Use choices struct field at creation time instead of using unsafe at …

…call time

Signed-off-by: Justus Fluegel <justusfluegel@gmail.com>

src/distributions/slice.rs

@@ -69,26 +69,28 @@ use alloc::string::String;
 pub struct Slice<'a, T> {
     slice: &'a [T],
     range: Uniform<usize>,
+    choices: NonZeroUsize,
 }
 
 impl<'a, T> Slice<'a, T> {
     /// Create a new `Slice` instance which samples uniformly from the slice.
     /// Returns `Err` if the slice is empty.
     pub fn new(slice: &'a [T]) -> Result<Self, EmptySlice> {
-        match slice.len() {
-            0 => Err(EmptySlice),
-            len => Ok(Self {
-                slice,
-                range: Uniform::new(0, len).unwrap(),
-            }),
-        }
+        let len = match NonZeroUsize::new(slice.len()) {
+            None => return Err(EmptySlice),
+            Some(len) => len,
+        };
+
+        Ok(Self {
+            slice,
+            range: Uniform::new(0, len.get()).unwrap(),
+            choices: len,
+        })
     }
 
     /// Returns the count of choices in this distribution
     pub fn num_choices(&self) -> NonZeroUsize {
-        // Safety: at construction time, it was ensured that the slice was
-        // non-empty, as such the length can never be 0.
-        unsafe { NonZeroUsize::new_unchecked(self.slice.len()) }
+        self.choices
     }
 }