1 use rustc::mir::visit::{PlaceContext, Visitor};
2 use rustc::mir::{Local, Location, Place, Statement, StatementKind, TerminatorKind};
4 use rustc_data_structures::fx::FxHashSet;
6 use crate::borrow_check::MirBorrowckCtxt;
8 impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
9 /// Walks the MIR adding to the set of `used_mut` locals that will be ignored for the purposes
10 /// of the `unused_mut` lint.
12 /// `temporary_used_locals` should contain locals that were found to be temporary, mutable and
13 /// used from borrow checking. This function looks for assignments into these locals from
14 /// user-declared locals and adds those user-defined locals to the `used_mut` set. This can
15 /// occur due to a rare case involving upvars in closures.
17 /// `never_initialized_mut_locals` should contain the set of user-declared mutable locals
18 /// (not arguments) that have not already been marked as being used.
19 /// This function then looks for assignments from statements or the terminator into the locals
20 /// from this set and removes them from the set. This leaves only those locals that have not
21 /// been assigned to - this set is used as a proxy for locals that were not initialized due to
22 /// unreachable code. These locals are then considered "used" to silence the lint for them.
23 /// See #55344 for context.
24 crate fn gather_used_muts(
26 temporary_used_locals: FxHashSet<Local>,
27 mut never_initialized_mut_locals: FxHashSet<Local>,
30 let mut visitor = GatherUsedMutsVisitor {
31 temporary_used_locals,
32 never_initialized_mut_locals: &mut never_initialized_mut_locals,
35 visitor.visit_body(visitor.mbcx.body);
38 // Take the union of the existed `used_mut` set with those variables we've found were
40 debug!("gather_used_muts: never_initialized_mut_locals={:?}", never_initialized_mut_locals);
41 self.used_mut = self.used_mut.union(&never_initialized_mut_locals).cloned().collect();
45 /// MIR visitor for collecting used mutable variables.
46 /// The 'visit lifetime represents the duration of the MIR walk.
47 struct GatherUsedMutsVisitor<'visit, 'cx, 'tcx> {
48 temporary_used_locals: FxHashSet<Local>,
49 never_initialized_mut_locals: &'visit mut FxHashSet<Local>,
50 mbcx: &'visit mut MirBorrowckCtxt<'cx, 'tcx>,
53 impl GatherUsedMutsVisitor<'_, '_, '_> {
54 fn remove_never_initialized_mut_locals(&mut self, into: &Place<'_>) {
55 // Remove any locals that we found were initialized from the
56 // `never_initialized_mut_locals` set. At the end, the only remaining locals will
57 // be those that were never initialized - we will consider those as being used as
58 // they will either have been removed by unreachable code optimizations; or linted
59 // as unused variables.
60 self.never_initialized_mut_locals.remove(&into.local);
64 impl<'visit, 'cx, 'tcx> Visitor<'tcx> for GatherUsedMutsVisitor<'visit, 'cx, 'tcx> {
65 fn visit_terminator_kind(&mut self, kind: &TerminatorKind<'tcx>, _location: Location) {
66 debug!("visit_terminator_kind: kind={:?}", kind);
68 TerminatorKind::Call { destination: Some((into, _)), .. } => {
69 self.remove_never_initialized_mut_locals(&into);
71 TerminatorKind::DropAndReplace { location, .. } => {
72 self.remove_never_initialized_mut_locals(&location);
78 fn visit_statement(&mut self, statement: &Statement<'tcx>, _location: Location) {
79 match &statement.kind {
80 StatementKind::Assign(box (into, _)) => {
82 "visit_statement: statement={:?} local={:?} \
83 never_initialized_mut_locals={:?}",
84 statement, into.local, self.never_initialized_mut_locals
86 self.remove_never_initialized_mut_locals(into);
92 fn visit_local(&mut self, local: &Local, place_context: PlaceContext, location: Location) {
93 if place_context.is_place_assignment() && self.temporary_used_locals.contains(local) {
94 // Propagate the Local assigned at this Location as a used mutable local variable
95 for moi in &self.mbcx.move_data.loc_map[location] {
96 let mpi = &self.mbcx.move_data.moves[*moi].path;
97 let path = &self.mbcx.move_data.move_paths[*mpi];
99 "assignment of {:?} to {:?}, adding {:?} to used mutable set",
100 path.place, local, path.place
102 if let Some(user_local) = path.place.as_local() {
103 self.mbcx.used_mut.insert(user_local);