Add regression test for need-mut diagnostic

Author: ChayimFriedman2

src/tools/rust-analyzer/crates/ide-diagnostics/src/handlers/mutability_errors.rs

@@ -1302,6 +1302,22 @@ fn main() {
 	let mut var = 1;
 	let mut func = || (var,) = (2,);
 	func();
+}
+        "#,
+        );
+    }
+
+    #[test]
+    fn regression_20662() {
+        check_diagnostics(
+            r#"
+//- minicore: index
+pub trait A: core::ops::IndexMut<usize> {
+    type T: A;
+}
+
+fn func(a: &mut impl A, b: &mut [i32]) {
+    b[0] += 1;
 }
         "#,
         );