Add const support for float rounding methods

Add const support for the float rounding methods floor, ceil, trunc,
fract, round and round_ties_even.
This works by moving the calculation logic from

     src/tools/miri/src/intrinsics/mod.rs

into

     compiler/rustc_const_eval/src/interpret/intrinsics.rs.

All relevant method definitions were adjusted to include the `const`
keyword for all supported float types: f16, f32, f64 and f128.

The constness is hidden behind the feature gate

     feature(const_float_round_methods)

which is tracked in

     https://github.com/rust-lang/rust/issues/141555

This commit is a squash of the following commits:
- test: add tests that we expect to pass when float rounding becomes const
- feat: make float rounding methods `const`
- fix: replace `rustc_allow_const_fn_unstable(core_intrinsics)` attribute with `#[rustc_const_unstable(feature = "f128", issue = "116909")]` in `library/core/src/num/f128.rs`
- revert: undo update to `library/stdarch`
- refactor: replace multiple `float_<mode>_intrinsic` rounding methods with a single, parametrized one
- fix: add `#[cfg(not(bootstrap))]` to new const method tests
- test: add extra sign tests to check `+0.0` and `-0.0`
- revert: undo accidental changes to `round` docs
- fix: gate `const` float round method behind `const_float_round_methods`
- fix: remove unnecessary `#![feature(const_float_methods)]`
- fix: remove unnecessary `#![feature(const_float_methods)]` [2]
- revert: undo changes to `tests/ui/consts/const-eval/float_methods.rs`
- fix: adjust after rebase
- test: fix float tests
- test: add tests for `fract`
- chore: add commented-out `const_float_round_methods` feature gates to `f16` and `f128`
- fix: adjust NaN when rounding floats
- chore: add FIXME comment for de-duplicating float tests
- test: remove unnecessary test file `tests/ui/consts/const-eval/float_methods.rs`
- test: fix tests after upstream simplification of how float tests are run

1 files changed, 18 insertions, 6 deletions

diff --git a/library/core/src/num/f128.rs b/library/core/src/num/f128.rs
index 0c2c4155d66..6b9b2ba8689 100644
--- a/library/core/src/num/f128.rs
+++ b/library/core/src/num/f128.rs
@@ -1447,8 +1447,10 @@ impl f128 {
     #[inline]
     #[rustc_allow_incoherent_impl]
     #[unstable(feature = "f128", issue = "116909")]
+    #[rustc_const_unstable(feature = "f128", issue = "116909")]
+    // #[rustc_const_unstable(feature = "const_float_round_methods", issue = "141555")]
     #[must_use = "method returns a new number and does not mutate the original value"]
-    pub fn floor(self) -> f128 {
+    pub const fn floor(self) -> f128 {
         // SAFETY: intrinsic with no preconditions
         unsafe { intrinsics::floorf128(self) }
     }
@@ -1477,8 +1479,10 @@ impl f128 {
     #[doc(alias = "ceiling")]
     #[rustc_allow_incoherent_impl]
     #[unstable(feature = "f128", issue = "116909")]
+    #[rustc_const_unstable(feature = "f128", issue = "116909")]
+    // #[rustc_const_unstable(feature = "const_float_round_methods", issue = "141555")]
     #[must_use = "method returns a new number and does not mutate the original value"]
-    pub fn ceil(self) -> f128 {
+    pub const fn ceil(self) -> f128 {
         // SAFETY: intrinsic with no preconditions
         unsafe { intrinsics::ceilf128(self) }
     }
@@ -1513,8 +1517,10 @@ impl f128 {
     #[inline]
     #[rustc_allow_incoherent_impl]
     #[unstable(feature = "f128", issue = "116909")]
+    #[rustc_const_unstable(feature = "f128", issue = "116909")]
+    // #[rustc_const_unstable(feature = "const_float_round_methods", issue = "141555")]
     #[must_use = "method returns a new number and does not mutate the original value"]
-    pub fn round(self) -> f128 {
+    pub const fn round(self) -> f128 {
         // SAFETY: intrinsic with no preconditions
         unsafe { intrinsics::roundf128(self) }
     }
@@ -1547,8 +1553,10 @@ impl f128 {
     #[inline]
     #[rustc_allow_incoherent_impl]
     #[unstable(feature = "f128", issue = "116909")]
+    #[rustc_const_unstable(feature = "f128", issue = "116909")]
+    // #[rustc_const_unstable(feature = "const_float_round_methods", issue = "141555")]
     #[must_use = "method returns a new number and does not mutate the original value"]
-    pub fn round_ties_even(self) -> f128 {
+    pub const fn round_ties_even(self) -> f128 {
         intrinsics::round_ties_even_f128(self)
     }
 
@@ -1579,8 +1587,10 @@ impl f128 {
     #[doc(alias = "truncate")]
     #[rustc_allow_incoherent_impl]
     #[unstable(feature = "f128", issue = "116909")]
+    #[rustc_const_unstable(feature = "f128", issue = "116909")]
+    // #[rustc_const_unstable(feature = "const_float_round_methods", issue = "141555")]
     #[must_use = "method returns a new number and does not mutate the original value"]
-    pub fn trunc(self) -> f128 {
+    pub const fn trunc(self) -> f128 {
         // SAFETY: intrinsic with no preconditions
         unsafe { intrinsics::truncf128(self) }
     }
@@ -1610,8 +1620,10 @@ impl f128 {
     #[inline]
     #[rustc_allow_incoherent_impl]
     #[unstable(feature = "f128", issue = "116909")]
+    #[rustc_const_unstable(feature = "f128", issue = "116909")]
+    // #[rustc_const_unstable(feature = "const_float_round_methods", issue = "141555")]
     #[must_use = "method returns a new number and does not mutate the original value"]
-    pub fn fract(self) -> f128 {
+    pub const fn fract(self) -> f128 {
         self - self.trunc()
     }