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::*;
   5 use rustc_middle::ty::TyCtxt;
   6
   7 use super::simplify::simplify_cfg;
   8
   9 pub struct RemoveUnneededDrops;
  10
  11 impl<'tcx> MirPass<'tcx> for RemoveUnneededDrops {
  12     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  13         trace!("Running RemoveUnneededDrops on {:?}", body.source);
  14
  15         let did = body.source.def_id();
  16         let param_env = tcx.param_env(did);
  17         let mut should_simplify = false;
  18
  19         let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
  20         for block in basic_blocks {
  21             let terminator = block.terminator_mut();
  22             if let TerminatorKind::Drop { place, target, .. } = terminator.kind {
  23                 let ty = place.ty(local_decls, tcx);
  24                 if ty.ty.needs_drop(tcx, param_env) {
  25                     continue;
  26                 }
  27                 if !tcx.consider_optimizing(|| format!("RemoveUnneededDrops {:?} ", did)) {
  28                     continue;
  29                 }
  30                 debug!("SUCCESS: replacing `drop` with goto({:?})", target);
  31                 terminator.kind = TerminatorKind::Goto { target };
  32                 should_simplify = true;
  33             }
  34         }
  35
  36         // if we applied optimizations, we potentially have some cfg to cleanup to
  37         // make it easier for further passes
  38         if should_simplify {
  39             simplify_cfg(body);
  40         }
  41     }
  42 }