increase mask robustness, add tests

Author: robamu

bitbybit/src/bitfield/mod.rs

@@ -452,6 +452,18 @@ pub fn bitfield(args: TokenStream, input: TokenStream) -> TokenStream {
     TokenStream::from(expanded)
 }
 
+const fn mask_for_width_and_offset(width: usize, offset: usize) -> u128 {
+    if width == 128 {
+        return u128::MAX;
+    }
+    if width + offset > 128 {
+        panic!("bitfield!: Internal error: width + offset > 128");
+    }
+    // The wrapping subtraction is important for the case where width is 128.
+    // A shift with 128 would yield a value of 0, and a regular subtraction of 1 could panic.
+    ((1_u128 << width) - 1) << offset
+}
+
 fn check_for_overlaps(
     field_definitions: &[FieldDefinition],
     input: &DeriveInput,
@@ -461,7 +473,7 @@ fn check_for_overlaps(
     for field in field_definitions {
         let mut check_and_update_bitmask = |offset: usize, width: usize| {
             // Create an all-ones bitmask for the given range, e.g. 0b1110 for range (1..=3).
-            let mask = ((1_u128 << width) - 1) << offset;
+            let mask = mask_for_width_and_offset(width, offset);
             if (current_bitmask & mask) != 0 {
                 return Err(syn::Error::new_spanned(
                     input,
@@ -556,3 +568,41 @@ fn const_name(field_name: &Ident, suffix: &str) -> Ident {
     syn::parse_str::<Ident>(&name)
         .unwrap_or_else(|_| panic!("bitfield!: Error creating {name} name"))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_mask_underflow() {
+        assert_eq!(mask_for_width_and_offset(128, 0), u128::MAX);
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_mask_error() {
+        assert_eq!(mask_for_width_and_offset(127, 1), u128::MAX - 1);
+        // This panics.
+        assert_eq!(mask_for_width_and_offset(127, 2), u128::MAX);
+    }
+
+    #[test]
+    fn test_mask_0() {
+        assert_eq!(mask_for_width_and_offset(3, 1), 0b1110);
+    }
+
+    #[test]
+    fn test_mask_1() {
+        assert_eq!(mask_for_width_and_offset(0, 0), 0);
+    }
+
+    #[test]
+    fn test_mask_2() {
+        assert_eq!(mask_for_width_and_offset(1, 0), 1);
+    }
+
+    #[test]
+    fn test_mask_3() {
+        assert_eq!(mask_for_width_and_offset(2, 0), 0b11);
+    }
+}