diff --git a/compiler/rustc_mir/src/transform/mod.rs b/compiler/rustc_mir/src/transform/mod.rs
index abe2dc496a630..f48ad039b4ff9 100644
--- a/compiler/rustc_mir/src/transform/mod.rs
+++ b/compiler/rustc_mir/src/transform/mod.rs
@@ -38,6 +38,7 @@ pub mod nrvo;
pub mod promote_consts;
pub mod qualify_min_const_fn;
pub mod remove_noop_landing_pads;
+pub mod remove_unneeded_drops;
pub mod required_consts;
pub mod rustc_peek;
pub mod simplify;
@@ -461,6 +462,7 @@ fn run_optimization_passes<'tcx>(
// The main optimizations that we do on MIR.
let optimizations: &[&dyn MirPass<'tcx>] = &[
+ &remove_unneeded_drops::RemoveUnneededDrops,
&match_branches::MatchBranchSimplification,
// inst combine is after MatchBranchSimplification to clean up Ne(_1, false)
&instcombine::InstCombine,
diff --git a/compiler/rustc_mir/src/transform/remove_unneeded_drops.rs b/compiler/rustc_mir/src/transform/remove_unneeded_drops.rs
new file mode 100644
index 0000000000000..f027f5e33a187
--- /dev/null
+++ b/compiler/rustc_mir/src/transform/remove_unneeded_drops.rs
@@ -0,0 +1,59 @@
+//! This pass replaces a drop of a type that does not need dropping, with a goto
+
+use crate::transform::{MirPass, MirSource};
+use rustc_hir::def_id::LocalDefId;
+use rustc_middle::mir::visit::Visitor;
+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>, source: MirSource<'tcx>, body: &mut Body<'tcx>) {
+ trace!("Running RemoveUnneededDrops on {:?}", source);
+ let mut opt_finder = RemoveUnneededDropsOptimizationFinder {
+ tcx,
+ body,
+ optimizations: vec![],
+ def_id: source.def_id().expect_local(),
+ };
+ opt_finder.visit_body(body);
+ let should_simplify = !opt_finder.optimizations.is_empty();
+ for (loc, target) in opt_finder.optimizations {
+ let terminator = body.basic_blocks_mut()[loc.block].terminator_mut();
+ debug!("SUCCESS: replacing `drop` with goto({:?})", target);
+ terminator.kind = TerminatorKind::Goto { target };
+ }
+
+ // if we applied optimizations, we potentially have some cfg to cleanup to
+ // make it easier for further passes
+ if should_simplify {
+ simplify_cfg(body);
+ }
+ }
+}
+
+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, .. }
+ | TerminatorKind::DropAndReplace { place, target, .. } => {
+ let ty = place.ty(self.body, self.tcx);
+ let needs_drop = ty.ty.needs_drop(self.tcx, self.tcx.param_env(self.def_id));
+ 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)>,
+ def_id: LocalDefId,
+}
diff --git a/src/test/mir-opt/dest-prop/cycle.main.DestinationPropagation.diff b/src/test/mir-opt/dest-prop/cycle.main.DestinationPropagation.diff
index dd717c1b9c324..881c296cee77a 100644
--- a/src/test/mir-opt/dest-prop/cycle.main.DestinationPropagation.diff
+++ b/src/test/mir-opt/dest-prop/cycle.main.DestinationPropagation.diff
@@ -57,10 +57,6 @@
- _6 = _1; // scope 3 at $DIR/cycle.rs:14:10: 14:11
+ _6 = _4; // scope 3 at $DIR/cycle.rs:14:10: 14:11
_5 = const (); // scope 4 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
- drop(_6) -> bb2; // scope 4 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
- }
-
- bb2: {
StorageDead(_6); // scope 3 at $DIR/cycle.rs:14:11: 14:12
StorageDead(_5); // scope 3 at $DIR/cycle.rs:14:12: 14:13
_0 = const (); // scope 0 at $DIR/cycle.rs:8:11: 15:2
diff --git a/src/test/mir-opt/dest-prop/union.main.DestinationPropagation.diff b/src/test/mir-opt/dest-prop/union.main.DestinationPropagation.diff
index 871f6e35043ec..f15e7bcb2fba6 100644
--- a/src/test/mir-opt/dest-prop/union.main.DestinationPropagation.diff
+++ b/src/test/mir-opt/dest-prop/union.main.DestinationPropagation.diff
@@ -32,10 +32,6 @@
StorageLive(_4); // scope 1 at $DIR/union.rs:15:10: 15:26
_4 = (_1.0: u32); // scope 2 at $DIR/union.rs:15:19: 15:24
_3 = const (); // scope 3 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
- drop(_4) -> bb2; // scope 3 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
- }
-
- bb2: {
StorageDead(_4); // scope 1 at $DIR/union.rs:15:26: 15:27
StorageDead(_3); // scope 1 at $DIR/union.rs:15:27: 15:28
_0 = const (); // scope 0 at $DIR/union.rs:8:11: 16:2
diff --git a/src/test/mir-opt/remove_unneeded_drops.cannot_opt_generic.RemoveUnneededDrops.diff b/src/test/mir-opt/remove_unneeded_drops.cannot_opt_generic.RemoveUnneededDrops.diff
new file mode 100644
index 0000000000000..787cf6f97c1f2
--- /dev/null
+++ b/src/test/mir-opt/remove_unneeded_drops.cannot_opt_generic.RemoveUnneededDrops.diff
@@ -0,0 +1,32 @@
+- // MIR for `cannot_opt_generic` before RemoveUnneededDrops
++ // MIR for `cannot_opt_generic` after RemoveUnneededDrops
+
+ fn cannot_opt_generic(_1: T) -> () {
+ debug x => _1; // in scope 0 at $DIR/remove_unneeded_drops.rs:20:26: 20:27
+ let mut _0: (); // return place in scope 0 at $DIR/remove_unneeded_drops.rs:20:32: 20:32
+ let _2: (); // in scope 0 at $DIR/remove_unneeded_drops.rs:21:5: 21:12
+ let mut _3: T; // in scope 0 at $DIR/remove_unneeded_drops.rs:21:10: 21:11
+ scope 1 {
+ debug _x => _3; // in scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
+ }
+
+ bb0: {
+ StorageLive(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:21:5: 21:12
+ StorageLive(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:21:10: 21:11
+ _3 = move _1; // scope 0 at $DIR/remove_unneeded_drops.rs:21:10: 21:11
+ _2 = const (); // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
+ drop(_3) -> [return: bb2, unwind: bb1]; // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
+ }
+
+ bb1 (cleanup): {
+ resume; // scope 0 at $DIR/remove_unneeded_drops.rs:20:1: 22:2
+ }
+
+ bb2: {
+ StorageDead(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:21:11: 21:12
+ StorageDead(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:21:12: 21:13
+ _0 = const (); // scope 0 at $DIR/remove_unneeded_drops.rs:20:32: 22:2
+ return; // scope 0 at $DIR/remove_unneeded_drops.rs:22:2: 22:2
+ }
+ }
+
diff --git a/src/test/mir-opt/remove_unneeded_drops.dont_opt.RemoveUnneededDrops.diff b/src/test/mir-opt/remove_unneeded_drops.dont_opt.RemoveUnneededDrops.diff
new file mode 100644
index 0000000000000..52e182eeb4a8a
--- /dev/null
+++ b/src/test/mir-opt/remove_unneeded_drops.dont_opt.RemoveUnneededDrops.diff
@@ -0,0 +1,32 @@
+- // MIR for `dont_opt` before RemoveUnneededDrops
++ // MIR for `dont_opt` after RemoveUnneededDrops
+
+ fn dont_opt(_1: Vec<bool>) -> () {
+ debug x => _1; // in scope 0 at $DIR/remove_unneeded_drops.rs:8:13: 8:14
+ let mut _0: (); // return place in scope 0 at $DIR/remove_unneeded_drops.rs:8:27: 8:27
+ let _2: (); // in scope 0 at $DIR/remove_unneeded_drops.rs:9:5: 9:12
+ let mut _3: std::vec::Vec<bool>; // in scope 0 at $DIR/remove_unneeded_drops.rs:9:10: 9:11
+ scope 1 {
+ debug _x => _3; // in scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
+ }
+
+ bb0: {
+ StorageLive(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:9:5: 9:12
+ StorageLive(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:9:10: 9:11
+ _3 = move _1; // scope 0 at $DIR/remove_unneeded_drops.rs:9:10: 9:11
+ _2 = const (); // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
+ drop(_3) -> [return: bb2, unwind: bb1]; // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
+ }
+
+ bb1 (cleanup): {
+ resume; // scope 0 at $DIR/remove_unneeded_drops.rs:8:1: 10:2
+ }
+
+ bb2: {
+ StorageDead(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:9:11: 9:12
+ StorageDead(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:9:12: 9:13
+ _0 = const (); // scope 0 at $DIR/remove_unneeded_drops.rs:8:27: 10:2
+ return; // scope 0 at $DIR/remove_unneeded_drops.rs:10:2: 10:2
+ }
+ }
+
diff --git a/src/test/mir-opt/remove_unneeded_drops.opt.RemoveUnneededDrops.diff b/src/test/mir-opt/remove_unneeded_drops.opt.RemoveUnneededDrops.diff
new file mode 100644
index 0000000000000..bc9e1344f3176
--- /dev/null
+++ b/src/test/mir-opt/remove_unneeded_drops.opt.RemoveUnneededDrops.diff
@@ -0,0 +1,28 @@
+- // MIR for `opt` before RemoveUnneededDrops
++ // MIR for `opt` after RemoveUnneededDrops
+
+ fn opt(_1: bool) -> () {
+ debug x => _1; // in scope 0 at $DIR/remove_unneeded_drops.rs:3:8: 3:9
+ let mut _0: (); // return place in scope 0 at $DIR/remove_unneeded_drops.rs:3:17: 3:17
+ let _2: (); // in scope 0 at $DIR/remove_unneeded_drops.rs:4:5: 4:12
+ let mut _3: bool; // in scope 0 at $DIR/remove_unneeded_drops.rs:4:10: 4:11
+ scope 1 {
+ debug _x => _3; // in scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
+ }
+
+ bb0: {
+ StorageLive(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:4:5: 4:12
+ StorageLive(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:4:10: 4:11
+ _3 = _1; // scope 0 at $DIR/remove_unneeded_drops.rs:4:10: 4:11
+ _2 = const (); // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
+- drop(_3) -> bb1; // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
+- }
+-
+- bb1: {
+ StorageDead(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:4:11: 4:12
+ StorageDead(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:4:12: 4:13
+ _0 = const (); // scope 0 at $DIR/remove_unneeded_drops.rs:3:17: 5:2
+ return; // scope 0 at $DIR/remove_unneeded_drops.rs:5:2: 5:2
+ }
+ }
+
diff --git a/src/test/mir-opt/remove_unneeded_drops.opt_generic_copy.RemoveUnneededDrops.diff b/src/test/mir-opt/remove_unneeded_drops.opt_generic_copy.RemoveUnneededDrops.diff
new file mode 100644
index 0000000000000..5c8b1d1372115
--- /dev/null
+++ b/src/test/mir-opt/remove_unneeded_drops.opt_generic_copy.RemoveUnneededDrops.diff
@@ -0,0 +1,28 @@
+- // MIR for `opt_generic_copy` before RemoveUnneededDrops
++ // MIR for `opt_generic_copy` after RemoveUnneededDrops
+
+ fn opt_generic_copy(_1: T) -> () {
+ debug x => _1; // in scope 0 at $DIR/remove_unneeded_drops.rs:13:30: 13:31
+ let mut _0: (); // return place in scope 0 at $DIR/remove_unneeded_drops.rs:13:36: 13:36
+ let _2: (); // in scope 0 at $DIR/remove_unneeded_drops.rs:14:5: 14:12
+ let mut _3: T; // in scope 0 at $DIR/remove_unneeded_drops.rs:14:10: 14:11
+ scope 1 {
+ debug _x => _3; // in scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
+ }
+
+ bb0: {
+ StorageLive(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:14:5: 14:12
+ StorageLive(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:14:10: 14:11
+ _3 = _1; // scope 0 at $DIR/remove_unneeded_drops.rs:14:10: 14:11
+ _2 = const (); // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
+- drop(_3) -> bb1; // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
+- }
+-
+- bb1: {
+ StorageDead(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:14:11: 14:12
+ StorageDead(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:14:12: 14:13
+ _0 = const (); // scope 0 at $DIR/remove_unneeded_drops.rs:13:36: 15:2
+ return; // scope 0 at $DIR/remove_unneeded_drops.rs:15:2: 15:2
+ }
+ }
+
diff --git a/src/test/mir-opt/remove_unneeded_drops.rs b/src/test/mir-opt/remove_unneeded_drops.rs
new file mode 100644
index 0000000000000..1052f2886770d
--- /dev/null
+++ b/src/test/mir-opt/remove_unneeded_drops.rs
@@ -0,0 +1,29 @@
+// ignore-wasm32-bare compiled with panic=abort by default
+// EMIT_MIR remove_unneeded_drops.opt.RemoveUnneededDrops.diff
+fn opt(x: bool) {
+ drop(x);
+}
+
+// EMIT_MIR remove_unneeded_drops.dont_opt.RemoveUnneededDrops.diff
+fn dont_opt(x: Vec<bool>) {
+ drop(x);
+}
+
+// EMIT_MIR remove_unneeded_drops.opt_generic_copy.RemoveUnneededDrops.diff
+fn opt_generic_copy<T: Copy>(x: T) {
+ drop(x);
+}
+
+// EMIT_MIR remove_unneeded_drops.cannot_opt_generic.RemoveUnneededDrops.diff
+// since the pass is not running on monomorphisized code,
+// we can't (but probably should) optimize this
+fn cannot_opt_generic<T>(x: T) {
+ drop(x);
+}
+
+fn main() {
+ opt(true);
+ opt_generic_copy(42);
+ cannot_opt_generic(42);
+ dont_opt(vec![true]);
+}