits_const)]
 #![cfg_attr(f16_enabled, feature(f16))]
 #![cfg_attr(f128_enabled, feature(f128))]
 #![no_builtins]
 #![no_std]
+#![allow(unstable_name_collisions)] // FIXME(float_bits_const): remove when stable
 #![allow(unused_features)]
 #![allow(internal_features)]
 // `mem::swap` cannot be used because it may generate references to memcpy in unoptimized code.

libm-test/src/lib.rs

@@ -1,6 +1,7 @@
 #![cfg_attr(f16_enabled, feature(f16))]
 #![cfg_attr(f128_enabled, feature(f128))]
 #![allow(clippy::unusual_byte_groupings)] // sometimes we group by sign_exp_sig
+#![allow(unstable_name_collisions)] // FIXME(float_bits_const): remove when stable
 
 pub mod domain;
 mod f8_impl;

libm/src/lib.rs

@@ -8,6 +8,7 @@
 )]
 #![cfg_attr(f128_enabled, feature(f128))]
 #![cfg_attr(f16_enabled, feature(f16))]
+#![allow(unstable_name_collisions)] // FIXME(float_bits_const): remove when stable
 #![allow(clippy::assign_op_pattern)]
 #![allow(clippy::deprecated_cfg_attr)]
 #![allow(clippy::eq_op)]