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Diffstat (limited to '')
-rw-r--r-- | compiler/rustc_mir_transform/src/remove_unneeded_drops.rs | 45 |
1 files changed, 45 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/remove_unneeded_drops.rs b/compiler/rustc_mir_transform/src/remove_unneeded_drops.rs new file mode 100644 index 000000000..84ccf6e1f --- /dev/null +++ b/compiler/rustc_mir_transform/src/remove_unneeded_drops.rs @@ -0,0 +1,45 @@ +//! This pass replaces a drop of a type that does not need dropping, with a goto. +//! +//! When the MIR is built, we check `needs_drop` before emitting a `Drop` for a place. This pass is +//! useful because (unlike MIR building) it runs after type checking, so it can make use of +//! `Reveal::All` to provide more precise type information. + +use crate::MirPass; +use rustc_middle::mir::*; +use rustc_middle::ty::TyCtxt; + +use super::simplify::simplify_cfg; + +pub struct RemoveUnneededDrops; + +impl<'tcx> MirPass<'tcx> for RemoveUnneededDrops { + fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) { + trace!("Running RemoveUnneededDrops on {:?}", body.source); + + let did = body.source.def_id(); + let param_env = tcx.param_env_reveal_all_normalized(did); + let mut should_simplify = false; + + for block in body.basic_blocks.as_mut() { + let terminator = block.terminator_mut(); + if let TerminatorKind::Drop { place, target, .. } = terminator.kind { + let ty = place.ty(&body.local_decls, tcx); + if ty.ty.needs_drop(tcx, param_env) { + continue; + } + if !tcx.consider_optimizing(|| format!("RemoveUnneededDrops {:?} ", did)) { + continue; + } + debug!("SUCCESS: replacing `drop` with goto({:?})", target); + terminator.kind = TerminatorKind::Goto { target }; + should_simplify = true; + } + } + + // if we applied optimizations, we potentially have some cfg to cleanup to + // make it easier for further passes + if should_simplify { + simplify_cfg(tcx, body); + } + } +} |