rename and comment the test for "Rule 5"-related mutability errors

This also makes it test the "structural" ruleset, in preparation for
additional tests where the rulesets disagree.

Author: dianne

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/ref_pat_eat_one_layer_2024_ref_mut_inside_and.fixed renamed to tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/ref-mut-inside-shared-ref-pat.classic.fixed

@@ -1,7 +1,11 @@
 //@ edition: 2024
 //@ run-rustfix
+//@ revisions: classic structural
+//! Tests for `&` patterns matched against `&mut` reference types where the inner pattern attempts
+//! to bind by mutable reference.
 #![allow(incomplete_features)]
-#![feature(ref_pat_eat_one_layer_2024)]
+#![cfg_attr(classic, feature(ref_pat_eat_one_layer_2024))]
+#![cfg_attr(structural, feature(ref_pat_eat_one_layer_2024_structural))]
 
 pub fn main() {
     if let Some(&mut Some(ref mut x)) = &mut Some(Some(0)) {

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/ref_pat_eat_one_layer_2024_ref_mut_inside_and.stderr renamed to tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/ref-mut-inside-shared-ref-pat.classic.stderr

@@ -1,37 +1,37 @@
 error[E0596]: cannot borrow as mutable inside an `&` pattern
-  --> $DIR/ref_pat_eat_one_layer_2024_ref_mut_inside_and.rs:7:31
+  --> $DIR/ref-mut-inside-shared-ref-pat.rs:11:31
    |
 LL |     if let Some(&Some(ref mut x)) = &mut Some(Some(0)) {
    |                 -             ^
    |                 |
    |                 help: replace this `&` with `&mut`: `&mut`
 
 error[E0596]: cannot borrow as mutable inside an `&` pattern
-  --> $DIR/ref_pat_eat_one_layer_2024_ref_mut_inside_and.rs:12:31
+  --> $DIR/ref-mut-inside-shared-ref-pat.rs:16:31
    |
 LL |     if let &Some(Some(ref mut x)) = &mut Some(Some(0)) {
    |            -                  ^
    |            |
    |            help: replace this `&` with `&mut`: `&mut`
 
 error[E0596]: cannot borrow as mutable inside an `&` pattern
-  --> $DIR/ref_pat_eat_one_layer_2024_ref_mut_inside_and.rs:20:15
+  --> $DIR/ref-mut-inside-shared-ref-pat.rs:24:15
    |
 LL |     let &pat!(x) = &mut 0;
    |         -     ^
    |         |
    |         help: replace this `&` with `&mut`: `&mut`
 
 error[E0596]: cannot borrow as mutable inside an `&` pattern
-  --> $DIR/ref_pat_eat_one_layer_2024_ref_mut_inside_and.rs:24:19
+  --> $DIR/ref-mut-inside-shared-ref-pat.rs:28:19
    |
 LL |     let &(ref mut a, ref mut b) = &mut (true, false);
    |         -         ^
    |         |
    |         help: replace this `&` with `&mut`: `&mut`
 
 error[E0596]: cannot borrow as mutable inside an `&` pattern
-  --> $DIR/ref_pat_eat_one_layer_2024_ref_mut_inside_and.rs:24:30
+  --> $DIR/ref-mut-inside-shared-ref-pat.rs:28:30
    |
 LL |     let &(ref mut a, ref mut b) = &mut (true, false);
    |         -                    ^

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/ref_pat_eat_one_layer_2024_ref_mut_inside_and.rs renamed to tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/ref-mut-inside-shared-ref-pat.rs

@@ -1,7 +1,11 @@
 //@ edition: 2024
 //@ run-rustfix
+//@ revisions: classic structural
+//! Tests for `&` patterns matched against `&mut` reference types where the inner pattern attempts
+//! to bind by mutable reference.
 #![allow(incomplete_features)]
-#![feature(ref_pat_eat_one_layer_2024)]
+#![cfg_attr(classic, feature(ref_pat_eat_one_layer_2024))]
+#![cfg_attr(structural, feature(ref_pat_eat_one_layer_2024_structural))]
 
 pub fn main() {
     if let Some(&Some(ref mut x)) = &mut Some(Some(0)) {

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/ref-mut-inside-shared-ref-pat.structural.fixed

@@ -0,0 +1,33 @@
+//@ edition: 2024
+//@ run-rustfix
+//@ revisions: classic structural
+//! Tests for `&` patterns matched against `&mut` reference types where the inner pattern attempts
+//! to bind by mutable reference.
+#![allow(incomplete_features)]
+#![cfg_attr(classic, feature(ref_pat_eat_one_layer_2024))]
+#![cfg_attr(structural, feature(ref_pat_eat_one_layer_2024_structural))]
+
+pub fn main() {
+    if let Some(&mut Some(ref mut x)) = &mut Some(Some(0)) {
+        //~^ ERROR: cannot borrow as mutable inside an `&` pattern
+        let _: &mut u8 = x;
+    }
+
+    if let &mut Some(Some(ref mut x)) = &mut Some(Some(0)) {
+        //~^ ERROR: cannot borrow as mutable inside an `&` pattern
+        let _: &mut u8 = x;
+    }
+
+    macro_rules! pat {
+        ($var:ident) => { ref mut $var };
+    }
+    let &mut pat!(x) = &mut 0;
+    //~^ ERROR: cannot borrow as mutable inside an `&` pattern
+    let _: &mut u8 = x;
+
+    let &mut (ref mut a, ref mut b) = &mut (true, false);
+    //~^ ERROR: cannot borrow as mutable inside an `&` pattern
+    //~| ERROR: cannot borrow as mutable inside an `&` pattern
+    let _: &mut bool = a;
+    let _: &mut bool = b;
+}

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/ref-mut-inside-shared-ref-pat.structural.stderr

@@ -0,0 +1,43 @@
+error[E0596]: cannot borrow as mutable inside an `&` pattern
+  --> $DIR/ref-mut-inside-shared-ref-pat.rs:11:31
+   |
+LL |     if let Some(&Some(ref mut x)) = &mut Some(Some(0)) {
+   |                 -             ^
+   |                 |
+   |                 help: replace this `&` with `&mut`: `&mut`
+
+error[E0596]: cannot borrow as mutable inside an `&` pattern
+  --> $DIR/ref-mut-inside-shared-ref-pat.rs:16:31
+   |
+LL |     if let &Some(Some(ref mut x)) = &mut Some(Some(0)) {
+   |            -                  ^
+   |            |
+   |            help: replace this `&` with `&mut`: `&mut`
+
+error[E0596]: cannot borrow as mutable inside an `&` pattern
+  --> $DIR/ref-mut-inside-shared-ref-pat.rs:24:15
+   |
+LL |     let &pat!(x) = &mut 0;
+   |         -     ^
+   |         |
+   |         help: replace this `&` with `&mut`: `&mut`
+
+error[E0596]: cannot borrow as mutable inside an `&` pattern
+  --> $DIR/ref-mut-inside-shared-ref-pat.rs:28:19
+   |
+LL |     let &(ref mut a, ref mut b) = &mut (true, false);
+   |         -         ^
+   |         |
+   |         help: replace this `&` with `&mut`: `&mut`
+
+error[E0596]: cannot borrow as mutable inside an `&` pattern
+  --> $DIR/ref-mut-inside-shared-ref-pat.rs:28:30
+   |
+LL |     let &(ref mut a, ref mut b) = &mut (true, false);
+   |         -                    ^
+   |         |
+   |         help: replace this `&` with `&mut`: `&mut`
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0596`.