Move the existing "unstable" feature to "unstable-intrinsics"

Currently there is a single feature called "unstable" that is used to
control whether intrinsics may be called. In anticipation of adding
other unstable features that we will want to control separately, create
a new feature called "unstable-intrinsics" that is enabled by
"unstable". Then move everything gated by "unstable" to
"unstable-intrinsics".

library/compiler-builtins/libm/Cargo.toml

@@ -18,7 +18,10 @@ default = []
 
 # This tells the compiler to assume that a Nightly toolchain is being used and
 # that it should activate any useful Nightly things accordingly.
-unstable = []
+unstable = ["unstable-intrinsics"]
+
+# Enable calls to functions in `core::intrinsics`
+unstable-intrinsics = []
 
 # Used to prevent using any intrinsics or arch-specific code.
 force-soft-floats = []

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

@@ -49,7 +49,7 @@ fi
 if [ "${BUILD_ONLY:-}" = "1" ]; then
     cmd="cargo build --target $target --package libm"
     $cmd
-    $cmd --features 'unstable'
+    $cmd --features "unstable-intrinsics"
 
     echo "can't run tests on $target"
 else
@@ -60,6 +60,6 @@ else
     $cmd --release
 
     # unstable with a feature
-    $cmd --features 'unstable'
-    $cmd --release --features 'unstable'
+    $cmd --features "unstable-intrinsics"
+    $cmd --release --features "unstable-intrinsics"
 fi

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

@@ -10,6 +10,8 @@ test = false
 bench = false
 
 [features]
+# Duplicated from libm's Cargo.toml
 unstable = []
+unstable-intrinsics = []
 checked = []
 force-soft-floats = []

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

@@ -1,7 +1,7 @@
 //! libm in pure Rust
 #![no_std]
-#![cfg_attr(feature = "unstable", allow(internal_features))]
-#![cfg_attr(feature = "unstable", feature(core_intrinsics))]
+#![cfg_attr(feature = "unstable-intrinsics", allow(internal_features))]
+#![cfg_attr(feature = "unstable-intrinsics", 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")))]
+#[cfg(any(debug_assertions, not(feature = "unstable-intrinsics")))]
 macro_rules! div {
     ($a:expr, $b:expr) => {
         $a / $b
     };
 }
 
-#[cfg(all(not(debug_assertions), feature = "unstable"))]
+#[cfg(all(not(debug_assertions), feature = "unstable-intrinsics"))]
 macro_rules! div {
     ($a:expr, $b:expr) => {
         unsafe { core::intrinsics::unchecked_div($a, $b) }
@@ -76,7 +76,9 @@ macro_rules! div {
 
 macro_rules! llvm_intrinsically_optimized {
     (#[cfg($($clause:tt)*)] $e:expr) => {
-        #[cfg(all(feature = "unstable", not(feature = "force-soft-floats"), $($clause)*))]
+        #[cfg(all(
+            feature = "unstable-intrinsics", not(feature = "force-soft-floats"), $($clause)*
+        ))]
         {
             if true { // thwart the dead code lint
                 $e