Extract helper, fix comment on DerefPure

Author: compiler-errors

Diff for: compiler/rustc_hir_typeck/src/pat.rs

@@ -2020,20 +2020,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // Check if the pattern has any `ref mut` bindings, which would require
         // `DerefMut` to be emitted in MIR building instead of just `Deref`.
-        let mut needs_mut = false;
-        inner.walk(|pat| {
-            if let hir::PatKind::Binding(_, id, _, _) = pat.kind
-                && let Some(ty::BindByReference(ty::Mutability::Mut)) =
-                    self.typeck_results.borrow().pat_binding_modes().get(id)
-            {
-                needs_mut = true;
-                // No need to continue recursing
-                false
-            } else {
-                true
-            }
-        });
-        if needs_mut {
+        // We do this *after* checking the inner pattern, since we want to make
+        // sure to apply any match-ergonomics adjustments.
+        if self.typeck_results.borrow().pat_has_ref_mut_binding(inner) {
             self.register_bound(
                 expected,
                 tcx.require_lang_item(hir::LangItem::DerefMut, Some(span)),

Diff for: compiler/rustc_middle/src/ty/typeck_results.rs

@@ -430,6 +430,31 @@ impl<'tcx> TypeckResults<'tcx> {
         LocalTableInContextMut { hir_owner: self.hir_owner, data: &mut self.pat_adjustments }
     }
 
+    /// Does the pattern recursively contain a `ref mut` binding in it?
+    ///
+    /// This is used to determined whether a `deref` pattern should emit a `Deref`
+    /// or `DerefMut` call for its pattern scrutinee.
+    ///
+    /// This is computed from the typeck results since we want to make
+    /// sure to apply any match-ergonomics adjustments, which we cannot
+    /// determine from the HIR alone.
+    pub fn pat_has_ref_mut_binding(&self, pat: &'tcx hir::Pat<'tcx>) -> bool {
+        let mut has_ref_mut = false;
+        pat.walk(|pat| {
+            if let hir::PatKind::Binding(_, id, _, _) = pat.kind
+                && let Some(ty::BindByReference(ty::Mutability::Mut)) =
+                    self.pat_binding_modes().get(id)
+            {
+                has_ref_mut = true;
+                // No need to continue recursing
+                false
+            } else {
+                true
+            }
+        });
+        has_ref_mut
+    }
+
     /// For a given closure, returns the iterator of `ty::CapturedPlace`s that are captured
     /// by the closure.
     pub fn closure_min_captures_flattened(

Diff for: library/core/src/ops/deref.rs

@@ -275,7 +275,15 @@ impl<T: ?Sized> DerefMut for &mut T {
     }
 }
 
-/// UwU
+/// Perma-unstable marker trait. Indicates that the type has a well-behaved [`Deref`]
+/// (and, if applicable, [`DerefMut`]) implementation. This is relied on for soundness
+/// of deref patterns.
+///
+/// FIXME(deref_patterns): The precise semantics are undecided; the rough idea is that
+/// successive calls to `deref`/`deref_mut` without intermediate mutation should be
+/// idempotent, in the sense that they return the same value as far as pattern-matching
+/// is concerned. Calls to `deref`/`deref_mut`` must leave the pointer itself likewise
+/// unchanged.
 #[unstable(feature = "deref_pure_trait", issue = "87121")]
 #[cfg_attr(not(bootstrap), lang = "deref_pure")]
 pub unsafe trait DerefPure {}