Do not create move paths that do not need dropping.

compiler/rustc_mir_transform/src/elaborate_drops.rs

@@ -54,7 +54,10 @@ impl<'tcx> MirPass<'tcx> for ElaborateDrops {
 
         let def_id = body.source.def_id();
         let param_env = tcx.param_env_reveal_all_normalized(def_id);
-        let move_data = MoveData::gather_moves(&body, tcx, param_env, |_| true);
+        // For types that do not need dropping, the behaviour is trivial. So we only need to track
+        // init/uninit for types that do need dropping.
+        let move_data =
+            MoveData::gather_moves(&body, tcx, param_env, |ty| ty.needs_drop(tcx, param_env));
         let elaborate_patch = {
             let env = MoveDataParamEnv { move_data, param_env };
 
@@ -338,6 +341,18 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
                 continue;
             };
 
+            // This place does not need dropping. It is not have an associated move-path, so the
+            // match below will conservatively keep an unconditional drop. As that drop is useless,
+            // just remove it here and now.
+            if !place
+                .ty(&self.body.local_decls, self.tcx)
+                .ty
+                .needs_drop(self.tcx, self.env.param_env)
+            {
+                self.patch.patch_terminator(bb, TerminatorKind::Goto { target });
+                continue;
+            }
+
             let path = self.move_data().rev_lookup.find(place.as_ref());
             match path {
                 LookupResult::Exact(path) => {

compiler/rustc_mir_transform/src/remove_uninit_drops.rs

@@ -24,7 +24,8 @@ pub struct RemoveUninitDrops;
 impl<'tcx> MirPass<'tcx> for RemoveUninitDrops {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         let param_env = tcx.param_env(body.source.def_id());
-        let move_data = MoveData::gather_moves(&body, tcx, param_env, |_| true);
+        let move_data =
+            MoveData::gather_moves(&body, tcx, param_env, |ty| ty.needs_drop(tcx, param_env));
 
         let mdpe = MoveDataParamEnv { move_data, param_env };
         let mut maybe_inits = MaybeInitializedPlaces::new(tcx, body, &mdpe)

tests/mir-opt/inline/unsized_argument.caller.Inline.diff

@@ -6,24 +6,18 @@
       let mut _0: ();
       let _2: ();
       let mut _3: std::boxed::Box<[i32]>;
-      let mut _4: &mut std::boxed::Box<[i32]>;
+      let mut _4: *const [i32];
-      let mut _5: ();
-      let mut _6: &mut std::boxed::Box<[i32]>;
-      let mut _7: ();
-      let mut _8: &mut std::boxed::Box<[i32]>;
-      let mut _9: ();
-      let mut _10: *const [i32];
   
       bb0: {
           StorageLive(_2);
           StorageLive(_3);
           _3 = move _1;
-          _10 = (((_3.0: std::ptr::Unique<[i32]>).0: std::ptr::NonNull<[i32]>).0: *const [i32]);
+          _4 = (((_3.0: std::ptr::Unique<[i32]>).0: std::ptr::NonNull<[i32]>).0: *const [i32]);
-          _2 = callee(move (*_10)) -> [return: bb3, unwind: bb4];
+          _2 = callee(move (*_4)) -> [return: bb1, unwind: bb3];
       }
   
-      bb1 (cleanup): {
+      bb1: {
-          resume;
+          drop(_3) -> [return: bb2, unwind: bb4];
       }
   
       bb2: {
@@ -33,14 +27,12 @@
           return;
       }
   
-      bb3: {
+      bb3 (cleanup): {
-          _4 = &mut _3;
+          drop(_3) -> [return: bb4, unwind terminate(cleanup)];
-          _5 = <Box<[i32]> as Drop>::drop(move _4) -> [return: bb2, unwind: bb1];
       }
   
       bb4 (cleanup): {
-          _8 = &mut _3;
+          resume;
-          _9 = <Box<[i32]> as Drop>::drop(move _8) -> [return: bb1, unwind terminate(cleanup)];
       }
   }
   

tests/mir-opt/issue_41888.main.ElaborateDrops.panic-abort.diff

@@ -10,9 +10,8 @@
       let mut _5: isize;
 +     let mut _7: bool;
 +     let mut _8: bool;
-+     let mut _9: bool;
++     let mut _9: isize;
 +     let mut _10: isize;
-+     let mut _11: isize;
       scope 1 {
           debug e => _1;
           scope 2 {
@@ -24,7 +23,6 @@
       bb0: {
 +         _7 = const false;
 +         _8 = const false;
-+         _9 = const false;
           StorageLive(_1);
           StorageLive(_2);
           _2 = cond() -> [return: bb1, unwind: bb11];
@@ -47,7 +45,6 @@
       bb3: {
 +         _7 = const true;
 +         _8 = const true;
-+         _9 = const true;
           _1 = move _3;
 -         drop(_3) -> [return: bb5, unwind: bb11];
 +         goto -> bb5;
@@ -56,7 +53,6 @@
       bb4 (cleanup): {
 +         _7 = const true;
 +         _8 = const true;
-+         _9 = const true;
           _1 = move _3;
 -         drop(_3) -> [return: bb11, unwind terminate(cleanup)];
 +         goto -> bb11;
@@ -70,7 +66,6 @@
   
       bb6: {
           StorageLive(_6);
-+         _9 = const false;
           _6 = move ((_1 as F).0: K);
           _0 = const ();
           StorageDead(_6);
@@ -90,13 +85,12 @@
       bb9: {
           StorageDead(_2);
 -         drop(_1) -> [return: bb10, unwind: bb12];
-+         goto -> bb18;
++         goto -> bb19;
       }
   
       bb10: {
 +         _7 = const false;
 +         _8 = const false;
-+         _9 = const false;
           StorageDead(_1);
           return;
       }
@@ -116,33 +110,37 @@
 +     }
 + 
 +     bb14 (cleanup): {
-+         goto -> bb12;
++         drop(((_1 as F).0: K)) -> [return: bb12, unwind terminate(cleanup)];
 +     }
 + 
-+     bb15: {
++     bb15 (cleanup): {
++         switchInt(_7) -> [0: bb12, otherwise: bb14];
++     }
++ 
++     bb16: {
 +         drop(_1) -> [return: bb13, unwind: bb12];
 +     }
 + 
-+     bb16 (cleanup): {
++     bb17 (cleanup): {
 +         drop(_1) -> [return: bb12, unwind terminate(cleanup)];
 +     }
 + 
-+     bb17: {
-+         _10 = discriminant(_1);
-+         switchInt(move _10) -> [0: bb13, otherwise: bb15];
-+     }
-+ 
 +     bb18: {
-+         switchInt(_7) -> [0: bb13, otherwise: bb17];
++         _9 = discriminant(_1);
++         switchInt(move _9) -> [0: bb13, otherwise: bb16];
 +     }
 + 
-+     bb19 (cleanup): {
++     bb19: {
-+         _11 = discriminant(_1);
++         switchInt(_7) -> [0: bb13, otherwise: bb18];
-+         switchInt(move _11) -> [0: bb14, otherwise: bb16];
 +     }
 + 
 +     bb20 (cleanup): {
-+         switchInt(_7) -> [0: bb12, otherwise: bb19];
++         _10 = discriminant(_1);
++         switchInt(move _10) -> [0: bb15, otherwise: bb17];
++     }
++ 
++     bb21 (cleanup): {
++         switchInt(_7) -> [0: bb12, otherwise: bb20];
       }
   }
   

tests/mir-opt/issue_41888.main.ElaborateDrops.panic-unwind.diff

@@ -10,9 +10,8 @@
       let mut _5: isize;
 +     let mut _7: bool;
 +     let mut _8: bool;
-+     let mut _9: bool;
++     let mut _9: isize;
 +     let mut _10: isize;
-+     let mut _11: isize;
       scope 1 {
           debug e => _1;
           scope 2 {
@@ -24,7 +23,6 @@
       bb0: {
 +         _7 = const false;
 +         _8 = const false;
-+         _9 = const false;
           StorageLive(_1);
           StorageLive(_2);
           _2 = cond() -> [return: bb1, unwind: bb11];
@@ -47,7 +45,6 @@
       bb3: {
 +         _7 = const true;
 +         _8 = const true;
-+         _9 = const true;
           _1 = move _3;
 -         drop(_3) -> [return: bb5, unwind: bb11];
 +         goto -> bb5;
@@ -56,7 +53,6 @@
       bb4 (cleanup): {
 +         _7 = const true;
 +         _8 = const true;
-+         _9 = const true;
           _1 = move _3;
 -         drop(_3) -> [return: bb11, unwind terminate(cleanup)];
 +         goto -> bb11;
@@ -70,7 +66,6 @@
   
       bb6: {
           StorageLive(_6);
-+         _9 = const false;
           _6 = move ((_1 as F).0: K);
           _0 = const ();
           StorageDead(_6);
@@ -90,13 +85,12 @@
       bb9: {
           StorageDead(_2);
 -         drop(_1) -> [return: bb10, unwind continue];
-+         goto -> bb18;
++         goto -> bb19;
       }
   
       bb10: {
 +         _7 = const false;
 +         _8 = const false;
-+         _9 = const false;
           StorageDead(_1);
           return;
       }
@@ -116,33 +110,37 @@
 +     }
 + 
 +     bb14 (cleanup): {
-+         goto -> bb12;
++         drop(((_1 as F).0: K)) -> [return: bb12, unwind terminate(cleanup)];
 +     }
 + 
-+     bb15: {
++     bb15 (cleanup): {
++         switchInt(_7) -> [0: bb12, otherwise: bb14];
++     }
++ 
++     bb16: {
 +         drop(_1) -> [return: bb13, unwind: bb12];
 +     }
 + 
-+     bb16 (cleanup): {
++     bb17 (cleanup): {
 +         drop(_1) -> [return: bb12, unwind terminate(cleanup)];
 +     }
 + 
-+     bb17: {
-+         _10 = discriminant(_1);
-+         switchInt(move _10) -> [0: bb13, otherwise: bb15];
-+     }
-+ 
 +     bb18: {
-+         switchInt(_7) -> [0: bb13, otherwise: bb17];
++         _9 = discriminant(_1);
++         switchInt(move _9) -> [0: bb13, otherwise: bb16];
 +     }
 + 
-+     bb19 (cleanup): {
++     bb19: {
-+         _11 = discriminant(_1);
++         switchInt(_7) -> [0: bb13, otherwise: bb18];
-+         switchInt(move _11) -> [0: bb14, otherwise: bb16];
 +     }
 + 
 +     bb20 (cleanup): {
-+         switchInt(_7) -> [0: bb12, otherwise: bb19];
++         _10 = discriminant(_1);
++         switchInt(move _10) -> [0: bb15, otherwise: bb17];
++     }
++ 
++     bb21 (cleanup): {
++         switchInt(_7) -> [0: bb12, otherwise: bb20];
       }
   }