remove Rule 3 from `ref_pat_eat_one_layer_2024`

The debug assertion ensuring that the pattern mutability cap holds
assumes the presence of Rule 3, so it now checks for that. I
considered going back to only tracking the mutability cap when Rule 3
is present, but since the mutability cap is used in Rule 5's
implementation too, the debug assertion would still need to check
which typing rules are present.

This also 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.

7 files changed, 22 insertions, 34 deletions

diff --git a/compiler/rustc_hir_typeck/src/pat.rs b/compiler/rustc_hir_typeck/src/pat.rs
index cbd1db2ca25..cf9921312d5 100644
--- a/compiler/rustc_hir_typeck/src/pat.rs
+++ b/compiler/rustc_hir_typeck/src/pat.rs
@@ -243,8 +243,7 @@ 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?
@@ -425,7 +424,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         max_ref_mutbl: MutblCap,
     ) -> (Ty<'tcx>, ByRef, MutblCap) {
         #[cfg(debug_assertions)]
-        if def_br == ByRef::Yes(Mutability::Mut) && max_ref_mutbl != MutblCap::Mut {
+        if def_br == ByRef::Yes(Mutability::Mut)
+            && max_ref_mutbl != MutblCap::Mut
+            && self.downgrade_mut_inside_shared()
+        {
             span_bug!(pat.span, "Pattern mutability cap violated!");
         }
         match adjust_mode {
diff --git a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/borrowck-errors.classic.stderr b/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/borrowck-errors.classic.stderr
index c6246114075..f26151fe4f7 100644
--- a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/borrowck-errors.classic.stderr
+++ b/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`.
diff --git a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/borrowck-errors.rs b/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/borrowck-errors.rs
index a01e9ca2657..79581936418 100644
--- a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/borrowck-errors.rs
+++ b/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;
+    }
 }
diff --git a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/pattern-errors.classic.stderr b/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/pattern-errors.classic.stderr
index 2bc3ecb7636..9b8a1e0ff39 100644
--- a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/pattern-errors.classic.stderr
+++ b/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/pattern-errors.classic.stderr
@@ -47,18 +47,6 @@ 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
    |
 LL |     if let Some(&Some(&mut _)) = &mut Some(&Some(0)) {
@@ -71,18 +59,6 @@ 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
    |
 LL |     if let Some(&mut Some(x)) = &Some(Some(0)) {
@@ -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`.
diff --git a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/pattern-errors.rs b/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/pattern-errors.rs
index 3535ba9c701..f9cc8504d55 100644
--- a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/pattern-errors.rs
+++ b/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
diff --git a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/ref_pat_eat_one_layer_2021.rs b/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/ref_pat_eat_one_layer_2021.rs
index 9372049a2b2..ab3264704ac 100644
--- a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/ref_pat_eat_one_layer_2021.rs
+++ b/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;
     }
diff --git a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/well-typed-edition-2024.rs b/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/well-typed-edition-2024.rs
index 077b52d8f27..28e008a779b 100644
--- a/tests/ui/pattern/rfc-3627-match-ergonomics-2024/experimental/well-typed-edition-2024.rs
+++ b/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;
     }