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]: 555e1d0386/library/std/build.rs (L84-L186)

tests/ui/cfg/disallowed-cli-cfgs.reliable_f128_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_has_reliable_f128` flag
+   |
+   = note: config `target_has_reliable_f128` is only supposed to be controlled by `--target`
+   = note: manually setting a built-in cfg can and does create incoherent behaviors
+   = note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.reliable_f128_math_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_has_reliable_f128_math` flag
+   |
+   = note: config `target_has_reliable_f128_math` is only supposed to be controlled by `--target`
+   = note: manually setting a built-in cfg can and does create incoherent behaviors
+   = note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.reliable_f16_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_has_reliable_f16` flag
+   |
+   = note: config `target_has_reliable_f16` is only supposed to be controlled by `--target`
+   = note: manually setting a built-in cfg can and does create incoherent behaviors
+   = note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.reliable_f16_math_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg target_has_reliable_f16_math` flag
+   |
+   = note: config `target_has_reliable_f16_math` is only supposed to be controlled by `--target`
+   = note: manually setting a built-in cfg can and does create incoherent behaviors
+   = note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.rs

@@ -8,6 +8,7 @@
 //@ revisions: target_thread_local_ relocation_model_
 //@ revisions: fmt_debug_
 //@ revisions: emscripten_wasm_eh_
+//@ revisions: reliable_f16_ reliable_f16_math_ reliable_f128_ reliable_f128_math_
 
 //@ [overflow_checks_]compile-flags: --cfg overflow_checks
 //@ [debug_assertions_]compile-flags: --cfg debug_assertions
@@ -35,6 +36,10 @@
 //@ [relocation_model_]compile-flags: --cfg relocation_model="a"
 //@ [fmt_debug_]compile-flags: --cfg fmt_debug="shallow"
 //@ [emscripten_wasm_eh_]compile-flags: --cfg emscripten_wasm_eh
+//@ [reliable_f16_]compile-flags: --cfg target_has_reliable_f16
+//@ [reliable_f16_math_]compile-flags: --cfg target_has_reliable_f16_math
+//@ [reliable_f128_]compile-flags: --cfg target_has_reliable_f128
+//@ [reliable_f128_math_]compile-flags: --cfg target_has_reliable_f128_math
 
 fn main() {}
 

tests/ui/feature-gates/feature-gate-cfg-target-has-reliable-f16-f128.rs

@@ -0,0 +1,12 @@
+//@ compile-flags: --check-cfg=cfg(target_has_reliable_f16,target_has_reliable_f16_math,target_has_reliable_f128,target_has_reliable_f128_math)
+
+fn main() {
+    cfg!(target_has_reliable_f16);
+    //~^ ERROR `cfg(target_has_reliable_f16)` is experimental and subject to change
+    cfg!(target_has_reliable_f16_math);
+    //~^ ERROR `cfg(target_has_reliable_f16_math)` is experimental and subject to change
+    cfg!(target_has_reliable_f128);
+    //~^ ERROR `cfg(target_has_reliable_f128)` is experimental and subject to change
+    cfg!(target_has_reliable_f128_math);
+    //~^ ERROR `cfg(target_has_reliable_f128_math)` is experimental and subject to change
+}

tests/ui/feature-gates/feature-gate-cfg-target-has-reliable-f16-f128.stderr

@@ -0,0 +1,39 @@
+error[E0658]: `cfg(target_has_reliable_f16)` is experimental and subject to change
+  --> $DIR/feature-gate-cfg-target-has-reliable-f16-f128.rs:4:10
+   |
+LL |     cfg!(target_has_reliable_f16);
+   |          ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(cfg_target_has_reliable_f16_f128)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: `cfg(target_has_reliable_f16_math)` is experimental and subject to change
+  --> $DIR/feature-gate-cfg-target-has-reliable-f16-f128.rs:6:10
+   |
+LL |     cfg!(target_has_reliable_f16_math);
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(cfg_target_has_reliable_f16_f128)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: `cfg(target_has_reliable_f128)` is experimental and subject to change
+  --> $DIR/feature-gate-cfg-target-has-reliable-f16-f128.rs:8:10
+   |
+LL |     cfg!(target_has_reliable_f128);
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(cfg_target_has_reliable_f16_f128)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: `cfg(target_has_reliable_f128_math)` is experimental and subject to change
+  --> $DIR/feature-gate-cfg-target-has-reliable-f16-f128.rs:10:10
+   |
+LL |     cfg!(target_has_reliable_f128_math);
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(cfg_target_has_reliable_f16_f128)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

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));
+    }
+}