rustc_borrowck/
used_muts.rs

1use rustc_data_structures::fx::FxIndexSet;
2use rustc_middle::mir::visit::{PlaceContext, Visitor};
3use rustc_middle::mir::{
4    Local, Location, Place, Statement, StatementKind, Terminator, TerminatorKind,
5};
6use tracing::debug;
7
8use crate::MirBorrowckCtxt;
9
10impl<'tcx> MirBorrowckCtxt<'_, '_, 'tcx> {
11    /// Walks the MIR adding to the set of `used_mut` locals that will be ignored for the purposes
12    /// of the `unused_mut` lint.
13    ///
14    /// `temporary_used_locals` should contain locals that were found to be temporary, mutable and
15    ///  used from borrow checking. This function looks for assignments into these locals from
16    ///  user-declared locals and adds those user-defined locals to the `used_mut` set. This can
17    ///  occur due to a rare case involving upvars in closures.
18    ///
19    /// `never_initialized_mut_locals` should contain the set of user-declared mutable locals
20    ///  (not arguments) that have not already been marked as being used.
21    ///  This function then looks for assignments from statements or the terminator into the locals
22    ///  from this set and removes them from the set. This leaves only those locals that have not
23    ///  been assigned to - this set is used as a proxy for locals that were not initialized due to
24    ///  unreachable code. These locals are then considered "used" to silence the lint for them.
25    ///  See #55344 for context.
26    pub(crate) fn gather_used_muts(
27        &mut self,
28        temporary_used_locals: FxIndexSet<Local>,
29        mut never_initialized_mut_locals: FxIndexSet<Local>,
30    ) {
31        {
32            let mut visitor = GatherUsedMutsVisitor {
33                temporary_used_locals,
34                never_initialized_mut_locals: &mut never_initialized_mut_locals,
35                mbcx: self,
36            };
37            visitor.visit_body(visitor.mbcx.body);
38        }
39
40        // Take the union of the existed `used_mut` set with those variables we've found were
41        // never initialized.
42        debug!("gather_used_muts: never_initialized_mut_locals={:?}", never_initialized_mut_locals);
43        self.used_mut = self.used_mut.union(&never_initialized_mut_locals).cloned().collect();
44    }
45}
46
47/// MIR visitor for collecting used mutable variables.
48/// The 'visit lifetime represents the duration of the MIR walk.
49struct GatherUsedMutsVisitor<'a, 'b, 'infcx, 'tcx> {
50    temporary_used_locals: FxIndexSet<Local>,
51    never_initialized_mut_locals: &'a mut FxIndexSet<Local>,
52    mbcx: &'a mut MirBorrowckCtxt<'b, 'infcx, 'tcx>,
53}
54
55impl GatherUsedMutsVisitor<'_, '_, '_, '_> {
56    fn remove_never_initialized_mut_locals(&mut self, into: Place<'_>) {
57        // Remove any locals that we found were initialized from the
58        // `never_initialized_mut_locals` set. At the end, the only remaining locals will
59        // be those that were never initialized - we will consider those as being used as
60        // they will either have been removed by unreachable code optimizations; or linted
61        // as unused variables.
62        // FIXME(#120456) - is `swap_remove` correct?
63        self.never_initialized_mut_locals.swap_remove(&into.local);
64    }
65}
66
67impl<'tcx> Visitor<'tcx> for GatherUsedMutsVisitor<'_, '_, '_, '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);
73            }
74            _ => {}
75        }
76
77        self.super_terminator(terminator, location);
78    }
79
80    fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
81        if let StatementKind::Assign(box (into, _)) = &statement.kind {
82            debug!(
83                "visit_statement: statement={:?} local={:?} \
84                    never_initialized_mut_locals={:?}",
85                statement, into.local, self.never_initialized_mut_locals
86            );
87            self.remove_never_initialized_mut_locals(*into);
88        }
89
90        self.super_statement(statement, location);
91    }
92
93    fn visit_local(&mut self, local: Local, place_context: PlaceContext, location: Location) {
94        if place_context.is_place_assignment() && self.temporary_used_locals.contains(&local) {
95            // Propagate the Local assigned at this Location as a used mutable local variable
96            for moi in &self.mbcx.move_data.loc_map[location] {
97                let mpi = &self.mbcx.move_data.moves[*moi].path;
98                let path = &self.mbcx.move_data.move_paths[*mpi];
99                debug!(
100                    "assignment of {:?} to {:?}, adding {:?} to used mutable set",
101                    path.place, local, path.place
102                );
103                if let Some(user_local) = path.place.as_local() {
104                    self.mbcx.used_mut.insert(user_local);
105                }
106            }
107        }
108    }
109}
rustc_borrowck/used_muts.rs

rustc_borrowck/
used_muts.rs
