Ignore unused variables when compiler-builtins is set

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

@@ -11,18 +11,18 @@ test = false
 bench = false
 
 [features]
-default = ["arch", "unstable-float"]
+default = ["arch", "compiler-builtins", "unstable-float"]
 
 # Copied from `libm`'s root `Cargo.toml`'
-unstable-float = []
 arch = []
+compiler-builtins = []
+unstable-float = []
 
 [lints.rust]
 unexpected_cfgs = { level = "warn", check-cfg = [
   "cfg(arch_enabled)",
   "cfg(assert_no_panic)",
   "cfg(intrinsics_enabled)",
-  'cfg(feature, values("compiler-builtins"))',
   'cfg(feature, values("force-soft-floats"))',
   'cfg(feature, values("unstable"))',
   'cfg(feature, values("unstable-intrinsics"))',

src/math/support/hex_float.rs

@@ -248,7 +248,8 @@ impl<F: Float> fmt::LowerHex for Hexf<F> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         cfg_if! {
             if #[cfg(feature = "compiler-builtins")] {
-                unreachable!()
+                let _ = f;
+                unimplemented!()
             } else {
                 fmt_any_hex(&self.0, f)
             }
@@ -272,7 +273,8 @@ impl fmt::LowerHex for Hexf<i32> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         cfg_if! {
             if #[cfg(feature = "compiler-builtins")] {
-                unreachable!()
+                let _ = f;
+                unimplemented!()
             } else {
                 fmt::LowerHex::fmt(&self.0, f)
             }