From 8121727283a945eec1ee342bfca227b17de0eb41 Mon Sep 17 00:00:00 2001
From: Chayim Refael Friedman <chayimfr@gmail.com>
Date: Sun, 5 Oct 2025 21:56:08 +0300
Subject: [PATCH] Add regression test for need-mut diagnostic

---
 .../src/handlers/mutability_errors.rs            | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/crates/ide-diagnostics/src/handlers/mutability_errors.rs b/crates/ide-diagnostics/src/handlers/mutability_errors.rs
index 6e30bf92dbaa..eefa1ac24add 100644
--- a/crates/ide-diagnostics/src/handlers/mutability_errors.rs
+++ b/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;
 }
         "#,
         );