Do the calloc optimization for Option<bool>

Inspired by <https://old.reddit.com/r/rust/comments/xtiqj8/why_is_this_functional_version_faster_than_my_for/iqqy37b/>.

library/alloc/src/vec/is_zero.rs

@@ -160,3 +160,25 @@ unsafe impl<T: IsZero> IsZero for Saturating<T> {
         self.0.is_zero()
     }
 }
+
+macro_rules! impl_for_optional_bool {
+    ($($t:ty,)+) => {$(
+        unsafe impl IsZero for $t {
+            #[inline]
+            fn is_zero(&self) -> bool {
+                // SAFETY: This is *not* a stable layout guarantee, but
+                // inside `core` we're allowed to rely on the current rustc
+                // behaviour that options of bools will be one byte with
+                // no padding, so long as they're nested less than 254 deep.
+                let raw: u8 = unsafe { core::mem::transmute(*self) };
+                raw == 0
+            }
+        }
+    )+};
+}
+impl_for_optional_bool! {
+    Option<bool>,
+    Option<Option<bool>>,
+    Option<Option<Option<bool>>>,
+    // Could go further, but not worth the metadata overhead
+}

src/test/codegen/vec-calloc.rs

@@ -1,4 +1,4 @@
-// compile-flags: -O
+// compile-flags: -O -Z merge-functions=disabled
 // only-x86_64
 // ignore-debug
 // min-llvm-version: 15.0
@@ -144,6 +144,23 @@ pub fn vec_non_zero_tuple(n: usize) -> Vec<(i16, u8, char)> {
     vec![(0, 0, 'A'); n]
 }
 
+// CHECK-LABEL: @vec_option_bool
+#[no_mangle]
+pub fn vec_option_bool(n: usize) -> Vec<Option<bool>> {
+    // CHECK-NOT: call {{.*}}alloc::vec::from_elem
+    // CHECK-NOT: call {{.*}}reserve
+    // CHECK-NOT: call {{.*}}__rust_alloc(
+
+    // CHECK: call {{.*}}__rust_alloc_zeroed(
+
+    // CHECK-NOT: call {{.*}}alloc::vec::from_elem
+    // CHECK-NOT: call {{.*}}reserve
+    // CHECK-NOT: call {{.*}}__rust_alloc(
+
+    // CHECK: ret void
+    vec![Some(false); n]
+}
+
 // Ensure that __rust_alloc_zeroed gets the right attributes for LLVM to optimize it away.
 // CHECK: declare noalias ptr @__rust_alloc_zeroed(i64, i64 allocalign) unnamed_addr [[RUST_ALLOC_ZEROED_ATTRS:#[0-9]+]]