compiler/rustc_mir/src/transform/check_consts/post_drop_elaboration.rs

   1 use rustc_middle::mir::visit::Visitor;
   2 use rustc_middle::mir::{self, BasicBlock, Location};
   3 use rustc_middle::ty::TyCtxt;
   4 use rustc_span::Span;
   5
   6 use super::ops::{self, NonConstOp};
   7 use super::qualifs::{NeedsDrop, Qualif};
   8 use super::validation::Qualifs;
   9 use super::ConstCx;
  10
  11 /// Returns `true` if we should use the more precise live drop checker that runs after drop
  12 /// elaboration.
  13 pub fn checking_enabled(ccx: &ConstCx<'_, '_>) -> bool {
  14     // Const-stable functions must always use the stable live drop checker.
  15     if ccx.is_const_stable_const_fn() {
  16         return false;
  17     }
  18
  19     ccx.tcx.features().const_precise_live_drops
  20 }
  21
  22 /// Look for live drops in a const context.
  23 ///
  24 /// This is separate from the rest of the const checking logic because it must run after drop
  25 /// elaboration.
  26 pub fn check_live_drops(tcx: TyCtxt<'tcx>, body: &mir::Body<'tcx>) {
  27     let def_id = body.source.def_id().expect_local();
  28     let const_kind = tcx.hir().body_const_context(def_id);
  29     if const_kind.is_none() {
  30         return;
  31     }
  32
  33     let ccx = ConstCx { body, tcx, const_kind, param_env: tcx.param_env(def_id) };
  34     if !checking_enabled(&ccx) {
  35         return;
  36     }
  37
  38     let mut visitor = CheckLiveDrops { ccx: &ccx, qualifs: Qualifs::default() };
  39
  40     visitor.visit_body(body);
  41 }
  42
  43 struct CheckLiveDrops<'mir, 'tcx> {
  44     ccx: &'mir ConstCx<'mir, 'tcx>,
  45     qualifs: Qualifs<'mir, 'tcx>,
  46 }
  47
  48 // So we can access `body` and `tcx`.
  49 impl std::ops::Deref for CheckLiveDrops<'mir, 'tcx> {
  50     type Target = ConstCx<'mir, 'tcx>;
  51
  52     fn deref(&self) -> &Self::Target {
  53         &self.ccx
  54     }
  55 }
  56
  57 impl CheckLiveDrops<'mir, 'tcx> {
  58     fn check_live_drop(&self, span: Span) {
  59         ops::LiveDrop { dropped_at: None }.build_error(self.ccx, span).emit();
  60     }
  61 }
  62
  63 impl Visitor<'tcx> for CheckLiveDrops<'mir, 'tcx> {
  64     fn visit_basic_block_data(&mut self, bb: BasicBlock, block: &mir::BasicBlockData<'tcx>) {
  65         trace!("visit_basic_block_data: bb={:?} is_cleanup={:?}", bb, block.is_cleanup);
  66
  67         // Ignore drop terminators in cleanup blocks.
  68         if block.is_cleanup {
  69             return;
  70         }
  71
  72         self.super_basic_block_data(bb, block);
  73     }
  74
  75     fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, location: Location) {
  76         trace!("visit_terminator: terminator={:?} location={:?}", terminator, location);
  77
  78         match &terminator.kind {
  79             mir::TerminatorKind::Drop { place: dropped_place, .. } => {
  80                 let dropped_ty = dropped_place.ty(self.body, self.tcx).ty;
  81                 if !NeedsDrop::in_any_value_of_ty(self.ccx, dropped_ty) {
  82                     bug!(
  83                         "Drop elaboration left behind a Drop for a type that does not need dropping"
  84                     );
  85                 }
  86
  87                 if dropped_place.is_indirect() {
  88                     self.check_live_drop(terminator.source_info.span);
  89                     return;
  90                 }
  91
  92                 // Drop elaboration is not precise enough to accept code like
  93                 // `src/test/ui/consts/control-flow/drop-pass.rs`; e.g., when an `Option<Vec<T>>` is
  94                 // initialized with `None` and never changed, it still emits drop glue.
  95                 // Hence we additionally check the qualifs here to allow more code to pass.
  96                 if self.qualifs.needs_drop(self.ccx, dropped_place.local, location) {
  97                     // Use the span where the dropped local was declared for the error.
  98                     let span = self.body.local_decls[dropped_place.local].source_info.span;
  99                     self.check_live_drop(span);
 100                 }
 101             }
 102
 103             mir::TerminatorKind::DropAndReplace { .. } => span_bug!(
 104                 terminator.source_info.span,
 105                 "`DropAndReplace` should be removed by drop elaboration",
 106             ),
 107
 108             mir::TerminatorKind::Abort
 109             | mir::TerminatorKind::Call { .. }
 110             | mir::TerminatorKind::Assert { .. }
 111             | mir::TerminatorKind::FalseEdge { .. }
 112             | mir::TerminatorKind::FalseUnwind { .. }
 113             | mir::TerminatorKind::GeneratorDrop
 114             | mir::TerminatorKind::Goto { .. }
 115             | mir::TerminatorKind::InlineAsm { .. }
 116             | mir::TerminatorKind::Resume
 117             | mir::TerminatorKind::Return
 118             | mir::TerminatorKind::SwitchInt { .. }
 119             | mir::TerminatorKind::Unreachable
 120             | mir::TerminatorKind::Yield { .. } => {}
 121         }
 122     }
 123 }