From d0a756719f86a8a304dad69803da2244473a9495 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tomasz=20Mi=C4=85sko?= <tomasz.miasko@gmail.com>
Date: Sun, 17 Jan 2021 00:00:00 +0000
Subject: [PATCH] Visit only terminators when removing unneeded drops

No functional changes intended
---
 .../src/transform/remove_unneeded_drops.rs    | 61 ++++++-------------
 1 file changed, 20 insertions(+), 41 deletions(-)

diff --git a/compiler/rustc_mir/src/transform/remove_unneeded_drops.rs b/compiler/rustc_mir/src/transform/remove_unneeded_drops.rs
index 221114eebaa45..5144d48750de7 100644
--- a/compiler/rustc_mir/src/transform/remove_unneeded_drops.rs
+++ b/compiler/rustc_mir/src/transform/remove_unneeded_drops.rs
@@ -1,9 +1,8 @@
 //! This pass replaces a drop of a type that does not need dropping, with a goto
 
 use crate::transform::MirPass;
-use rustc_middle::mir::visit::Visitor;
 use rustc_middle::mir::*;
-use rustc_middle::ty::{ParamEnv, TyCtxt};
+use rustc_middle::ty::TyCtxt;
 
 use super::simplify::simplify_cfg;
 
@@ -12,24 +11,26 @@ 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 mut opt_finder = RemoveUnneededDropsOptimizationFinder {
-            tcx,
-            body,
-            param_env: tcx.param_env(body.source.def_id()),
-            optimizations: vec![],
-        };
-        opt_finder.visit_body(body);
-        let should_simplify = !opt_finder.optimizations.is_empty();
-        for (loc, target) in opt_finder.optimizations {
-            if !tcx
-                .consider_optimizing(|| format!("RemoveUnneededDrops {:?} ", body.source.def_id()))
-            {
-                break;
-            }
 
-            let terminator = body.basic_blocks_mut()[loc.block].terminator_mut();
-            debug!("SUCCESS: replacing `drop` with goto({:?})", target);
-            terminator.kind = TerminatorKind::Goto { target };
+        let did = body.source.def_id();
+        let param_env = tcx.param_env(did);
+        let mut should_simplify = false;
+
+        let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
+        for block in basic_blocks {
+            let terminator = block.terminator_mut();
+            if let TerminatorKind::Drop { place, target, .. } = terminator.kind {
+                let ty = place.ty(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
@@ -39,25 +40,3 @@ impl<'tcx> MirPass<'tcx> for RemoveUnneededDrops {
         }
     }
 }
-
-impl<'a, 'tcx> Visitor<'tcx> for RemoveUnneededDropsOptimizationFinder<'a, 'tcx> {
-    fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
-        match terminator.kind {
-            TerminatorKind::Drop { place, target, .. } => {
-                let ty = place.ty(self.body, self.tcx);
-                let needs_drop = ty.ty.needs_drop(self.tcx, self.param_env);
-                if !needs_drop {
-                    self.optimizations.push((location, target));
-                }
-            }
-            _ => {}
-        }
-        self.super_terminator(terminator, location);
-    }
-}
-pub struct RemoveUnneededDropsOptimizationFinder<'a, 'tcx> {
-    tcx: TyCtxt<'tcx>,
-    body: &'a Body<'tcx>,
-    optimizations: Vec<(Location, BasicBlock)>,
-    param_env: ParamEnv<'tcx>,
-}