Implement the internal feature `cfg_target_has_reliable_f16_f128`

Support for `f16` and `f128` is varied across targets, backends, and
backend versions. Eventually we would like to reach a point where all
backends support these approximately equally, but until then we have to
work around some of these nuances of support being observable.

Introduce the `cfg_target_has_reliable_f16_f128` internal feature, which
provides the following new configuration gates:

* `cfg(target_has_reliable_f16)`
* `cfg(target_has_reliable_f16_math)`
* `cfg(target_has_reliable_f128)`
* `cfg(target_has_reliable_f128_math)`

`reliable_f16` and `reliable_f128` indicate that basic arithmetic for
the type works correctly. The `_math` versions indicate that anything
relying on `libm` works correctly, since sometimes this hits a separate
class of codegen bugs.

These options match configuration set by the build script at [1]. The
logic for LLVM support is duplicated as-is from the same script. There
are a few possible updates that will come as a follow up.

The config introduced here is not planned to ever become stable, it is
only intended to replace the build scripts for `std` tests and
`compiler-builtins` that don't have any way to configure based on the
codegen backend.

MCP: https://github.com/rust-lang/compiler-team/issues/866
Closes: https://github.com/rust-lang/compiler-team/issues/866

[1]: https://github.com/rust-lang/rust/blob/555e1d0386f024a8359645c3217f4b3eae9be042/library/std/build.rs#L84-L186

1 files changed, 31 insertions, 0 deletions

diff --git a/tests/ui/float/target-has-reliable-nightly-float.rs b/tests/ui/float/target-has-reliable-nightly-float.rs
new file mode 100644
index 00000000000..ad8600fc635
--- /dev/null
+++ b/tests/ui/float/target-has-reliable-nightly-float.rs
@@ -0,0 +1,31 @@
+//@ run-pass
+//@ compile-flags: --check-cfg=cfg(target_has_reliable_f16,target_has_reliable_f16_math,target_has_reliable_f128,target_has_reliable_f128_math)
+// Verify that the feature gates and config work and are registered as known config
+// options.
+
+#![deny(unexpected_cfgs)]
+#![feature(cfg_target_has_reliable_f16_f128)]
+
+#[cfg(target_has_reliable_f16)]
+pub fn has_f16() {}
+
+#[cfg(target_has_reliable_f16_math)]
+pub fn has_f16_math() {}
+
+#[cfg(target_has_reliable_f128 )]
+pub fn has_f128() {}
+
+#[cfg(target_has_reliable_f128_math)]
+pub fn has_f128_math() {}
+
+fn main() {
+    if cfg!(target_arch = "aarch64") && cfg!(target_os = "linux") {
+        // Aarch64+Linux is one target that has support for all features, so use it to spot
+        // check that the compiler does indeed enable these gates.
+
+        assert!(cfg!(target_has_reliable_f16));
+        assert!(cfg!(target_has_reliable_f16_math));
+        assert!(cfg!(target_has_reliable_f128));
+        assert!(cfg!(target_has_reliable_f128_math));
+    }
+}