compiler/rustc_mir/src/borrow_check/used_muts.rs

   1 use rustc_middle::mir::visit::{PlaceContext, Visitor};
   2 use rustc_middle::mir::{
   3     Local, Location, Place, Statement, StatementKind, Terminator, TerminatorKind,
   4 };
   5
   6 use rustc_data_structures::fx::FxHashSet;
   7
   8 use crate::borrow_check::MirBorrowckCtxt;
   9
  10 impl<'cx, 'tcx> MirBorrowckCtxt<'cx, '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     crate fn gather_used_muts(
  27         &mut self,
  28         temporary_used_locals: FxHashSet<Local>,
  29         mut never_initialized_mut_locals: FxHashSet<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.
  49 struct GatherUsedMutsVisitor<'visit, 'cx, 'tcx> {
  50     temporary_used_locals: FxHashSet<Local>,
  51     never_initialized_mut_locals: &'visit mut FxHashSet<Local>,
  52     mbcx: &'visit mut MirBorrowckCtxt<'cx, 'tcx>,
  53 }
  54
  55 impl 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         self.never_initialized_mut_locals.remove(&into.local);
  63     }
  64 }
  65
  66 impl<'visit, 'cx, 'tcx> Visitor<'tcx> for GatherUsedMutsVisitor<'visit, 'cx, 'tcx> {
  67     fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
  68         debug!("visit_terminator: terminator={:?}", terminator);
  69         match &terminator.kind {
  70             TerminatorKind::Call { destination: Some((into, _)), .. } => {
  71                 self.remove_never_initialized_mut_locals(*into);
  72             }
  73             TerminatorKind::DropAndReplace { place, .. } => {
  74                 self.remove_never_initialized_mut_locals(*place);
  75             }
  76             _ => {}
  77         }
  78
  79         self.super_terminator(terminator, location);
  80     }
  81
  82     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
  83         if let StatementKind::Assign(box (into, _)) = &statement.kind {
  84             debug!(
  85                 "visit_statement: statement={:?} local={:?} \
  86                     never_initialized_mut_locals={:?}",
  87                 statement, into.local, self.never_initialized_mut_locals
  88             );
  89             self.remove_never_initialized_mut_locals(*into);
  90         }
  91
  92         self.super_statement(statement, location);
  93     }
  94
  95     fn visit_local(&mut self, local: &Local, place_context: PlaceContext, location: Location) {
  96         if place_context.is_place_assignment() && self.temporary_used_locals.contains(local) {
  97             // Propagate the Local assigned at this Location as a used mutable local variable
  98             for moi in &self.mbcx.move_data.loc_map[location] {
  99                 let mpi = &self.mbcx.move_data.moves[*moi].path;
 100                 let path = &self.mbcx.move_data.move_paths[*mpi];
 101                 debug!(
 102                     "assignment of {:?} to {:?}, adding {:?} to used mutable set",
 103                     path.place, local, path.place
 104                 );
 105                 if let Some(user_local) = path.place.as_local() {
 106                     self.mbcx.used_mut.insert(user_local);
 107                 }
 108             }
 109         }
 110     }
 111 }