Replace feature = "unstable-intrinsics" with intrinsics_enabled

We currently have a non-additive feature, "force-soft-floats", and we
will need to gain another "no-f16-f128". This makes `cfg` usage in code
somewhat confusing and redundant.

Use `build.rs` to figure out if "unstable-intrinsics" is enabled while
"force-soft-floats" is not enabled and if so, emit a cfg
`intrinsics_enabled`. This is cleaner to use and should make adding more
features easier to reason about.

Also use this as an opportunity to eliminate the build.rs from the
compiler-builtins test crate, replaced with the `[lints]` table in
Cargo.toml.

library/compiler-builtins/libm/Cargo.toml

@@ -24,6 +24,11 @@ unstable = ["unstable-intrinsics"]
 unstable-intrinsics = []
 
 # Used to prevent using any intrinsics or arch-specific code.
+#
+# HACK: this is a negative feature which is generally a bad idea in Cargo, but
+# we need it to be able to forbid other features when this crate is used in
+# Rust dependencies. Setting this overrides all features that may enable
+# hard float operations.
 force-soft-floats = []
 
 [workspace]

library/compiler-builtins/libm/build.rs

@@ -3,7 +3,6 @@ use std::env;
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rustc-check-cfg=cfg(assert_no_panic)");
-    println!("cargo:rustc-check-cfg=cfg(feature, values(\"unstable\"))");
 
     println!("cargo:rustc-check-cfg=cfg(feature, values(\"checked\"))");
 
@@ -14,4 +13,18 @@ fn main() {
             println!("cargo:rustc-cfg=assert_no_panic");
         }
     }
+
+    configure_intrinsics();
+}
+
+/// Simplify the feature logic for enabling intrinsics so code only needs to use
+/// `cfg(intrinsics_enabled)`.
+fn configure_intrinsics() {
+    println!("cargo:rustc-check-cfg=cfg(intrinsics_enabled)");
+
+    // Disabled by default; `unstable-intrinsics` enables again; `force-soft-floats` overrides
+    // to disable.
+    if cfg!(feature = "unstable-intrinsics") && !cfg!(feature = "force-soft-floats") {
+        println!("cargo:rustc-cfg=intrinsics_enabled");
+    }
 }

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

@@ -46,6 +46,10 @@ if [ "$(uname -a)" = "Linux" ]; then
     extra_flags="$extra_flags --features libm-test/test-musl-serialized"
 fi
 
+# Make sure we can build with overriding features. We test the indibidual
+# features it controls separately.
+cargo check --features "force-soft-floats"
+
 if [ "${BUILD_ONLY:-}" = "1" ]; then
     cmd="cargo build --target $target --package libm"
     $cmd

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

@@ -15,3 +15,9 @@ unstable = []
 unstable-intrinsics = []
 checked = []
 force-soft-floats = []
+
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = [
+  "cfg(assert_no_panic)",
+  "cfg(intrinsics_enabled)",
+] }

library/compiler-builtins/libm/crates/compiler-builtins-smoke-test/build.rs

@@ -1,3 +0,0 @@
-fn main() {
-    println!("cargo::rustc-check-cfg=cfg(assert_no_panic)");
-}

library/compiler-builtins/libm/src/lib.rs

@@ -1,7 +1,7 @@
 //! libm in pure Rust
 #![no_std]
-#![cfg_attr(feature = "unstable-intrinsics", allow(internal_features))]
-#![cfg_attr(feature = "unstable-intrinsics", feature(core_intrinsics))]
+#![cfg_attr(intrinsics_enabled, allow(internal_features))]
+#![cfg_attr(intrinsics_enabled, feature(core_intrinsics))]
 #![allow(clippy::assign_op_pattern)]
 #![allow(clippy::deprecated_cfg_attr)]
 #![allow(clippy::eq_op)]

library/compiler-builtins/libm/src/math/mod.rs

@@ -60,14 +60,14 @@ macro_rules! i {
 // the time of this writing this is only used in a few places, and once
 // rust-lang/rust#72751 is fixed then this macro will no longer be necessary and
 // the native `/` operator can be used and panics won't be codegen'd.
-#[cfg(any(debug_assertions, not(feature = "unstable-intrinsics")))]
+#[cfg(any(debug_assertions, not(intrinsics_enabled)))]
 macro_rules! div {
     ($a:expr, $b:expr) => {
         $a / $b
     };
 }
 
-#[cfg(all(not(debug_assertions), feature = "unstable-intrinsics"))]
+#[cfg(all(not(debug_assertions), intrinsics_enabled))]
 macro_rules! div {
     ($a:expr, $b:expr) => {
         unsafe { core::intrinsics::unchecked_div($a, $b) }
@@ -76,9 +76,7 @@ macro_rules! div {
 
 macro_rules! llvm_intrinsically_optimized {
     (#[cfg($($clause:tt)*)] $e:expr) => {
-        #[cfg(all(
-            feature = "unstable-intrinsics", not(feature = "force-soft-floats"), $($clause)*
-        ))]
+        #[cfg(all(intrinsics_enabled, not(feature = "force-soft-floats"), $($clause)*))]
         {
             if true { // thwart the dead code lint
                 $e