compiler/rustc_mir/src/transform/remove_unneeded_drops.rs

   1 //! This pass replaces a drop of a type that does not need dropping, with a goto
   2
   3 use crate::transform::MirPass;
   4 use rustc_middle::mir::visit::Visitor;
   5 use rustc_middle::mir::*;
   6 use rustc_middle::ty::{ParamEnv, TyCtxt};
   7
   8 use super::simplify::simplify_cfg;
   9
  10 pub struct RemoveUnneededDrops;
  11
  12 impl<'tcx> MirPass<'tcx> for RemoveUnneededDrops {
  13     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  14         trace!("Running RemoveUnneededDrops on {:?}", body.source);
  15         let mut opt_finder = RemoveUnneededDropsOptimizationFinder {
  16             tcx,
  17             body,
  18             param_env: tcx.param_env(body.source.def_id()),
  19             optimizations: vec![],
  20         };
  21         opt_finder.visit_body(body);
  22         let should_simplify = !opt_finder.optimizations.is_empty();
  23         for (loc, target) in opt_finder.optimizations {
  24             if !tcx
  25                 .consider_optimizing(|| format!("RemoveUnneededDrops {:?} ", body.source.def_id()))
  26             {
  27                 break;
  28             }
  29
  30             let terminator = body.basic_blocks_mut()[loc.block].terminator_mut();
  31             debug!("SUCCESS: replacing `drop` with goto({:?})", target);
  32             terminator.kind = TerminatorKind::Goto { target };
  33         }
  34
  35         // if we applied optimizations, we potentially have some cfg to cleanup to
  36         // make it easier for further passes
  37         if should_simplify {
  38             simplify_cfg(body);
  39         }
  40     }
  41 }
  42
  43 impl<'a, 'tcx> Visitor<'tcx> for RemoveUnneededDropsOptimizationFinder<'a, 'tcx> {
  44     fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
  45         match terminator.kind {
  46             TerminatorKind::Drop { place, target, .. } => {
  47                 let ty = place.ty(self.body, self.tcx);
  48                 let needs_drop = ty.ty.needs_drop(self.tcx, self.param_env);
  49                 if !needs_drop {
  50                     self.optimizations.push((location, target));
  51                 }
  52             }
  53             _ => {}
  54         }
  55         self.super_terminator(terminator, location);
  56     }
  57 }
  58 pub struct RemoveUnneededDropsOptimizationFinder<'a, 'tcx> {
  59     tcx: TyCtxt<'tcx>,
  60     body: &'a Body<'tcx>,
  61     optimizations: Vec<(Location, BasicBlock)>,
  62     param_env: ParamEnv<'tcx>,
  63 }