Empty Unstructured::choose param should return Err, not panic.

CHANGELOG.md

@@ -37,6 +37,7 @@ Released YYYY-MM-DD.
 ### Changed
 
 * Rename `Unstructured#get_bytes` to `Unstructured#bytes`. [#70](https://github.com/rust-fuzz/arbitrary/pull/70)
+* Passing an empty slice of choices to `Unstructured#choose` returns an error. Previously it would panic. [71](https://github.com/rust-fuzz/arbitrary/pull/71)
 
 ### Removed
 

src/error.rs

@@ -4,6 +4,8 @@ use std::{error, fmt};
 #[derive(Debug, Clone, Copy)]
 #[non_exhaustive]
 pub enum Error {
+    /// No choices were provided to the Unstructured::choose call
+    EmptyChoose,
     /// There was not enough underlying data to fulfill some request for raw
     /// bytes.
     NotEnoughData,
@@ -14,6 +16,10 @@ pub enum Error {
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
+            Error::EmptyChoose => write!(
+                f,
+                "`arbitrary::Unstructured::choose` must be given a non-empty set of choices"
+            ),
             Error::NotEnoughData => write!(
                 f,
                 "There is not enough underlying raw data to construct an `Arbitrary` instance"

src/unstructured.rs

@@ -345,29 +345,39 @@ impl<'a> Unstructured<'a> {
     /// This should only be used inside of `Arbitrary` implementations.
     ///
     /// Returns an error if there is not enough underlying data to make a
-    /// choice.
+    /// choice or if no choices are provided.
     ///
-    /// # Panics
+    /// # Examples
     ///
-    /// Panics if `choices` is empty.
+    /// Selecting from an array of choices:
     ///
-    /// # Example
+    /// ```
+    /// use arbitrary::Unstructured;
+    ///
+    /// let mut u = Unstructured::new(&[1, 2, 3, 4, 5, 6, 7, 8, 9, 0]);
+    /// let choices = ['a', 'b', 'c', 'd', 'e', 'f', 'g'];
+    ///
+    /// let choice = u.choose(&choices).unwrap();
+    ///
+    /// println!("chose {}", choice);
+    /// ```
+    ///
+    /// An error is returned if no choices are provided:
     ///
     /// ```
     /// use arbitrary::Unstructured;
     ///
     /// let mut u = Unstructured::new(&[1, 2, 3, 4, 5, 6, 7, 8, 9, 0]);
+    /// let choices: [char; 0] = [];
     ///
-    /// let choices = ['a', 'b', 'c', 'd', 'e', 'f', 'g'];
-    /// if let Ok(ch) = u.choose(&choices) {
-    ///     println!("chose {}", ch);
-    /// }
+    /// let result = u.choose(&choices);
+    ///
+    /// assert!(result.is_err());
     /// ```
     pub fn choose<'b, T>(&mut self, choices: &'b [T]) -> Result<&'b T> {
-        assert!(
-            !choices.is_empty(),
-            "`arbitrary::Unstructured::choose` must be given a non-empty set of choices"
-        );
+        if choices.is_empty() {
+            return Err(Error::EmptyChoose);
+        }
         let idx = self.int_in_range(0..=choices.len() - 1)?;
         Ok(&choices[idx])
     }