diff --git a/crates/core_simd/src/lane_count.rs b/crates/core_simd/src/lane_count.rs
index 280b27bc9bc6..c62b9d3b7d9b 100644
--- a/crates/core_simd/src/lane_count.rs
+++ b/crates/core_simd/src/lane_count.rs
@@ -18,7 +18,11 @@ impl<const N: usize> LaneCount<N> {
 /// Only SIMD vectors with supported lane counts are constructable.
 pub trait SupportedLaneCount: Sealed {
     #[doc(hidden)]
-    type BitMask: Copy + Default + AsRef<[u8]> + AsMut<[u8]>;
+    type BitMask: Copy + AsRef<[u8]> + AsMut<[u8]>;
+    #[doc(hidden)]
+    const EMPTY_BIT_MASK: Self::BitMask;
+    #[doc(hidden)]
+    const FULL_BIT_MASK: Self::BitMask;
 }
 
 impl<const N: usize> Sealed for LaneCount<N> {}
@@ -28,6 +32,15 @@ macro_rules! supported_lane_count {
         $(
             impl SupportedLaneCount for LaneCount<$lanes> {
                 type BitMask = [u8; ($lanes + 7) / 8];
+                const EMPTY_BIT_MASK: Self::BitMask = [0; ($lanes + 7) / 8];
+                const FULL_BIT_MASK: Self::BitMask = {
+                    const LEN: usize = ($lanes + 7) / 8;
+                    let mut array = [!0u8; LEN];
+                    if $lanes % 8 > 0 {
+                        array[LEN - 1] = (!0) >> (8 - $lanes % 8);
+                    }
+                    array
+                };
             }
         )+
     };
diff --git a/crates/core_simd/src/masks.rs b/crates/core_simd/src/masks.rs
index 19d45f4d3b31..2b6be3676279 100644
--- a/crates/core_simd/src/masks.rs
+++ b/crates/core_simd/src/masks.rs
@@ -139,7 +139,8 @@ where
 {
     /// Constructs a mask by setting all elements to the given value.
     #[inline]
-    pub fn splat(value: bool) -> Self {
+    #[rustc_const_unstable(feature = "portable_simd", issue = "86656")]
+    pub const fn splat(value: bool) -> Self {
         Self(mask_impl::Mask::splat(value))
     }
 
diff --git a/crates/core_simd/src/masks/bitmask.rs b/crates/core_simd/src/masks/bitmask.rs
index 8221d8f17e90..ef03ec7f7159 100644
--- a/crates/core_simd/src/masks/bitmask.rs
+++ b/crates/core_simd/src/masks/bitmask.rs
@@ -78,17 +78,16 @@ where
 {
     #[inline]
     #[must_use = "method returns a new mask and does not mutate the original value"]
-    pub(crate) fn splat(value: bool) -> Self {
-        let mut mask = <LaneCount<N> as SupportedLaneCount>::BitMask::default();
-        if value {
-            mask.as_mut().fill(u8::MAX)
-        } else {
-            mask.as_mut().fill(u8::MIN)
-        }
-        if N % 8 > 0 {
-            *mask.as_mut().last_mut().unwrap() &= u8::MAX >> (8 - N % 8);
-        }
-        Self(mask, PhantomData)
+    #[rustc_const_unstable(feature = "portable_simd", issue = "86656")]
+    pub(crate) const fn splat(value: bool) -> Self {
+        Self(
+            if value {
+                <LaneCount<N> as SupportedLaneCount>::FULL_BIT_MASK
+            } else {
+                <LaneCount<N> as SupportedLaneCount>::EMPTY_BIT_MASK
+            },
+            PhantomData,
+        )
     }
 
     #[inline]
@@ -131,7 +130,7 @@ where
 
     #[inline]
     pub(crate) fn from_bitmask_integer(bitmask: u64) -> Self {
-        let mut bytes = <LaneCount<N> as SupportedLaneCount>::BitMask::default();
+        let mut bytes = <LaneCount<N> as SupportedLaneCount>::BitMask::EMPTY_BIT_MASK;
         let len = bytes.as_mut().len();
         bytes
             .as_mut()
diff --git a/crates/core_simd/src/masks/full_masks.rs b/crates/core_simd/src/masks/full_masks.rs
index 4e98db4070a9..ddd7fb69b189 100644
--- a/crates/core_simd/src/masks/full_masks.rs
+++ b/crates/core_simd/src/masks/full_masks.rs
@@ -102,7 +102,8 @@ where
 {
     #[inline]
     #[must_use = "method returns a new mask and does not mutate the original value"]
-    pub(crate) fn splat(value: bool) -> Self {
+    #[rustc_const_unstable(feature = "portable_simd", issue = "86656")]
+    pub(crate) const fn splat(value: bool) -> Self {
         Self(Simd::splat(if value { T::TRUE } else { T::FALSE }))
     }