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);
+        }
+    }
+}