lower deref patterns on boxes using built-in derefs

This allows deref patterns to move out of boxes.

Implementation-wise, I've opted to put the information of whether a
deref pattern uses a built-in deref or a method call in the THIR. It'd
be a bit less code to check `.is_box()` everywhere, but I think this way
feels more robust (and we don't have a `mutability` field in the THIR
that we ignore when the smart pointer's a box). I'm not sure about the
naming (or using `ByRef`), though.

2 files changed, 21 insertions, 1 deletions

diff --git a/compiler/rustc_middle/src/thir.rs b/compiler/rustc_middle/src/thir.rs
index c168142fb1e..086ec529f33 100644
--- a/compiler/rustc_middle/src/thir.rs
+++ b/compiler/rustc_middle/src/thir.rs
@@ -799,7 +799,12 @@ pub enum PatKind<'tcx> {
     /// Deref pattern, written `box P` for now.
     DerefPattern {
         subpattern: Box<Pat<'tcx>>,
-        mutability: hir::Mutability,
+        /// Whether the pattern scrutinee needs to be borrowed in order to call `Deref::deref` or
+        /// `DerefMut::deref_mut`, and if so, which. This is `ByRef::No` for deref patterns on
+        /// boxes; they are lowered using a built-in deref rather than a method call, thus they
+        /// don't borrow the scrutinee.
+        #[type_visitable(ignore)]
+        borrow: ByRef,
     },
 
     /// One of the following:
diff --git a/compiler/rustc_middle/src/ty/typeck_results.rs b/compiler/rustc_middle/src/ty/typeck_results.rs
index 4c5c669771f..8c5827d36df 100644
--- a/compiler/rustc_middle/src/ty/typeck_results.rs
+++ b/compiler/rustc_middle/src/ty/typeck_results.rs
@@ -475,6 +475,21 @@ impl<'tcx> TypeckResults<'tcx> {
         has_ref_mut
     }
 
+    /// How should a deref pattern find the place for its inner pattern to match on?
+    ///
+    /// In most cases, if the pattern recursively contains a `ref mut` binding, we find the inner
+    /// pattern's scrutinee by calling `DerefMut::deref_mut`, and otherwise we call `Deref::deref`.
+    /// However, for boxes we can use a built-in deref instead, which doesn't borrow the scrutinee;
+    /// in this case, we return `ByRef::No`.
+    pub fn deref_pat_borrow_mode(&self, pointer_ty: Ty<'_>, inner: &hir::Pat<'_>) -> ByRef {
+        if pointer_ty.is_box() {
+            ByRef::No
+        } else {
+            let mutable = self.pat_has_ref_mut_binding(inner);
+            ByRef::Yes(if mutable { Mutability::Mut } else { Mutability::Not })
+        }
+    }
+
     /// For a given closure, returns the iterator of `ty::CapturedPlace`s that are captured
     /// by the closure.
     pub fn closure_min_captures_flattened(