compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs

   1 use super::*;
   2
   3 use crate::{AnalysisDomain, CallReturnPlaces, GenKill, GenKillAnalysis};
   4 use rustc_middle::mir::visit::Visitor;
   5 use rustc_middle::mir::*;
   6
   7 /// A dataflow analysis that tracks whether a pointer or reference could possibly exist that points
   8 /// to a given local.
   9 ///
  10 /// At present, this is used as a very limited form of alias analysis. For example,
  11 /// `MaybeBorrowedLocals` is used to compute which locals are live during a yield expression for
  12 /// immovable generators.
  13 pub struct MaybeBorrowedLocals;
  14
  15 impl MaybeBorrowedLocals {
  16     fn transfer_function<'a, T>(&'a self, trans: &'a mut T) -> TransferFunction<'a, T> {
  17         TransferFunction { trans }
  18     }
  19 }
  20
  21 impl<'tcx> AnalysisDomain<'tcx> for MaybeBorrowedLocals {
  22     type Domain = BitSet<Local>;
  23     const NAME: &'static str = "maybe_borrowed_locals";
  24
  25     fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
  26         // bottom = unborrowed
  27         BitSet::new_empty(body.local_decls().len())
  28     }
  29
  30     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {
  31         // No locals are aliased on function entry
  32     }
  33 }
  34
  35 impl<'tcx> GenKillAnalysis<'tcx> for MaybeBorrowedLocals {
  36     type Idx = Local;
  37
  38     fn statement_effect(
  39         &self,
  40         trans: &mut impl GenKill<Self::Idx>,
  41         statement: &mir::Statement<'tcx>,
  42         location: Location,
  43     ) {
  44         self.transfer_function(trans).visit_statement(statement, location);
  45     }
  46
  47     fn terminator_effect(
  48         &self,
  49         trans: &mut impl GenKill<Self::Idx>,
  50         terminator: &mir::Terminator<'tcx>,
  51         location: Location,
  52     ) {
  53         self.transfer_function(trans).visit_terminator(terminator, location);
  54     }
  55
  56     fn call_return_effect(
  57         &self,
  58         _trans: &mut impl GenKill<Self::Idx>,
  59         _block: mir::BasicBlock,
  60         _return_places: CallReturnPlaces<'_, 'tcx>,
  61     ) {
  62     }
  63 }
  64
  65 /// A `Visitor` that defines the transfer function for `MaybeBorrowedLocals`.
  66 struct TransferFunction<'a, T> {
  67     trans: &'a mut T,
  68 }
  69
  70 impl<'tcx, T> Visitor<'tcx> for TransferFunction<'_, T>
  71 where
  72     T: GenKill<Local>,
  73 {
  74     fn visit_statement(&mut self, stmt: &Statement<'tcx>, location: Location) {
  75         self.super_statement(stmt, location);
  76
  77         // When we reach a `StorageDead` statement, we can assume that any pointers to this memory
  78         // are now invalid.
  79         if let StatementKind::StorageDead(local) = stmt.kind {
  80             self.trans.kill(local);
  81         }
  82     }
  83
  84     fn visit_rvalue(&mut self, rvalue: &mir::Rvalue<'tcx>, location: Location) {
  85         self.super_rvalue(rvalue, location);
  86
  87         match rvalue {
  88             mir::Rvalue::AddressOf(_, borrowed_place) | mir::Rvalue::Ref(_, _, borrowed_place) => {
  89                 if !borrowed_place.is_indirect() {
  90                     self.trans.gen(borrowed_place.local);
  91                 }
  92             }
  93
  94             mir::Rvalue::Cast(..)
  95             | mir::Rvalue::ShallowInitBox(..)
  96             | mir::Rvalue::Use(..)
  97             | mir::Rvalue::ThreadLocalRef(..)
  98             | mir::Rvalue::Repeat(..)
  99             | mir::Rvalue::Len(..)
 100             | mir::Rvalue::BinaryOp(..)
 101             | mir::Rvalue::CheckedBinaryOp(..)
 102             | mir::Rvalue::NullaryOp(..)
 103             | mir::Rvalue::UnaryOp(..)
 104             | mir::Rvalue::Discriminant(..)
 105             | mir::Rvalue::Aggregate(..)
 106             | mir::Rvalue::CopyForDeref(..) => {}
 107         }
 108     }
 109
 110     fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, location: Location) {
 111         self.super_terminator(terminator, location);
 112
 113         match terminator.kind {
 114             mir::TerminatorKind::Drop { place: dropped_place, .. }
 115             | mir::TerminatorKind::DropAndReplace { place: dropped_place, .. } => {
 116                 // Drop terminators may call custom drop glue (`Drop::drop`), which takes `&mut
 117                 // self` as a parameter. In the general case, a drop impl could launder that
 118                 // reference into the surrounding environment through a raw pointer, thus creating
 119                 // a valid `*mut` pointing to the dropped local. We are not yet willing to declare
 120                 // this particular case UB, so we must treat all dropped locals as mutably borrowed
 121                 // for now. See discussion on [#61069].
 122                 //
 123                 // [#61069]: https://github.com/rust-lang/rust/pull/61069
 124                 self.trans.gen(dropped_place.local);
 125             }
 126
 127             TerminatorKind::Abort
 128             | TerminatorKind::Assert { .. }
 129             | TerminatorKind::Call { .. }
 130             | TerminatorKind::FalseEdge { .. }
 131             | TerminatorKind::FalseUnwind { .. }
 132             | TerminatorKind::GeneratorDrop
 133             | TerminatorKind::Goto { .. }
 134             | TerminatorKind::InlineAsm { .. }
 135             | TerminatorKind::Resume
 136             | TerminatorKind::Return
 137             | TerminatorKind::SwitchInt { .. }
 138             | TerminatorKind::Unreachable
 139             | TerminatorKind::Yield { .. } => {}
 140         }
 141     }
 142 }
 143
 144 /// The set of locals that are borrowed at some point in the MIR body.
 145 pub fn borrowed_locals(body: &Body<'_>) -> BitSet<Local> {
 146     struct Borrowed(BitSet<Local>);
 147
 148     impl GenKill<Local> for Borrowed {
 149         #[inline]
 150         fn gen(&mut self, elem: Local) {
 151             self.0.gen(elem)
 152         }
 153         #[inline]
 154         fn kill(&mut self, _: Local) {
 155             // Ignore borrow invalidation.
 156         }
 157     }
 158
 159     let mut borrowed = Borrowed(BitSet::new_empty(body.local_decls.len()));
 160     TransferFunction { trans: &mut borrowed }.visit_body(body);
 161     borrowed.0
 162 }