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

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::new(def_id),
         &match_branches::MatchBranchSimplification,
         // inst combine is after MatchBranchSimplification to clean up Ne(_1, false)
         &instcombine::InstCombine,

compiler/rustc_mir/src/transform/remove_unneeded_drops.rs

@@ -0,0 +1,57 @@
+//! 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;
+
+pub struct RemoveUnneededDrops {
+    def_id: LocalDefId,
+}
+
+impl RemoveUnneededDrops {
+    pub fn new(def_id: LocalDefId) -> Self {
+        Self { def_id }
+    }
+}
+
+impl<'tcx> MirPass<'tcx> for RemoveUnneededDrops {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut Body<'tcx>) {
+        trace!("Running SimplifyComparisonIntegral on {:?}", source);
+        let mut opt_finder = RemoveUnneededDropsOptimizationFinder {
+            tcx,
+            body,
+            optimizations: vec![],
+            def_id: self.def_id,
+        };
+        opt_finder.visit_body(body);
+        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 };
+        }
+    }
+}
+
+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.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,
+}

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:19:26: 19:27
+      let mut _0: ();                      // return place in scope 0 at $DIR/remove_unneeded_drops.rs:19:32: 19:32
+      let _2: ();                          // in scope 0 at $DIR/remove_unneeded_drops.rs:20:5: 20:12
+      let mut _3: T;                       // in scope 0 at $DIR/remove_unneeded_drops.rs:20:10: 20:11
+      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:20:5: 20:12
+          StorageLive(_3);                 // scope 0 at $DIR/remove_unneeded_drops.rs:20:10: 20:11
+          _3 = move _1;                    // scope 0 at $DIR/remove_unneeded_drops.rs:20:10: 20:11
+          _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:19:1: 21:2
+      }
+
+      bb2: {
+          StorageDead(_3);                 // scope 0 at $DIR/remove_unneeded_drops.rs:20:11: 20:12
+          StorageDead(_2);                 // scope 0 at $DIR/remove_unneeded_drops.rs:20:12: 20:13
+          _0 = const ();                   // scope 0 at $DIR/remove_unneeded_drops.rs:19:32: 21:2
+          return;                          // scope 0 at $DIR/remove_unneeded_drops.rs:21:2: 21:2
+      }
+  }
+

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:7:13: 7:14
+      let mut _0: ();                      // return place in scope 0 at $DIR/remove_unneeded_drops.rs:7:27: 7:27
+      let _2: ();                          // in scope 0 at $DIR/remove_unneeded_drops.rs:8:5: 8:12
+      let mut _3: std::vec::Vec<bool>;     // in scope 0 at $DIR/remove_unneeded_drops.rs:8:10: 8:11
+      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:8:5: 8:12
+          StorageLive(_3);                 // scope 0 at $DIR/remove_unneeded_drops.rs:8:10: 8:11
+          _3 = move _1;                    // scope 0 at $DIR/remove_unneeded_drops.rs:8:10: 8:11
+          _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:7:1: 9:2
+      }
+
+      bb2: {
+          StorageDead(_3);                 // scope 0 at $DIR/remove_unneeded_drops.rs:8:11: 8:12
+          StorageDead(_2);                 // scope 0 at $DIR/remove_unneeded_drops.rs:8:12: 8:13
+          _0 = const ();                   // scope 0 at $DIR/remove_unneeded_drops.rs:7:27: 9:2
+          return;                          // scope 0 at $DIR/remove_unneeded_drops.rs:9:2: 9:2
+      }
+  }
+

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

@@ -0,0 +1,29 @@
+- // 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:2:8: 2:9
+      let mut _0: ();                      // return place in scope 0 at $DIR/remove_unneeded_drops.rs:2:17: 2:17
+      let _2: ();                          // in scope 0 at $DIR/remove_unneeded_drops.rs:3:5: 3:12
+      let mut _3: bool;                    // in scope 0 at $DIR/remove_unneeded_drops.rs:3:10: 3:11
+      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:3:5: 3:12
+          StorageLive(_3);                 // scope 0 at $DIR/remove_unneeded_drops.rs:3:10: 3:11
+          _3 = _1;                         // scope 0 at $DIR/remove_unneeded_drops.rs:3:10: 3:11
+          _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
++         goto -> bb1;                     // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
+      }
+
+      bb1: {
+          StorageDead(_3);                 // scope 0 at $DIR/remove_unneeded_drops.rs:3:11: 3:12
+          StorageDead(_2);                 // scope 0 at $DIR/remove_unneeded_drops.rs:3:12: 3:13
+          _0 = const ();                   // scope 0 at $DIR/remove_unneeded_drops.rs:2:17: 4:2
+          return;                          // scope 0 at $DIR/remove_unneeded_drops.rs:4:2: 4:2
+      }
+  }
+

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

@@ -0,0 +1,29 @@
+- // 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:12:30: 12:31
+      let mut _0: ();                      // return place in scope 0 at $DIR/remove_unneeded_drops.rs:12:36: 12:36
+      let _2: ();                          // in scope 0 at $DIR/remove_unneeded_drops.rs:13:5: 13:12
+      let mut _3: T;                       // in scope 0 at $DIR/remove_unneeded_drops.rs:13:10: 13:11
+      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:13:5: 13:12
+          StorageLive(_3);                 // scope 0 at $DIR/remove_unneeded_drops.rs:13:10: 13:11
+          _3 = _1;                         // scope 0 at $DIR/remove_unneeded_drops.rs:13:10: 13:11
+          _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
++         goto -> bb1;                     // scope 1 at $SRC_DIR/core/src/mem/mod.rs:LL:COL
+      }
+
+      bb1: {
+          StorageDead(_3);                 // scope 0 at $DIR/remove_unneeded_drops.rs:13:11: 13:12
+          StorageDead(_2);                 // scope 0 at $DIR/remove_unneeded_drops.rs:13:12: 13:13
+          _0 = const ();                   // scope 0 at $DIR/remove_unneeded_drops.rs:12:36: 14:2
+          return;                          // scope 0 at $DIR/remove_unneeded_drops.rs:14:2: 14:2
+      }
+  }
+

src/test/mir-opt/remove_unneeded_drops.rs

@@ -0,0 +1,28 @@
+// 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]);
+}