Properly downgrade inherited mutability

Author: Jules-Bertholet

compiler/rustc_hir_typeck/src/pat.rs

@@ -299,7 +299,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     if mutbls_match {
                         (expected, INITIAL_BM, true)
                     } else {
-                        (expected, def_bm, false)
+                        let mut new_bm = def_bm;
+                        if new_bm.0 == ByRef::Yes(Mutability::Mut) && mutbl == Mutability::Not {
+                            new_bm.0 = ByRef::Yes(Mutability::Not);
+                        }
+                        (expected, new_bm, false)
                     }
                 } else {
                     (expected, INITIAL_BM, mutbls_match)

tests/ui/match/ref_pat_eat_one_layer_2024/ref_pat_eat_one_layer_2024.rs

@@ -26,7 +26,13 @@ pub fn main() {
     if let Some(&mut Some(&x)) = &Some(&mut Some(0)) {
         let _: u32 = x;
     }
-    if let Some(&Some(&mut x)) = &mut Some(& Some(0)) {
+    if let Some(&Some(&x)) = &mut Some(&Some(0)) {
         let _: u32 = x;
     }
+    if let Some(&Some(x)) = &mut Some(&Some(0)) {
+        let _: &u32 = x;
+    }
+    if let Some(&Some(&mut ref x)) = Some(&Some(&mut 0)) {
+        let _: &u32 = x;
+    }
 }

tests/ui/match/ref_pat_eat_one_layer_2024/ref_pat_eat_one_layer_2024_fail.rs

@@ -10,4 +10,11 @@ pub fn main() {
     if let Some(&Some(&_)) = &Some(&mut Some(0)) {
         //~^ ERROR: mismatched types
     }
+    if let Some(&Some(x)) = &mut Some(&Some(0)) {
+        let _: &mut u32 = x;
+        //~^ ERROR: mismatched types
+    }
+    if let Some(&Some(&x)) = Some(&Some(&mut 0)) {
+        //~^ ERROR: mismatched types
+    }
 }

tests/ui/match/ref_pat_eat_one_layer_2024/ref_pat_eat_one_layer_2024_fail.stderr

@@ -20,6 +20,32 @@ LL |     if let Some(&Some(&_)) = &Some(&mut Some(0)) {
    = note:   expected type `{integer}`
            found reference `&_`
 
-error: aborting due to 2 previous errors
+error[E0308]: mismatched types
+  --> $DIR/ref_pat_eat_one_layer_2024_fail.rs:14:27
+   |
+LL |         let _: &mut u32 = x;
+   |                --------   ^ types differ in mutability
+   |                |
+   |                expected due to this
+   |
+   = note: expected mutable reference `&mut u32`
+                      found reference `&{integer}`
+
+error[E0308]: mismatched types
+  --> $DIR/ref_pat_eat_one_layer_2024_fail.rs:17:23
+   |
+LL |     if let Some(&Some(&x)) = Some(&Some(&mut 0)) {
+   |                       ^^     ------------------- this expression has type `Option<&Option<&mut {integer}>>`
+   |                       |
+   |                       types differ in mutability
+   |
+   = note: expected mutable reference `&mut {integer}`
+                      found reference `&_`
+help: consider removing `&` from the pattern
+   |
+LL |     if let Some(&Some(x)) = Some(&Some(&mut 0)) {
+   |                       ~
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0308`.

tests/ui/match/ref_pat_eat_one_layer_2024/ref_pat_eat_one_layer_2024_fail2.rs

@@ -0,0 +1,11 @@
+//@ edition: 2024
+//@ compile-flags: -Zunstable-options
+#![allow(incomplete_features)]
+#![feature(ref_pat_eat_one_layer_2024)]
+
+pub fn main() {
+    if let Some(&Some(x)) = Some(&Some(&mut 0)) {
+        //~^ ERROR: cannot move out of a shared reference [E0507]
+        let _: &u32 = x;
+    }
+}

tests/ui/match/ref_pat_eat_one_layer_2024/ref_pat_eat_one_layer_2024_fail2.stderr

@@ -0,0 +1,17 @@
+error[E0507]: cannot move out of a shared reference
+  --> $DIR/ref_pat_eat_one_layer_2024_fail2.rs:7:29
+   |
+LL |     if let Some(&Some(x)) = Some(&Some(&mut 0)) {
+   |                       -     ^^^^^^^^^^^^^^^^^^^
+   |                       |
+   |                       data moved here
+   |                       move occurs because `x` has type `&mut u32`, which does not implement the `Copy` trait
+   |
+help: consider borrowing the pattern binding
+   |
+LL |     if let Some(&Some(ref x)) = Some(&Some(&mut 0)) {
+   |                       +++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0507`.