Add comment for regression rust-lang#136223 on borrowck-errors.rs

Signed-off-by: Shunpoco <[email protected]>

Author: Shunpoco

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/borrowck-errors.classic2024.stderr

@@ -48,19 +48,19 @@ LL |     let &ref mut x = &0;
    |          ^^^^^^^^^ cannot borrow as mutable
 
 error[E0596]: cannot borrow data in a `&` reference as mutable
-  --> $DIR/borrowck-errors.rs:35:23
+  --> $DIR/borrowck-errors.rs:37:23
    |
 LL |     if let &Some(Some(x)) = &Some(&mut Some(0)) {
    |                       ^ cannot borrow as mutable
 
 error[E0596]: cannot borrow data in a `&` reference as mutable
-  --> $DIR/borrowck-errors.rs:40:11
+  --> $DIR/borrowck-errors.rs:42:11
    |
 LL |     let &[x] = &&mut [0];
    |           ^ cannot borrow as mutable
 
 error[E0508]: cannot move out of type `[&mut i32; 1]`, a non-copy array
-  --> $DIR/borrowck-errors.rs:44:20
+  --> $DIR/borrowck-errors.rs:46:20
    |
 LL |     let [&mut x] = &mut [&mut 0];
    |               -    ^^^^^^^^^^^^^ cannot move out of here

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/borrowck-errors.rs

@@ -32,6 +32,8 @@ pub fn main() {
     let &ref mut x = &0;
     //~^ cannot borrow data in a `&` reference as mutable [E0596]
 
+    // For 2021 edition, this is also a regression test for #136223
+    // since the maximum mutability is downgraded during the pattern check process.
     if let &Some(Some(x)) = &Some(&mut Some(0)) {
         //[stable2021,classic2024]~^ ERROR: cannot borrow data in a `&` reference as mutable
         let _: &u32 = x;

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/borrowck-errors.stable2021.stderr

@@ -52,13 +52,13 @@ LL |     let &ref mut x = &0;
    |          ^^^^^^^^^ cannot borrow as mutable
 
 error[E0596]: cannot borrow data in a `&` reference as mutable
-  --> $DIR/borrowck-errors.rs:35:23
+  --> $DIR/borrowck-errors.rs:37:23
    |
 LL |     if let &Some(Some(x)) = &Some(&mut Some(0)) {
    |                       ^ cannot borrow as mutable
 
 error[E0596]: cannot borrow data in a `&` reference as mutable
-  --> $DIR/borrowck-errors.rs:40:11
+  --> $DIR/borrowck-errors.rs:42:11
    |
 LL |     let &[x] = &&mut [0];
    |           ^ cannot borrow as mutable