register DerefMut bounds for implicit mutable derefs

Author: dianne

compiler/rustc_hir_typeck/src/pat.rs

@@ -407,6 +407,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         self.write_ty(pat.hir_id, ty);
 
+        // If we implicitly inserted overloaded dereferences before matching, check the pattern to
+        // see if the dereferenced types need `DerefMut` bounds.
+        if let Some(derefed_tys) = self.typeck_results.borrow().pat_adjustments().get(pat.hir_id)
+            && derefed_tys.iter().any(|ty| !ty.is_ref())
+        {
+            self.register_deref_mut_bounds_if_needed(
+                pat.span,
+                pat,
+                derefed_tys.iter().filter(|ty| !ty.is_ref()).copied(),
+            );
+        }
+
         // (note_1): In most of the cases where (note_1) is referenced
         // (literals and constants being the exception), we relate types
         // using strict equality, even though subtyping would be sufficient.
@@ -604,7 +616,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     // requirement that `expected: DerefPure`.
                     self.deref_pat_target(pat.span, expected)
                     // Once we've checked `pat`, we'll add a `DerefMut` bound if it contains any
-                    // `ref mut` bindings. TODO: implement that, then reference here.
+                    // `ref mut` bindings. See `Self::register_deref_mut_bounds_if_needed`.
                 } else {
                     // Bail, so we get a normal type error.
                     break;
@@ -2343,20 +2355,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ) -> Ty<'tcx> {
         let target_ty = self.deref_pat_target(span, expected);
         self.check_pat(inner, target_ty, pat_info);
-
-        // Check if the pattern has any `ref mut` bindings, which would require
-        // `DerefMut` to be emitted in MIR building instead of just `Deref`.
-        // We do this *after* checking the inner pattern, since we want to make
-        // sure to apply any match-ergonomics adjustments.
-        // TODO: move this to a separate definition to share it with implicit deref pats
-        if self.typeck_results.borrow().pat_has_ref_mut_binding(inner) {
-            self.register_bound(
-                expected,
-                self.tcx.require_lang_item(hir::LangItem::DerefMut, Some(span)),
-                self.misc(span),
-            );
-        }
-
+        self.register_deref_mut_bounds_if_needed(span, inner, [expected]);
         expected
     }
 
@@ -2378,6 +2377,27 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self.try_structurally_resolve_type(span, target_ty)
     }
 
+    /// Check if the interior of a deref pattern (either explicit or implicit) has any `ref mut`
+    /// bindings, which would require `DerefMut` to be emitted in MIR building instead of just
+    /// `Deref`. We do this *after* checking the inner pattern, since we want to make sure to
+    /// account for `ref mut` binding modes inherited from implicitly dereferencing `&mut` refs.
+    fn register_deref_mut_bounds_if_needed(
+        &self,
+        span: Span,
+        inner: &'tcx Pat<'tcx>,
+        derefed_tys: impl IntoIterator<Item = Ty<'tcx>>,
+    ) {
+        if self.typeck_results.borrow().pat_has_ref_mut_binding(inner) {
+            for mutably_derefed_ty in derefed_tys {
+                self.register_bound(
+                    mutably_derefed_ty,
+                    self.tcx.require_lang_item(hir::LangItem::DerefMut, Some(span)),
+                    self.misc(span),
+                );
+            }
+        }
+    }
+
     // Precondition: Pat is Ref(inner)
     fn check_pat_ref(
         &self,

tests/ui/pattern/deref-patterns/bindings.rs

@@ -51,6 +51,7 @@ fn nested_vec(vecvec: Vec<Vec<u32>>) -> u32 {
     }
 }
 
+#[cfg(explicit)]
 fn ref_mut(val: u32) -> u32 {
     let mut b = Box::new(0u32);
     match &mut b {
@@ -64,6 +65,21 @@ fn ref_mut(val: u32) -> u32 {
     *x
 }
 
+#[cfg(implicit)]
+fn ref_mut(val: u32) -> u32 {
+    let mut b = Box::new((0u32,));
+    match &mut b {
+        (_x,) if false => unreachable!(),
+        (x,) => {
+            *x = val;
+        }
+        _ => unreachable!(),
+    }
+    let (x,) = &b else { unreachable!() };
+    *x
+}
+
+#[cfg(explicit)]
 #[rustfmt::skip]
 fn or_and_guard(tuple: (u32, u32)) -> u32 {
     let mut sum = 0;
@@ -75,6 +91,18 @@ fn or_and_guard(tuple: (u32, u32)) -> u32 {
     sum
 }
 
+#[cfg(implicit)]
+#[rustfmt::skip]
+fn or_and_guard(tuple: (u32, u32)) -> u32 {
+    let mut sum = 0;
+    let b = Box::new(tuple);
+    match b {
+        (x, _) | (_, x) if { sum += x; false } => {},
+        _ => {},
+    }
+    sum
+}
+
 fn main() {
     assert_eq!(simple_vec(vec![1]), 1);
     assert_eq!(simple_vec(vec![1, 2]), 202);

tests/ui/pattern/deref-patterns/fake_borrows.rs

@@ -11,4 +11,11 @@ fn main() {
         deref!(false) => {}
         _ => {},
     }
+    match b {
+        true => {}
+        _ if { *b = true; false } => {}
+        //~^ ERROR cannot assign `*b` in match guard
+        false => {}
+        _ => {},
+    }
 }

tests/ui/pattern/deref-patterns/fake_borrows.stderr

@@ -7,6 +7,15 @@ LL |         deref!(true) => {}
 LL |         _ if { *b = true; false } => {}
    |                ^^^^^^^^^ cannot assign
 
-error: aborting due to 1 previous error
+error[E0510]: cannot assign `*b` in match guard
+  --> $DIR/fake_borrows.rs:16:16
+   |
+LL |     match b {
+   |           - value is immutable in match guard
+LL |         true => {}
+LL |         _ if { *b = true; false } => {}
+   |                ^^^^^^^^^ cannot assign
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0510`.

tests/ui/pattern/deref-patterns/ref-mut.rs

@@ -8,10 +8,19 @@ fn main() {
         deref!(x) => {}
         _ => {}
     }
+    match &mut vec![1] {
+        [x] => {}
+        _ => {}
+    }
 
     match &mut Rc::new(1) {
         deref!(x) => {}
         //~^ ERROR the trait bound `Rc<{integer}>: DerefMut` is not satisfied
         _ => {}
     }
+    match &mut Rc::new((1,)) {
+        (x,) => {}
+        //~^ ERROR the trait bound `Rc<({integer},)>: DerefMut` is not satisfied
+        _ => {}
+    }
 }

tests/ui/pattern/deref-patterns/ref-mut.stderr

@@ -8,13 +8,19 @@ LL | #![feature(deref_patterns)]
    = note: `#[warn(incomplete_features)]` on by default
 
 error[E0277]: the trait bound `Rc<{integer}>: DerefMut` is not satisfied
-  --> $DIR/ref-mut.rs:13:9
+  --> $DIR/ref-mut.rs:17:9
    |
 LL |         deref!(x) => {}
    |         ^^^^^^^^^ the trait `DerefMut` is not implemented for `Rc<{integer}>`
    |
    = note: this error originates in the macro `deref` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 1 previous error; 1 warning emitted
+error[E0277]: the trait bound `Rc<({integer},)>: DerefMut` is not satisfied
+  --> $DIR/ref-mut.rs:22:9
+   |
+LL |         (x,) => {}
+   |         ^^^^ the trait `DerefMut` is not implemented for `Rc<({integer},)>`
+
+error: aborting due to 2 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0277`.