remove Rule 3 from ref_pat_eat_one_layer_2024

This required some changes to tests:
- `ref_pat_eat_one_layer_2021.rs` had a test for Rule 3; I'll be
handling tests for earlier editions in a later commit, so as a stopgap
I've #[cfg]ed it out.
- One test case had to be moved from `well-typed-edition-2024.rs` to
`borrowck-errors.rs` in order to get borrowck to run on it and emit an
error.

Author: dianne

compiler/rustc_hir_typeck/src/pat.rs

@@ -237,8 +237,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     fn downgrade_mut_inside_shared(&self) -> bool {
         // NB: RFC 3627 proposes stabilizing Rule 3 in all editions. If we adopt the same behavior
         // across all editions, this may be removed.
-        self.tcx.features().ref_pat_eat_one_layer_2024()
-            || self.tcx.features().ref_pat_eat_one_layer_2024_structural()
+        self.tcx.features().ref_pat_eat_one_layer_2024_structural()
     }
 
     /// Experimental pattern feature: when do reference patterns match against inherited references?

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

@@ -19,7 +19,13 @@ error[E0596]: cannot borrow data in a `&` reference as mutable
 LL |     let &ref mut x = &0;
    |          ^^^^^^^^^ cannot borrow as mutable
 
-error: aborting due to 2 previous errors
+error[E0596]: cannot borrow data in a `&` reference as mutable
+  --> $DIR/borrowck-errors.rs:17:23
+   |
+LL |     if let &Some(Some(x)) = &Some(&mut Some(0)) {
+   |                       ^ cannot borrow as mutable
+
+error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0507, E0596.
 For more information about an error, try `rustc --explain E0507`.

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

@@ -13,4 +13,9 @@ pub fn main() {
 
     let &ref mut x = &0;
     //~^ cannot borrow data in a `&` reference as mutable [E0596]
+
+    if let &Some(Some(x)) = &Some(&mut Some(0)) {
+        //[classic]~^ ERROR: cannot borrow data in a `&` reference as mutable
+        let _: &u32 = x;
+    }
 }

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

@@ -46,18 +46,6 @@ help: replace this `&mut` pattern with `&`
 LL |     if let Some(&Some(&_)) = &Some(&Some(0)) {
    |                 ~
 
-error[E0308]: mismatched types
-  --> $DIR/pattern-errors.rs:31:23
-   |
-LL |     if let Some(&Some(&mut _)) = &Some(&mut Some(0)) {
-   |                       ^^^^^
-   |
-   = note: cannot match inherited `&` with `&mut` pattern
-help: replace this `&mut` pattern with `&`
-   |
-LL |     if let Some(&Some(&_)) = &Some(&mut Some(0)) {
-   |                       ~
-
 error[E0308]: mismatched types
   --> $DIR/pattern-errors.rs:34:23
    |
@@ -70,18 +58,6 @@ help: replace this `&mut` pattern with `&`
 LL |     if let Some(&Some(&_)) = &mut Some(&Some(0)) {
    |                       ~
 
-error[E0308]: mismatched types
-  --> $DIR/pattern-errors.rs:37:29
-   |
-LL |     if let Some(&Some(Some((&mut _)))) = &Some(Some(&mut Some(0))) {
-   |                             ^^^^^
-   |
-   = note: cannot match inherited `&` with `&mut` pattern
-help: replace this `&mut` pattern with `&`
-   |
-LL |     if let Some(&Some(Some((&_)))) = &Some(Some(&mut Some(0))) {
-   |                             ~
-
 error[E0308]: mismatched types
   --> $DIR/pattern-errors.rs:40:17
    |
@@ -106,6 +82,6 @@ help: replace this `&mut` pattern with `&`
 LL |     if let Some(&Some(x)) = &Some(Some(0)) {
    |                 ~
 
-error: aborting due to 9 previous errors
+error: aborting due to 7 previous errors
 
 For more information about this error, try `rustc --explain E0308`.

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

@@ -29,13 +29,13 @@ pub fn main() {
         //~^ ERROR: mismatched types
     }
     if let Some(&Some(&mut _)) = &Some(&mut Some(0)) {
-        //~^ ERROR: mismatched types
+        //[structural]~^ ERROR: mismatched types
     }
     if let Some(&Some(&mut _)) = &mut Some(&Some(0)) {
         //~^ ERROR: mismatched types
     }
     if let Some(&Some(Some((&mut _)))) = &Some(Some(&mut Some(0))) {
-        //~^ ERROR: mismatched types
+        //[structural]~^ ERROR: mismatched types
     }
     if let Some(&mut Some(x)) = &Some(Some(0)) {
         //~^ ERROR: mismatched types

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/ref_pat_eat_one_layer_2021.rs

@@ -6,9 +6,11 @@
 #![cfg_attr(structural, feature(ref_pat_eat_one_layer_2024_structural))]
 
 pub fn main() {
+    #[cfg(structural)]
     if let &Some(Some(x)) = &Some(&mut Some(0)) {
         let _: &u32 = x;
     }
+
     if let Some(&x) = Some(&mut 0) {
         let _: u32 = x;
     }

tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/well-typed-edition-2024.rs

@@ -35,9 +35,6 @@ pub fn main() {
     if let Some(&Some(&mut ref x)) = Some(&Some(&mut 0)) {
         let _: &u32 = x;
     }
-    if let &Some(Some(x)) = &Some(&mut Some(0)) {
-        let _: &u32 = x;
-    }
     if let Some(&Some(&x)) = &Some(&mut Some(0)) {
         let _: u32 = x;
     }