Add fast path for trivial drops.

Author: cjgillot

compiler/rustc_mir_transform/src/inline.rs

@@ -611,6 +611,23 @@ fn try_inlining<'tcx, I: Inliner<'tcx>>(
     caller_body: &mut Body<'tcx>,
     callsite: &CallSite<'tcx>,
 ) -> Result<std::ops::Range<BasicBlock>, &'static str> {
+    // Fast path to inline trivial drops.
+    if let InstanceKind::DropGlue(_, None) = callsite.callee.def {
+        let terminator = caller_body[callsite.block].terminator_mut();
+        let target = match terminator.kind {
+            TerminatorKind::Call { target, .. } => target,
+            TerminatorKind::Drop { target, .. } => Some(target),
+            _ => bug!("unexpected terminator kind {:?}", terminator.kind),
+        };
+        if let Some(target) = target {
+            terminator.kind = TerminatorKind::Goto { target };
+        } else {
+            terminator.kind = TerminatorKind::Unreachable;
+        }
+        let next_block = caller_body.basic_blocks.next_index();
+        return Ok(next_block..next_block);
+    }
+
     let tcx = inliner.tcx();
     check_mir_is_available(inliner, caller_body, callsite.callee)?;
 

tests/mir-opt/dest-prop/union.main.DestinationPropagation.panic-abort.diff

@@ -9,8 +9,6 @@
           debug un => _1;
           scope 3 (inlined std::mem::drop::<u32>) {
               debug _x => _2;
-              scope 4 (inlined drop_in_place::<u32> - shim(None)) {
-              }
           }
       }
       scope 2 (inlined val) {

tests/mir-opt/dest-prop/union.main.DestinationPropagation.panic-unwind.diff

@@ -9,8 +9,6 @@
           debug un => _1;
           scope 3 (inlined std::mem::drop::<u32>) {
               debug _x => _2;
-              scope 4 (inlined drop_in_place::<u32> - shim(None)) {
-              }
           }
       }
       scope 2 (inlined val) {

tests/mir-opt/inline/inline_diverging.h.Inline.panic-abort.diff

@@ -7,8 +7,6 @@
 +     let mut _2: fn() -> ! {sleep};
 +     let mut _7: ();
 +     let mut _8: ();
-+     let mut _9: *mut fn() -> ! {sleep};
-+     let mut _10: ();
 +     scope 1 (inlined call_twice::<!, fn() -> ! {sleep}>) {
 +         debug f => _2;
 +         let mut _3: &fn() -> ! {sleep};
@@ -29,8 +27,6 @@
 +             scope 5 (inlined sleep) {
 +             }
 +         }
-+         scope 8 (inlined drop_in_place::<fn() -> ! {sleep}> - shim(None)) {
-+         }
 +     }
 
       bb0: {

tests/mir-opt/inline/inline_diverging.h.Inline.panic-unwind.diff

@@ -7,8 +7,6 @@
 +     let mut _2: fn() -> ! {sleep};
 +     let mut _8: ();
 +     let mut _9: ();
-+     let mut _10: *mut fn() -> ! {sleep};
-+     let mut _11: ();
 +     scope 1 (inlined call_twice::<!, fn() -> ! {sleep}>) {
 +         debug f => _2;
 +         let mut _3: &fn() -> ! {sleep};
@@ -30,8 +28,6 @@
 +             scope 5 (inlined sleep) {
 +             }
 +         }
-+         scope 8 (inlined drop_in_place::<fn() -> ! {sleep}> - shim(None)) {
-+         }
 +     }
 
       bb0: {

tests/mir-opt/pre-codegen/checked_ops.saturating_sub_at_home.PreCodegen.after.panic-abort.mir

@@ -13,8 +13,6 @@ fn saturating_sub_at_home(_1: u32, _2: u32) -> u32 {
         let _6: u32;
         scope 3 {
         }
-        scope 4 (inlined drop_in_place::<u32> - shim(None)) {
-        }
     }
 
     bb0: {

tests/mir-opt/pre-codegen/checked_ops.saturating_sub_at_home.PreCodegen.after.panic-unwind.mir

@@ -13,8 +13,6 @@ fn saturating_sub_at_home(_1: u32, _2: u32) -> u32 {
         let _6: u32;
         scope 3 {
         }
-        scope 4 (inlined drop_in_place::<u32> - shim(None)) {
-        }
     }
 
     bb0: {

tests/mir-opt/pre-codegen/simple_option_map.ezmap.PreCodegen.after.mir

@@ -8,11 +8,9 @@ fn ezmap(_1: Option<i32>) -> Option<i32> {
         let _3: i32;
         let mut _4: i32;
         scope 2 {
-            scope 4 (inlined ezmap::{closure#0}) {
+            scope 3 (inlined ezmap::{closure#0}) {
             }
         }
-        scope 3 (inlined drop_in_place::<{closure@$DIR/simple_option_map.rs:23:12: 23:15}> - shim(None)) {
-        }
     }
 
     bb0: {