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