Use saturating arithmetics for adjust_max_width

src/config/config_type.rs

@@ -435,22 +435,22 @@ macro_rules! create_config {
             /// arms within a declarative macro.
             #[allow(unreachable_pub)]
             pub fn reduce_max_width(&mut self, delta: usize) {
-                self.adjust_max_width(-(delta as isize));
+                self.adjust_max_width(-(delta.min(isize::MAX as usize) as isize));
             }
 
             /// Increases the maximum width of the configuration.
             ///
             /// This method is intended to be used when creating a forked
             /// configuration for a particular formatting context in which the
-            /// total available width needs to be reduced. This increases the
+            /// total available width needs to be increased. This increases the
             /// size of max_width, and all other properties affected by small
             /// heuristics, without treating those adjustments as overrides.
             ///
             /// As an example use case, this method is used when formatting
             /// arms within a declarative macro.
             #[allow(unreachable_pub)]
             pub fn increase_max_width(&mut self, delta: usize) {
-                self.adjust_max_width(delta as isize);
+                self.adjust_max_width(delta.min(isize::MAX as usize) as isize);
             }
 
             /// Adjusts the maximum width of the configuration.
@@ -464,7 +464,9 @@ macro_rules! create_config {
             /// As an example use case, this method is used when formatting
             /// arms within a declarative macro.
             fn adjust_max_width(&mut self, delta: isize) {
-                let adjust = |value: usize| (value as isize + delta) as usize;
+                // NB: we need to either saturate or report an error (or some kind of testable
+                // warning). I picked to saturate here.
+                let adjust = |value: usize| value.saturating_add_signed(delta);
 
                 self.array_width.2 = adjust(self.array_width.2);
                 self.attr_fn_like_width.2 = adjust(self.attr_fn_like_width.2);

src/config/mod.rs

@@ -698,6 +698,23 @@ mod test {
         assert_eq!(config.verbose(), Verbosity::Normal);
     }
 
+    #[test]
+    fn test_adjust_max_width() {
+        // Regression tests for #7147, where we didn't handle overflows / integral cast outside of
+        // range properly.
+
+        // Reduce with huge amount saturates to zero instead of wraparound.
+        let mut config = Config::default();
+        config.reduce_max_width(usize::MAX);
+        assert_eq!(config.max_width(), 0);
+
+        // Increase with huge amount saturates instead of wraparound.
+        let mut config = Config::default();
+        config.set().max_width(usize::MAX - 1);
+        config.increase_max_width(2);
+        assert_eq!(config.max_width(), usize::MAX);
+    }
+
     #[test]
     fn test_config_used_to_toml() {
         let config = Config::default();