1.41 compatible

src/lib.rs

@@ -1021,6 +1021,14 @@ where
 {
 }
 
+/// Generate bit mask with `len` 1 bits. (Rust 1.41.0 compatible)
+const fn ones(len: usize) -> usize {
+    let shr = usize::BITS - len as u32;
+    let first_shr = shr / 2;
+    // right shift in two passes to avoid overflow
+    0usize.wrapping_sub(1) >> first_shr >> (shr - first_shr)
+}
+
 // === pack ===
 
 pub(crate) trait Pack<C: cfg::Config>: Sized {
@@ -1048,14 +1056,8 @@ pub(crate) trait Pack<C: cfg::Config>: Sized {
     /// left by `Self::SHIFT` bits to calculate this type's `MASK`.
     ///
     /// This is computed automatically based on `Self::LEN`.
-    const BITS: usize = {
-        if Self::LEN == 0 {
-            0
-        } else {
-            let shift = 1 << (Self::LEN - 1);
-            shift | (shift - 1)
-        }
-    };
+    const BITS: usize = ones(Self::LEN);
+
     /// The number of bits to shift a number to pack it into a usize with other
     /// values.
     ///