add : new UI test

tests/ui/borrowck/borrowck-for-loop-deref-pattern-assignment.stderr

@@ -8,8 +8,9 @@ LL |         num *= 2;
    |
 help: consider making this binding mutable
    |
-LL |     for &(mut num) num in nums {
-   |          +++++++++
+LL -     for &num in nums {
+LL +     for &(mut num) in nums {
+   |
 
 error: aborting due to 1 previous error
 

tests/ui/borrowck/borrowck_for_loop_pattern_assignment.rs

@@ -0,0 +1,9 @@
+//! regression test for <https://github.com/rust-lang/rust/issues/155030>
+
+fn main() {
+    let nums: [u32; 3] = [1, 2, 3];
+    for num in nums {
+        num *= 2; //~ ERROR cannot assign twice to immutable variable `num`
+        println!("{num}");
+    }
+}

tests/ui/borrowck/borrowck_for_loop_pattern_assignment.stderr

@@ -0,0 +1,16 @@
+error[E0384]: cannot assign twice to immutable variable `num`
+  --> $DIR/borrowck_for_loop_pattern_assignment.rs:6:9
+   |
+LL |     for num in nums {
+   |         --- first assignment to `num`
+LL |         num *= 2;
+   |         ^^^^^^^^ cannot assign twice to immutable variable
+   |
+help: consider making this binding mutable
+   |
+LL |     for mut num in nums {
+   |         +++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0384`.