Add build.rs config for reliable f16 and f128

There are some complexities about what platforms we can test f16 and
f128 on.  Put this in build.rs so we have an easy way to configure tests
with a single attribute, and keep it up to date.

std/build.rs

@@ -7,6 +7,10 @@ fn main() {
     let target_vendor =
         env::var("CARGO_CFG_TARGET_VENDOR").expect("CARGO_CFG_TARGET_VENDOR was not set");
     let target_env = env::var("CARGO_CFG_TARGET_ENV").expect("CARGO_CFG_TARGET_ENV was not set");
+    let target_pointer_width: u32 = env::var("CARGO_CFG_TARGET_POINTER_WIDTH")
+        .expect("CARGO_CFG_TARGET_POINTER_WIDTH was not set")
+        .parse()
+        .unwrap();
 
     println!("cargo:rustc-check-cfg=cfg(netbsd10)");
     if target_os == "netbsd" && env::var("RUSTC_STD_NETBSD10").is_ok() {
@@ -70,4 +74,62 @@ fn main() {
     println!("cargo:rustc-cfg=backtrace_in_libstd");
 
     println!("cargo:rustc-env=STD_ENV_ARCH={}", env::var("CARGO_CFG_TARGET_ARCH").unwrap());
+
+    // Emit these on platforms that have no known ABI bugs, LLVM selection bugs, lowering bugs,
+    // missing symbols, or other problems, to determine when tests get run.
+    // If more broken platforms are found, please update the tracking issue at
+    // <https://github.com/rust-lang/rust/issues/116909>
+    //
+    // Some of these match arms are redundant; the goal is to separate reasons that the type is
+    // unreliable, even when multiple reasons might fail the same platform.
+    println!("cargo:rustc-check-cfg=cfg(reliable_f16)");
+    println!("cargo:rustc-check-cfg=cfg(reliable_f128)");
+
+    let has_reliable_f16 = match (target_arch.as_str(), target_os.as_str()) {
+        // Selection failure until recent LLVM <https://github.com/llvm/llvm-project/issues/93894>
+        // FIXME(llvm19): can probably be removed at the version bump
+        ("loongarch64", _) => false,
+        // Selection failure <https://github.com/llvm/llvm-project/issues/50374>
+        ("s390x", _) => false,
+        // Unsupported <https://github.com/llvm/llvm-project/issues/94434>
+        ("arm64ec", _) => false,
+        // MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
+        ("x86", "windows") => false,
+        // x86 has ABI bugs that show up with optimizations. This should be partially fixed with
+        // the compiler-builtins update. <https://github.com/rust-lang/rust/issues/123885>
+        ("x86" | "x86_64", _) => false,
+        // Missing `__gnu_h2f_ieee` and `__gnu_f2h_ieee`
+        ("powerpc" | "powerpc64" | "powerpc64le", _) => false,
+        // Missing `__extendhfsf` and `__truncsfhf`
+        ("riscv32" | "riscv64", _) => false,
+        // Most OSs are missing `__extendhfsf` and `__truncsfhf`
+        (_, "linux" | "macos") => true,
+        // Almost all OSs besides Linux and MacOS are missing symbols until compiler-builtins can
+        // be updated. <https://github.com/rust-lang/rust/pull/125016> will get some of these, the
+        // next CB update should get the rest.
+        _ => false,
+    };
+
+    let has_reliable_f128 = match (target_arch.as_str(), target_os.as_str()) {
+        // Unsupported <https://github.com/llvm/llvm-project/issues/94434>
+        ("arm64ec", _) => false,
+        // ABI and precision bugs <https://github.com/rust-lang/rust/issues/125109>
+        // <https://github.com/rust-lang/rust/issues/125102>
+        ("powerpc" | "powerpc64", _) => false,
+        // Selection bug <https://github.com/llvm/llvm-project/issues/95471>
+        ("nvptx64", _) => false,
+        // ABI unsupported  <https://github.com/llvm/llvm-project/issues/41838>
+        ("sparc", _) => false,
+        // 64-bit Linux is about the only platform to have f128 symbols by default
+        (_, "linux") if target_pointer_width == 64 => true,
+        // Same as for f16, except MacOS is also missing f128 symbols.
+        _ => false,
+    };
+
+    if has_reliable_f16 {
+        println!("cargo:rustc-cfg=reliable_f16");
+    }
+    if has_reliable_f128 {
+        println!("cargo:rustc-cfg=reliable_f128");
+    }
 }