1 files changed, 81 insertions, 0 deletions
diff --git a/compiler/rustc_borrowck/src/place_ext.rs b/compiler/rustc_borrowck/src/place_ext.rs
new file mode 100644
index 000000000..93d202e49
--- /dev/null
+++ b/compiler/rustc_borrowck/src/place_ext.rs
@@ -0,0 +1,81 @@
+use crate::borrow_set::LocalsStateAtExit;
+use rustc_hir as hir;
+use rustc_middle::mir::ProjectionElem;
+use rustc_middle::mir::{Body, Mutability, Place};
+use rustc_middle::ty::{self, TyCtxt};
+
+/// Extension methods for the `Place` type.
+pub(crate) trait PlaceExt<'tcx> {
+    /// Returns `true` if we can safely ignore borrows of this place.
+    /// This is true whenever there is no action that the user can do
+    /// to the place `self` that would invalidate the borrow. This is true
+    /// for borrows of raw pointer dereferents as well as shared references.
+    fn ignore_borrow(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        body: &Body<'tcx>,
+        locals_state_at_exit: &LocalsStateAtExit,
+    ) -> bool;
+}
+
+impl<'tcx> PlaceExt<'tcx> for Place<'tcx> {
+    fn ignore_borrow(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        body: &Body<'tcx>,
+        locals_state_at_exit: &LocalsStateAtExit,
+    ) -> bool {
+        // If a local variable is immutable, then we only need to track borrows to guard
+        // against two kinds of errors:
+        // * The variable being dropped while still borrowed (e.g., because the fn returns
+        //   a reference to a local variable)
+        // * The variable being moved while still borrowed
+        //
+        // In particular, the variable cannot be mutated -- the "access checks" will fail --
+        // so we don't have to worry about mutation while borrowed.
+        if let LocalsStateAtExit::SomeAreInvalidated { has_storage_dead_or_moved } =
+            locals_state_at_exit
+        {
+            let ignore = !has_storage_dead_or_moved.contains(self.local)
+                && body.local_decls[self.local].mutability == Mutability::Not;
+            debug!("ignore_borrow: local {:?} => {:?}", self.local, ignore);
+            if ignore {
+                return true;
+            }
+        }
+
+        for (i, elem) in self.projection.iter().enumerate() {
+            let proj_base = &self.projection[..i];
+
+            if elem == ProjectionElem::Deref {
+                let ty = Place::ty_from(self.local, proj_base, body, tcx).ty;
+                match ty.kind() {
+                    ty::Ref(_, _, hir::Mutability::Not) if i == 0 => {
+                        // For references to thread-local statics, we do need
+                        // to track the borrow.
+                        if body.local_decls[self.local].is_ref_to_thread_local() {
+                            continue;
+                        }
+                        return true;
+                    }
+                    ty::RawPtr(..) | ty::Ref(_, _, hir::Mutability::Not) => {
+                        // For both derefs of raw pointers and `&T`
+                        // references, the original path is `Copy` and
+                        // therefore not significant.  In particular,
+                        // there is nothing the user can do to the
+                        // original path that would invalidate the
+                        // newly created reference -- and if there
+                        // were, then the user could have copied the
+                        // original path into a new variable and
+                        // borrowed *that* one, leaving the original
+                        // path unborrowed.
+                        return true;
+                    }
+                    _ => {}
+                }
+            }
+        }
+
+        false
+    }
+}