Avoid panic from deserialized Uniform<char> where range == 0

CHANGELOG.md

@@ -13,9 +13,11 @@ You may also find the [Upgrade Guide](https://rust-random.github.io/book/update.
 ### Fixes
 - Fix `WeightedIndex` panic when the sum of float weights is infinite; return `Error::Overflow` instead ([#1808])
 - Fix spurious `Error::NonFinite` from `Uniform::new_inclusive` on large finite float ranges such as `0.0..=f64::MAX` ([#1821])
+- Fix possible panic due to sampling a deserialized `Uniform<char>` ([#1831])
 
 [#1808]: https://github.com/rust-random/rand/pull/1808
 [#1821]: https://github.com/rust-random/rand/pull/1821
+[#1831]: https://github.com/rust-random/rand/pull/1831
 
 ## [0.10.2] — 2026-07-02
 

src/distr/uniform_int.rs

@@ -79,7 +79,15 @@ macro_rules! uniform_int_impl {
             #[allow(unused)]
             #[inline]
             pub(crate) fn max(&self) -> $ty {
-                self.range.wrapping_sub(1).wrapping_add(self.low)
+                if self.range == 0 {
+                    return <$ty>::MAX;
+                } else {
+                    // Wrapping through <$ty>::MIN is possible with a valid
+                    // sampler over signed types. Wrapping through <$ty>::MAX is
+                    // possible with a bad sampler (constructible using serde).
+                    let max = self.low.wrapping_add(self.range.wrapping_sub(1));
+                    if max < self.low { <$ty>::MAX } else { max }
+                }
             }
         }
 

src/distr/uniform_other.rs

@@ -316,19 +316,26 @@ mod tests {
     #[test]
     #[cfg(feature = "serde")]
     fn test_char_bad_deser() {
-        let json = r#"{"sampler":{"low":4294967200,"range":0,"thresh":0}}"#;
-        let result = serde_json::from_str::<Uniform<char>>(json);
-        assert!(result.is_err());
-        let err = result.unwrap_err();
-        assert_eq!(err.classify(), serde_json::error::Category::Data);
-
-        #[cfg(feature = "alloc")]
-        {
-            assert_eq!(
-                alloc::string::ToString::to_string(&err),
-                "bad sampler range for UniformChar at line 1 column 51"
-            );
+        fn do_test(json: &str, col: usize) {
+            let result = serde_json::from_str::<Uniform<char>>(json);
+            assert!(result.is_err());
+            let err = result.unwrap_err();
+            assert_eq!(err.classify(), serde_json::error::Category::Data);
+
+            #[cfg(feature = "alloc")]
+            {
+                let msg =
+                    alloc::format!("bad sampler range for UniformChar at line 1 column {col}");
+                assert_eq!(alloc::string::ToString::to_string(&err), msg);
+            }
         }
+
+        do_test(r#"{"sampler":{"low":5,"range":0,"thresh":0}}"#, 42);
+        do_test(r#"{"sampler":{"low":4294967200,"range":0,"thresh":0}}"#, 51);
+        do_test(
+            r#"{"sampler":{"low":4294967280,"range":32,"thresh":0}}"#,
+            52,
+        );
     }
 
     #[test]