Add an "arch" Cargo feature that is on by default

Introduce a Cargo feature to enable or disable architecture-specific
features (SIMD, assembly), which is on by default. This allows for more
fine grained control compared to relying on the `force-soft-floats`
feature.

Similar to "unstable-intrinsics", introduce a build.rs config option for
`unstable-intrinsics AND NOT force-soft-floats`, which makes this easier
to work with in code.

Effectively, this allows moving our non-additive Cargo feature
(force-soft-floats) to a positive one by default, allowing for an
override when needed.

library/compiler-builtins/libm/Cargo.toml

@@ -14,7 +14,10 @@ exclude = ["/ci/", "/.github/workflows/"]
 rust-version = "1.63"
 
 [features]
-default = []
+default = ["arch"]
+
+# Enable architecture-specific features such as SIMD or assembly routines.
+arch = []
 
 # This tells the compiler to assume that a Nightly toolchain is being used and
 # that it should activate any useful Nightly things accordingly.

library/compiler-builtins/libm/build.rs

@@ -15,6 +15,7 @@ fn main() {
     }
 
     configure_intrinsics();
+    configure_arch();
 }
 
 /// Simplify the feature logic for enabling intrinsics so code only needs to use
@@ -28,3 +29,14 @@ fn configure_intrinsics() {
         println!("cargo:rustc-cfg=intrinsics_enabled");
     }
 }
+
+/// Simplify the feature logic for enabling arch-specific features so code only needs to use
+/// `cfg(arch_enabled)`.
+fn configure_arch() {
+    println!("cargo:rustc-check-cfg=cfg(arch_enabled)");
+
+    // Enabled by default via the "arch" feature, `force-soft-floats` overrides to disable.
+    if cfg!(feature = "arch") && !cfg!(feature = "force-soft-floats") {
+        println!("cargo:rustc-cfg=arch_enabled");
+    }
+}

library/compiler-builtins/libm/ci/run.sh

@@ -64,6 +64,7 @@ fi
 
 # Make sure we can build with overriding features. We test the indibidual
 # features it controls separately.
+cargo check --no-default-features
 cargo check --features "force-soft-floats"
 
 if [ "${BUILD_ONLY:-}" = "1" ]; then

library/compiler-builtins/libm/crates/compiler-builtins-smoke-test/Cargo.toml

@@ -18,6 +18,7 @@ force-soft-floats = []
 
 [lints.rust]
 unexpected_cfgs = { level = "warn", check-cfg = [
+  "cfg(arch_enabled)",
   "cfg(assert_no_panic)",
   "cfg(intrinsics_enabled)",
 ] }