Compute mutability of closure captures

When `capture_disjoint_fields` is not enabled, checking if the root variable binding is mutable would suffice. However with the feature enabled, the captured place might be mutable because it dereferences a mutable reference. This PR computes the mutability of each capture after capture analysis in rustc_typeck. We store this in `ty::CapturedPlace` and then use `ty::CapturedPlace::mutability` in mir_build and borrow_check.
rust-lang · jonas-schievink · Jan 21, 2021 · Jan 24, 2021 · Dec 27, 2020 · Dec 28, 2020
commit 34880825828260fad0a74621e4a13fe7da9a4a9d
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
@@ -661,11 +661,17 @@ pub type RootVariableMinCaptureList<'tcx> = FxIndexMap<hir::HirId, MinCaptureLis
 /// Part of `MinCaptureInformationMap`; List of `CapturePlace`s.
 pub type MinCaptureList<'tcx> = Vec<CapturedPlace<'tcx>>;
 
-/// A `Place` and the corresponding `CaptureInfo`.
+/// A composite describing a `Place` that is captured by a closure.
 #[derive(PartialEq, Clone, Debug, TyEncodable, TyDecodable, TypeFoldable, HashStable)]
 pub struct CapturedPlace<'tcx> {
+    /// The `Place` that is captured.
     pub place: HirPlace<'tcx>,
+
+    /// `CaptureKind` and expression(s) that resulted in such capture of `place`.
     pub info: CaptureInfo<'tcx>,
+
+    /// Represents if `place` can be mutated or not.
+    pub mutability: hir::Mutability,
 }
 
 pub fn place_to_string_for_capture(tcx: TyCtxt<'tcx>, place: &HirPlace<'tcx>) -> String {

diff --git a/compiler/rustc_mir/src/borrow_check/mod.rs b/compiler/rustc_mir/src/borrow_check/mod.rs
@@ -170,17 +170,12 @@ fn do_mir_borrowck<'a, 'tcx>(
                 ty::UpvarCapture::ByValue(_) => false,
                 ty::UpvarCapture::ByRef(..) => true,
             };
-            let mut upvar = Upvar {
+            Upvar {
                 name: tcx.hir().name(var_hir_id),
                 var_hir_id,
                 by_ref,
-                mutability: Mutability::Not,
-            };
-            let bm = *tables.pat_binding_modes().get(var_hir_id).expect("missing binding mode");
-            if bm == ty::BindByValue(hir::Mutability::Mut) {
-                upvar.mutability = Mutability::Mut;
+                mutability: captured_place.mutability,
             }
-            upvar
         })
         .collect();
 

diff --git a/compiler/rustc_mir_build/src/build/mod.rs b/compiler/rustc_mir_build/src/build/mod.rs
@@ -851,22 +851,13 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         _ => bug!("Expected an upvar")
                     };
 
-                    let mut mutability = Mutability::Not;
+                    let mutability = captured_place.mutability;
 
                     // FIXME(project-rfc-2229#8): Store more precise information
                     let mut name = kw::Empty;
                     if let Some(Node::Binding(pat)) = tcx_hir.find(var_id) {
                         if let hir::PatKind::Binding(_, _, ident, _) = pat.kind {
                             name = ident.name;
-                            match hir_typeck_results
-                                .extract_binding_mode(tcx.sess, pat.hir_id, pat.span)
-                            {
-                                Some(ty::BindByValue(hir::Mutability::Mut)) => {
-                                    mutability = Mutability::Mut;
-                                }
-                                Some(_) => mutability = Mutability::Not,
-                                _ => {}
-                            }
                         }
                     }
 

diff --git a/compiler/rustc_typeck/src/check/upvar.rs b/compiler/rustc_typeck/src/check/upvar.rs
@@ -252,8 +252,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 let capture = captured_place.info.capture_kind;
 
                 debug!(
-                    "place={:?} upvar_ty={:?} capture={:?}",
-                    captured_place.place, upvar_ty, capture
+                    "final_upvar_tys: place={:?} upvar_ty={:?} capture={:?}, mutability={:?}",
+                    captured_place.place, upvar_ty, capture, captured_place.mutability,
                 );
 
                 match capture {
@@ -423,7 +423,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
             let min_cap_list = match root_var_min_capture_list.get_mut(&var_hir_id) {
                 None => {
-                    let min_cap_list = vec![ty::CapturedPlace { place, info: capture_info }];
+                    let mutability = self.determine_capture_mutability(&place);
+                    let min_cap_list =
+                        vec![ty::CapturedPlace { place, info: capture_info, mutability }];
                     root_var_min_capture_list.insert(var_hir_id, min_cap_list);
                     continue;
                 }
@@ -486,8 +488,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
             // Only need to insert when we don't have an ancestor in the existing min capture list
             if !ancestor_found {
+                let mutability = self.determine_capture_mutability(&place);
                 let captured_place =
-                    ty::CapturedPlace { place: place.clone(), info: updated_capture_info };
+                    ty::CapturedPlace { place, info: updated_capture_info, mutability };
                 min_cap_list.push(captured_place);
             }
         }
@@ -607,6 +610,49 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         }
     }
+
+    /// A captured place is mutable if
+    /// 1. Projections don't include a Deref of an immut-borrow, **and**
+    /// 2. PlaceBase is mut or projections include a Deref of a mut-borrow.
+    fn determine_capture_mutability(&self, place: &Place<'tcx>) -> hir::Mutability {
+        let var_hir_id = match place.base {
+            PlaceBase::Upvar(upvar_id) => upvar_id.var_path.hir_id,
+            _ => unreachable!(),
+        };
+
+        let bm = *self
+            .typeck_results
+            .borrow()
+            .pat_binding_modes()
+            .get(var_hir_id)
+            .expect("missing binding mode");
+
+        let mut is_mutbl = match bm {
+            ty::BindByValue(mutability) => mutability,
+            ty::BindByReference(_) => hir::Mutability::Not,
+        };
+
+        for pointer_ty in place.deref_tys() {
+            match pointer_ty.kind() {
+                // We don't capture derefs of raw ptrs
+                ty::RawPtr(_) => unreachable!(),
+
+                // Derefencing a mut-ref allows us to mut the Place if we don't deref
+                // an immut-ref after on top of this.
+                ty::Ref(.., hir::Mutability::Mut) => is_mutbl = hir::Mutability::Mut,
+
+                // The place isn't mutable once we dereference a immutable reference.
+                ty::Ref(.., hir::Mutability::Not) => return hir::Mutability::Not,
+
+                // Dereferencing a box doesn't change mutability
+                ty::Adt(def, ..) if def.is_box() => {}
+
+                unexpected_ty => bug!("deref of unexpected pointer type {:?}", unexpected_ty),
+            }
+        }
+
+        is_mutbl
+    }
 }
 
 struct InferBorrowKind<'a, 'tcx> {