1 files changed, 110 insertions, 0 deletions
diff --git a/compiler/rustc_borrowck/src/used_muts.rs b/compiler/rustc_borrowck/src/used_muts.rs
new file mode 100644
index 000000000..8833753b1
--- /dev/null
+++ b/compiler/rustc_borrowck/src/used_muts.rs
@@ -0,0 +1,110 @@
+use rustc_data_structures::fx::FxHashSet;
+use rustc_middle::mir::visit::{PlaceContext, Visitor};
+use rustc_middle::mir::{
+    Local, Location, Place, Statement, StatementKind, Terminator, TerminatorKind,
+};
+
+use crate::MirBorrowckCtxt;
+
+impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
+    /// Walks the MIR adding to the set of `used_mut` locals that will be ignored for the purposes
+    /// of the `unused_mut` lint.
+    ///
+    /// `temporary_used_locals` should contain locals that were found to be temporary, mutable and
+    ///  used from borrow checking. This function looks for assignments into these locals from
+    ///  user-declared locals and adds those user-defined locals to the `used_mut` set. This can
+    ///  occur due to a rare case involving upvars in closures.
+    ///
+    /// `never_initialized_mut_locals` should contain the set of user-declared mutable locals
+    ///  (not arguments) that have not already been marked as being used.
+    ///  This function then looks for assignments from statements or the terminator into the locals
+    ///  from this set and removes them from the set. This leaves only those locals that have not
+    ///  been assigned to - this set is used as a proxy for locals that were not initialized due to
+    ///  unreachable code. These locals are then considered "used" to silence the lint for them.
+    ///  See #55344 for context.
+    pub(crate) fn gather_used_muts(
+        &mut self,
+        temporary_used_locals: FxHashSet<Local>,
+        mut never_initialized_mut_locals: FxHashSet<Local>,
+    ) {
+        {
+            let mut visitor = GatherUsedMutsVisitor {
+                temporary_used_locals,
+                never_initialized_mut_locals: &mut never_initialized_mut_locals,
+                mbcx: self,
+            };
+            visitor.visit_body(&visitor.mbcx.body);
+        }
+
+        // Take the union of the existed `used_mut` set with those variables we've found were
+        // never initialized.
+        debug!("gather_used_muts: never_initialized_mut_locals={:?}", never_initialized_mut_locals);
+        self.used_mut = self.used_mut.union(&never_initialized_mut_locals).cloned().collect();
+    }
+}
+
+/// MIR visitor for collecting used mutable variables.
+/// The 'visit lifetime represents the duration of the MIR walk.
+struct GatherUsedMutsVisitor<'visit, 'cx, 'tcx> {
+    temporary_used_locals: FxHashSet<Local>,
+    never_initialized_mut_locals: &'visit mut FxHashSet<Local>,
+    mbcx: &'visit mut MirBorrowckCtxt<'cx, 'tcx>,
+}
+
+impl GatherUsedMutsVisitor<'_, '_, '_> {
+    fn remove_never_initialized_mut_locals(&mut self, into: Place<'_>) {
+        // Remove any locals that we found were initialized from the
+        // `never_initialized_mut_locals` set. At the end, the only remaining locals will
+        // be those that were never initialized - we will consider those as being used as
+        // they will either have been removed by unreachable code optimizations; or linted
+        // as unused variables.
+        self.never_initialized_mut_locals.remove(&into.local);
+    }
+}
+
+impl<'visit, 'cx, 'tcx> Visitor<'tcx> for GatherUsedMutsVisitor<'visit, 'cx, 'tcx> {
+    fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
+        debug!("visit_terminator: terminator={:?}", terminator);
+        match &terminator.kind {
+            TerminatorKind::Call { destination, .. } => {
+                self.remove_never_initialized_mut_locals(*destination);
+            }
+            TerminatorKind::DropAndReplace { place, .. } => {
+                self.remove_never_initialized_mut_locals(*place);
+            }
+            _ => {}
+        }
+
+        self.super_terminator(terminator, location);
+    }
+
+    fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
+        if let StatementKind::Assign(box (into, _)) = &statement.kind {
+            debug!(
+                "visit_statement: statement={:?} local={:?} \
+                    never_initialized_mut_locals={:?}",
+                statement, into.local, self.never_initialized_mut_locals
+            );
+            self.remove_never_initialized_mut_locals(*into);
+        }
+
+        self.super_statement(statement, location);
+    }
+
+    fn visit_local(&mut self, local: Local, place_context: PlaceContext, location: Location) {
+        if place_context.is_place_assignment() && self.temporary_used_locals.contains(&local) {
+            // Propagate the Local assigned at this Location as a used mutable local variable
+            for moi in &self.mbcx.move_data.loc_map[location] {
+                let mpi = &self.mbcx.move_data.moves[*moi].path;
+                let path = &self.mbcx.move_data.move_paths[*mpi];
+                debug!(
+                    "assignment of {:?} to {:?}, adding {:?} to used mutable set",
+                    path.place, local, path.place
+                );
+                if let Some(user_local) = path.place.as_local() {
+                    self.mbcx.used_mut.insert(user_local);
+                }
+            }
+        }
+    }
+}