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simonvandelsimonvandel
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MIR pass to remove unneeded drops on types not needing drop
This is heavily dependent on MIR inlining running to actually see the drop statement
1 parent e0bc267 commit 290dca1

7 files changed

+209
-0
lines changed

‎compiler/rustc_mir/src/transform/mod.rs

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@@ -38,6 +38,7 @@ pub mod nrvo;
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pub mod promote_consts;
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pub mod qualify_min_const_fn;
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pub mod remove_noop_landing_pads;
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pub mod remove_unneeded_drops;
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pub mod required_consts;
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pub mod rustc_peek;
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pub mod simplify;
@@ -461,6 +462,7 @@ fn run_optimization_passes<'tcx>(
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// The main optimizations that we do on MIR.
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let optimizations: &[&dyn MirPass<'tcx>] = &[
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&remove_unneeded_drops::RemoveUnneededDrops::new(def_id),
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&match_branches::MatchBranchSimplification,
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// inst combine is after MatchBranchSimplification to clean up Ne(_1, false)
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&instcombine::InstCombine,

‎compiler/rustc_mir/src/transform/remove_unneeded_drops.rs

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//! This pass replaces a drop of a type that does not need dropping, with a goto
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use crate::transform::{MirPass, MirSource};
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use rustc_hir::def_id::LocalDefId;
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use rustc_middle::mir::visit::Visitor;
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use rustc_middle::mir::*;
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use rustc_middle::ty::TyCtxt;
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pub struct RemoveUnneededDrops {
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def_id: LocalDefId,
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}
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impl RemoveUnneededDrops {
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pub fn new(def_id: LocalDefId) -> Self {
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Self { def_id }
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}
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}
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impl<'tcx> MirPass<'tcx> for RemoveUnneededDrops {
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fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut Body<'tcx>) {
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trace!("Running SimplifyComparisonIntegral on {:?}", source);
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let mut opt_finder = RemoveUnneededDropsOptimizationFinder {
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tcx,
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body,
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optimizations: vec![],
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def_id: self.def_id,
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};
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opt_finder.visit_body(body);
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for (loc, target) in opt_finder.optimizations {
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let terminator = body.basic_blocks_mut()[loc.block].terminator_mut();
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debug!("SUCCESS: replacing `drop` with goto({:?})", target);
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terminator.kind = TerminatorKind::Goto { target };
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}
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}
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}
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impl<'a, 'tcx> Visitor<'tcx> for RemoveUnneededDropsOptimizationFinder<'a, 'tcx> {
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fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
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match terminator.kind {
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TerminatorKind::Drop { place, target, .. } => {
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let ty = place.ty(self.body, self.tcx);
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let needs_drop = ty.ty.needs_drop(self.tcx, self.tcx.param_env(self.def_id));
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if !needs_drop {
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self.optimizations.push((location, target));
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}
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}
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_ => {}
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}
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self.super_terminator(terminator, location);
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}
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}
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pub struct RemoveUnneededDropsOptimizationFinder<'a, 'tcx> {
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tcx: TyCtxt<'tcx>,
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body: &'a Body<'tcx>,
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optimizations: Vec<(Location, BasicBlock)>,
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def_id: LocalDefId,
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}

‎src/test/mir-opt/remove_unneeded_drops.cannot_opt_generic.RemoveUnneededDrops.diff

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- // MIR for `cannot_opt_generic` before RemoveUnneededDrops
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+ // MIR for `cannot_opt_generic` after RemoveUnneededDrops
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fn cannot_opt_generic(_1: T) -> () {
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debug x => _1; // in scope 0 at $DIR/remove_unneeded_drops.rs:19:26: 19:27
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let mut _0: (); // return place in scope 0 at $DIR/remove_unneeded_drops.rs:19:32: 19:32
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let _2: (); // in scope 0 at $DIR/remove_unneeded_drops.rs:20:5: 20:12
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let mut _3: T; // in scope 0 at $DIR/remove_unneeded_drops.rs:20:10: 20:11
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scope 1 {
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debug _x => _3; // in scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
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}
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bb0: {
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StorageLive(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:20:5: 20:12
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StorageLive(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:20:10: 20:11
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_3 = move _1; // scope 0 at $DIR/remove_unneeded_drops.rs:20:10: 20:11
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_2 = const (); // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
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drop(_3) -> [return: bb2, unwind: bb1]; // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
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}
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bb1 (cleanup): {
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resume; // scope 0 at $DIR/remove_unneeded_drops.rs:19:1: 21:2
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}
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bb2: {
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StorageDead(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:20:11: 20:12
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StorageDead(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:20:12: 20:13
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_0 = const (); // scope 0 at $DIR/remove_unneeded_drops.rs:19:32: 21:2
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return; // scope 0 at $DIR/remove_unneeded_drops.rs:21:2: 21:2
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}
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}
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‎src/test/mir-opt/remove_unneeded_drops.dont_opt.RemoveUnneededDrops.diff

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- // MIR for `dont_opt` before RemoveUnneededDrops
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+ // MIR for `dont_opt` after RemoveUnneededDrops
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fn dont_opt(_1: Vec<bool>) -> () {
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debug x => _1; // in scope 0 at $DIR/remove_unneeded_drops.rs:7:13: 7:14
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let mut _0: (); // return place in scope 0 at $DIR/remove_unneeded_drops.rs:7:27: 7:27
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let _2: (); // in scope 0 at $DIR/remove_unneeded_drops.rs:8:5: 8:12
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let mut _3: std::vec::Vec<bool>; // in scope 0 at $DIR/remove_unneeded_drops.rs:8:10: 8:11
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scope 1 {
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debug _x => _3; // in scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
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}
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bb0: {
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StorageLive(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:8:5: 8:12
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StorageLive(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:8:10: 8:11
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_3 = move _1; // scope 0 at $DIR/remove_unneeded_drops.rs:8:10: 8:11
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_2 = const (); // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
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drop(_3) -> [return: bb2, unwind: bb1]; // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
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}
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bb1 (cleanup): {
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resume; // scope 0 at $DIR/remove_unneeded_drops.rs:7:1: 9:2
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}
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bb2: {
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StorageDead(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:8:11: 8:12
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StorageDead(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:8:12: 8:13
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_0 = const (); // scope 0 at $DIR/remove_unneeded_drops.rs:7:27: 9:2
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return; // scope 0 at $DIR/remove_unneeded_drops.rs:9:2: 9:2
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}
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}
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‎src/test/mir-opt/remove_unneeded_drops.opt.RemoveUnneededDrops.diff

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@@ -0,0 +1,29 @@
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- // MIR for `opt` before RemoveUnneededDrops
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+ // MIR for `opt` after RemoveUnneededDrops
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fn opt(_1: bool) -> () {
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debug x => _1; // in scope 0 at $DIR/remove_unneeded_drops.rs:2:8: 2:9
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let mut _0: (); // return place in scope 0 at $DIR/remove_unneeded_drops.rs:2:17: 2:17
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let _2: (); // in scope 0 at $DIR/remove_unneeded_drops.rs:3:5: 3:12
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let mut _3: bool; // in scope 0 at $DIR/remove_unneeded_drops.rs:3:10: 3:11
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scope 1 {
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debug _x => _3; // in scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
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}
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bb0: {
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StorageLive(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:3:5: 3:12
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StorageLive(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:3:10: 3:11
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_3 = _1; // scope 0 at $DIR/remove_unneeded_drops.rs:3:10: 3:11
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_2 = const (); // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
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- drop(_3) -> bb1; // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
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+ goto -> bb1; // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
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}
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bb1: {
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StorageDead(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:3:11: 3:12
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StorageDead(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:3:12: 3:13
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_0 = const (); // scope 0 at $DIR/remove_unneeded_drops.rs:2:17: 4:2
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return; // scope 0 at $DIR/remove_unneeded_drops.rs:4:2: 4:2
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}
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}
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‎src/test/mir-opt/remove_unneeded_drops.opt_generic_copy.RemoveUnneededDrops.diff

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- // MIR for `opt_generic_copy` before RemoveUnneededDrops
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+ // MIR for `opt_generic_copy` after RemoveUnneededDrops
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fn opt_generic_copy(_1: T) -> () {
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debug x => _1; // in scope 0 at $DIR/remove_unneeded_drops.rs:12:30: 12:31
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let mut _0: (); // return place in scope 0 at $DIR/remove_unneeded_drops.rs:12:36: 12:36
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let _2: (); // in scope 0 at $DIR/remove_unneeded_drops.rs:13:5: 13:12
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let mut _3: T; // in scope 0 at $DIR/remove_unneeded_drops.rs:13:10: 13:11
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scope 1 {
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debug _x => _3; // in scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
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}
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bb0: {
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StorageLive(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:13:5: 13:12
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StorageLive(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:13:10: 13:11
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_3 = _1; // scope 0 at $DIR/remove_unneeded_drops.rs:13:10: 13:11
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_2 = const (); // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
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- drop(_3) -> bb1; // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
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+ goto -> bb1; // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
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}
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bb1: {
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StorageDead(_3); // scope 0 at $DIR/remove_unneeded_drops.rs:13:11: 13:12
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StorageDead(_2); // scope 0 at $DIR/remove_unneeded_drops.rs:13:12: 13:13
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_0 = const (); // scope 0 at $DIR/remove_unneeded_drops.rs:12:36: 14:2
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return; // scope 0 at $DIR/remove_unneeded_drops.rs:14:2: 14:2
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}
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}
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‎src/test/mir-opt/remove_unneeded_drops.rs

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// EMIT_MIR remove_unneeded_drops.opt.RemoveUnneededDrops.diff
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fn opt(x: bool) {
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drop(x);
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}
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// EMIT_MIR remove_unneeded_drops.dont_opt.RemoveUnneededDrops.diff
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fn dont_opt(x: Vec<bool>) {
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drop(x);
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}
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// EMIT_MIR remove_unneeded_drops.opt_generic_copy.RemoveUnneededDrops.diff
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fn opt_generic_copy<T: Copy>(x: T) {
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drop(x);
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}
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// EMIT_MIR remove_unneeded_drops.cannot_opt_generic.RemoveUnneededDrops.diff
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// since the pass is not running on monomorphisized code,
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// we can't (but probably should) optimize this
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fn cannot_opt_generic<T>(x: T) {
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drop(x);
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}
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fn main() {
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opt(true);
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opt_generic_copy(42);
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cannot_opt_generic(42);
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dont_opt(vec![true]);
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}

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