diff --git a/lib/SILGen/LValue.h b/lib/SILGen/LValue.h
index 9b16da86bedcb..32d02f9efccfe 100644
--- a/lib/SILGen/LValue.h
+++ b/lib/SILGen/LValue.h
@@ -105,7 +105,6 @@ class PathComponent {
     CoroutineAccessorKind,      // coroutine accessor
     ValueKind,                  // random base pointer as an lvalue
     PhysicalKeyPathApplicationKind, // applying a key path
-    ABISafeConversionKind,        // unchecked_addr_cast
 
     // Logical LValue kinds
     GetterSetterKind,           // property or subscript getter/setter
@@ -118,6 +117,7 @@ class PathComponent {
     // Translation LValue kinds (a subtype of logical)
     OrigToSubstKind,            // generic type substitution
     SubstToOrigKind,            // generic type substitution
+    UncheckedConversionKind,    // unchecked_X_cast
 
     FirstLogicalKind = GetterSetterKind,
     FirstTranslationKind = OrigToSubstKind,
diff --git a/lib/SILGen/SILGenBuilder.cpp b/lib/SILGen/SILGenBuilder.cpp
index 9a1508a94e6f1..afb59e47fbdd2 100644
--- a/lib/SILGen/SILGenBuilder.cpp
+++ b/lib/SILGen/SILGenBuilder.cpp
@@ -620,7 +620,8 @@ ManagedValue SILGenBuilder::createUncheckedBitCast(SILLocation loc,
   // updated.
   assert((isa<UncheckedTrivialBitCastInst>(cast) ||
           isa<UncheckedRefCastInst>(cast) ||
-          isa<UncheckedBitwiseCastInst>(cast)) &&
+          isa<UncheckedBitwiseCastInst>(cast) ||
+          isa<ConvertFunctionInst>(cast)) &&
          "SILGenBuilder is out of sync with SILBuilder.");
 
   // If we have a trivial inst, just return early.
diff --git a/lib/SILGen/SILGenLValue.cpp b/lib/SILGen/SILGenLValue.cpp
index 6c15d8d30bc02..973d47ef04c98 100644
--- a/lib/SILGen/SILGenLValue.cpp
+++ b/lib/SILGen/SILGenLValue.cpp
@@ -2204,26 +2204,82 @@ namespace {
     }
   };
 
-  /// A physical component which performs an unchecked_addr_cast
-  class ABISafeConversionComponent final : public PhysicalPathComponent {
+  /// A translation component that performs \c unchecked_*_cast 's as-needed.
+  class UncheckedConversionComponent final : public TranslationPathComponent {
+  private:
+    Type OrigType;
+
+    /// \returns the type this component is trying to convert \b to
+    CanType getTranslatedType() const {
+      return getTypeData().SubstFormalType->getCanonicalType();
+    }
+
+    /// \returns the type this component is trying to convert \b from
+    CanType getUntranslatedType() const {
+      return OrigType->getRValueType()->getCanonicalType();
+    }
+
+    /// perform a conversion of ManagedValue -> ManagedValue
+    ManagedValue doUncheckedConversion(SILGenFunction &SGF, SILLocation loc,
+                                       ManagedValue val, CanType toType) {
+      auto toTy = SGF.getLoweredType(toType);
+      auto fromTy = val.getType();
+
+      if (fromTy == toTy)
+        return val; // nothing to do.
+
+      // otherwise emit the right kind of cast based on whether it's an address.
+      assert(fromTy.isAddress() == toTy.isAddress());
+
+      if (toTy.isAddress())
+        return SGF.B.createUncheckedAddrCast(loc, val, toTy);
+
+      return SGF.B.createUncheckedBitCast(loc, val, toTy);
+    }
+
+    /// perform a conversion of RValue -> RValue
+    RValue doUncheckedConversion(SILGenFunction &SGF, SILLocation loc,
+                                 RValue &&rv, CanType toType) {
+      auto val = std::move(rv).getAsSingleValue(SGF, loc);
+      val = doUncheckedConversion(SGF, loc, val, toType);
+      return RValue(SGF, loc, toType, val);
+    }
+
   public:
-    ABISafeConversionComponent(LValueTypeData typeData)
-      : PhysicalPathComponent(typeData, ABISafeConversionKind,
-                              /*actorIsolation=*/None) {}
+    /// \param OrigType is the type we are converting \b from
+    /// \param typeData will contain the type we are converting \b to
+    UncheckedConversionComponent(LValueTypeData typeData, Type OrigType)
+      : TranslationPathComponent(typeData, UncheckedConversionKind),
+      OrigType(OrigType) {}
 
-    ManagedValue project(SILGenFunction &SGF, SILLocation loc,
-                         ManagedValue base) && override {
-      auto toType = SGF.getLoweredType(getTypeData().SubstFormalType)
-                       .getAddressType();
+    bool isLoadingPure() const override { return true; }
 
-      if (base.getType() == toType)
-        return base; // nothing to do
+    /// Used during write operations to convert the value prior to writing to
+    /// the base.
+    RValue untranslate(SILGenFunction &SGF, SILLocation loc,
+                       RValue &&rv, SGFContext c) && override {
+      return doUncheckedConversion(SGF, loc, std::move(rv),
+                getUntranslatedType());
+    }
 
-      return SGF.B.createUncheckedAddrCast(loc, base, toType);
+    /// Used during read operations to convert the value after reading the base.
+    RValue translate(SILGenFunction &SGF, SILLocation loc,
+                     RValue &&rv, SGFContext c) && override {
+      return doUncheckedConversion(SGF, loc, std::move(rv),
+                getTranslatedType());
+    }
+
+    std::unique_ptr<LogicalPathComponent>
+    clone(SILGenFunction &SGF, SILLocation loc) const override {
+      return std::make_unique<UncheckedConversionComponent>(getTypeData(),
+                                                            OrigType);
     }
 
     void dump(raw_ostream &OS, unsigned indent) const override {
-      OS.indent(indent) << "ABISafeConversionComponent\n";
+      OS.indent(indent) << "UncheckedConversionComponent"
+                        << "\n\tfromType: " << getUntranslatedType()
+                        << "\n\ttoType: " << getTranslatedType()
+                        << "\n";
     }
   };
 } // end anonymous namespace
@@ -3741,7 +3797,9 @@ LValue SILGenLValue::visitABISafeConversionExpr(ABISafeConversionExpr *e,
   LValue lval = visitRec(e->getSubExpr(), accessKind, options);
   auto typeData = getValueTypeData(SGF, accessKind, e);
 
-  lval.add<ABISafeConversionComponent>(typeData);
+  auto OrigType = e->getSubExpr()->getType();
+
+  lval.add<UncheckedConversionComponent>(typeData, OrigType);
 
   return lval;
 }
diff --git a/test/SILGen/objc_preconcurrency.swift b/test/SILGen/objc_preconcurrency.swift
index 69fa8b9474703..3b577a984b184 100644
--- a/test/SILGen/objc_preconcurrency.swift
+++ b/test/SILGen/objc_preconcurrency.swift
@@ -7,6 +7,13 @@
   @preconcurrency var sendyHandler: @Sendable () -> Void { get set }
 }
 
+@preconcurrency class OldWorld {
+  @preconcurrency var handler: (@Sendable () -> Void)?
+  @preconcurrency var mainHandler: (@MainActor () -> Void)?
+  @preconcurrency var nonOptionalHandler: @Sendable () -> Void = {}
+  @preconcurrency var nonOptionalMainHandler: @MainActor () -> Void = {}
+}
+
 // CHECK-LABEL: sil hidden [ossa] @$s19objc_preconcurrency19testDynamicDispatch1p17completionHandleryAA1P_p_yyctF
 // CHECK: dynamic_method_br
 // CHECK: bb{{[0-9]+}}(%{{[0-9]+}} : $@convention(objc_method) (@convention(block) @Sendable () -> (), @opened
@@ -19,18 +26,121 @@ func testDynamicDispatch(p: P, completionHandler: @escaping () -> Void) {
 }
 
 // CHECK-LABEL: sil hidden [ossa] @$s19objc_preconcurrency21testOptionalVarAccessyySo12NSTouchGrassCF
-// CHECK:         unchecked_addr_cast {{.*}} : $*Optional<@Sendable @callee_guaranteed () -> ()> to $*Optional<@callee_guaranteed () -> ()>
+// CHECK:         unchecked_bitwise_cast {{.*}} : $Optional<@Sendable @callee_guaranteed () -> ()> to $Optional<@callee_guaranteed () -> ()>
 // CHECK:       } // end sil function '$s19objc_preconcurrency21testOptionalVarAccessyySo12NSTouchGrassCF'
 func testOptionalVarAccess(_ grass: NSTouchGrass) {
   grass.cancellationHandler?()
 }
 
+// CHECK-LABEL: sil hidden [ossa] @$s19objc_preconcurrency33testOptionalVarAccessPartialApplyyyycSgSo12NSTouchGrassCF
+// CHECK:         unchecked_bitwise_cast {{.*}} : $Optional<@Sendable @callee_guaranteed () -> ()> to $Optional<@callee_guaranteed () -> ()>
+// CHECK: } // end sil function '$s19objc_preconcurrency33testOptionalVarAccessPartialApplyyyycSgSo12NSTouchGrassCF'
+func testOptionalVarAccessPartialApply(_ grass: NSTouchGrass) -> (() -> Void)? {
+  let handler = grass.cancellationHandler
+  if let unwrapped = handler {
+    unwrapped()
+  }
+  return handler
+}
+
+// CHECK-LABEL: sil hidden [ossa] @$s19objc_preconcurrency16testObjCVarWriteyySo12NSTouchGrassCF
+// CHECK:         unchecked_bitwise_cast {{.*}} : $Optional<@callee_guaranteed () -> ()> to $Optional<@Sendable @callee_guaranteed () -> ()>
+// CHECK:         objc_method {{.*}} : $NSTouchGrass, #NSTouchGrass.cancellationHandler!setter.foreign : (NSTouchGrass) -> ((@Sendable () -> ())?) -> (), $@convention(objc_method) (Optional<@convention(block) @Sendable () -> ()>, NSTouchGrass) -> ()
+// CHECK:       } // end sil function '$s19objc_preconcurrency16testObjCVarWriteyySo12NSTouchGrassCF'
+func testObjCVarWrite(_ grass: NSTouchGrass) {
+  grass.cancellationHandler = {}
+}
+
+// the below looks kinda long and wonky, but is expected. for a summary, the steps are:
+// 1. objc to native
+// 2. Sendable to non-Sendable (major part of this test)
+// 3. non-optional to optional
+// 4. from non-Sendable to Sendable (major part of this test)
+// 5. from native to objc (which involves unwrapping and rewrapping that optional; kinda silly but optimization will clean it up)
+//
+// CHECK-LABEL: sil hidden [ossa] @$s19objc_preconcurrency22testObjCVarWriteAcrossyySo12NSTouchGrassCF
+// CHECK:         [[GET_EXCEPTION:%[0-9]+]] = objc_method {{.*}} : $NSTouchGrass, #NSTouchGrass.exceptionHandler!getter.foreign
+// CHECK:         [[SENDABLE_BLOCK:%[0-9]+]] = apply [[GET_EXCEPTION]]({{.*}}) : $@convention(objc_method) (NSTouchGrass) -> @autoreleased @convention(block) @Sendable () -> ()
+//                           << step 1 >>
+// CHECK:         [[NATIVE_THUNK:%[0-9]+]] = function_ref @$sIeyBh_Iegh_TR : $@convention(thin) @Sendable (@guaranteed @convention(block) @Sendable () -> ()) -> ()
+// CHECK:         [[NATIVE_SENDABLE_EXCEPTION:%[0-9]+]] = partial_apply [callee_guaranteed] [[NATIVE_THUNK]]([[SENDABLE_BLOCK]])
+//                           << step 2 >>
+// CHECK:         [[NATIVE_EXCEPTION:%[0-9]+]] = convert_function [[NATIVE_SENDABLE_EXCEPTION]] : $@Sendable @callee_guaranteed () -> () to $@callee_guaranteed () -> ()
+//                           << step 3 >>
+// CHECK:         [[OPTIONAL_NATIVE_EXCEPTION:%[0-9]+]] = enum $Optional<@callee_guaranteed () -> ()>, #Optional.some!enumelt, [[NATIVE_EXCEPTION]] : $@callee_guaranteed () -> ()
+//                           << step 4 >>
+// CHECK:         = unchecked_bitwise_cast [[OPTIONAL_NATIVE_EXCEPTION]] : $Optional<@callee_guaranteed () -> ()> to $Optional<@Sendable @callee_guaranteed () -> ()>
+//                           << step 5 >>
+// CHECK:         switch_enum {{.*}} : $Optional<@Sendable @callee_guaranteed () -> ()>
+//
+// CHECK:         bb1({{.*}} : @owned $@Sendable @callee_guaranteed () -> ()):
+// CHECK:           init_block_storage_header {{.*}} : $*@block_storage @Sendable @callee_guaranteed () -> ()
+// CHECK:       } // end sil function '$s19objc_preconcurrency22testObjCVarWriteAcrossyySo12NSTouchGrassCF'
+func testObjCVarWriteAcross(_ grass: NSTouchGrass) {
+  grass.cancellationHandler = grass.exceptionHandler // *slaps roof of assignment* this bad boy can fit so much conversion in it!
+}
+
+// CHECK-LABEL: sil hidden [ossa] @$s19objc_preconcurrency25testOptionalAssignSetter1yyAA8OldWorldCF
+// CHECK:  unchecked_bitwise_cast {{.*}} : $Optional<@callee_guaranteed () -> ()> to $Optional<@Sendable @callee_guaranteed () -> ()>
+// CHECK:  #OldWorld.handler!setter : (OldWorld) -> ((@Sendable () -> ())?) -> ()
+// CHECK: } // end sil function '$s19objc_preconcurrency25testOptionalAssignSetter1yyAA8OldWorldCF'
+func testOptionalAssignSetter1(_ oldWorld: OldWorld) {
+  oldWorld.handler = {}
+}
+
+// CHECK-LABEL: sil hidden [ossa] @$s19objc_preconcurrency25testOptionalAssignSetter2yyAA8OldWorldCF
+// CHECK:  convert_function {{.*}} : $@callee_guaranteed () -> () to $@Sendable @callee_guaranteed () -> ()
+// CHECK:  $OldWorld, #OldWorld.nonOptionalHandler!setter : (OldWorld) -> (@escaping @Sendable () -> ()) -> ()
+// CHECK: } // end sil function '$s19objc_preconcurrency25testOptionalAssignSetter2yyAA8OldWorldCF'
+func testOptionalAssignSetter2(_ oldWorld: OldWorld) {
+  oldWorld.nonOptionalHandler = {}
+}
+
+// CHECK-LABEL: sil hidden [ossa] @$s19objc_preconcurrency21testMainHandlerWritesyyAA8OldWorldCF
+// CHECK:         = unchecked_bitwise_cast {{.*}} : $Optional<@callee_guaranteed () -> ()> to $Optional<@Sendable @callee_guaranteed () -> ()>
+// CHECK:         = unchecked_bitwise_cast {{.*}} : $Optional<@Sendable @callee_guaranteed () -> ()> to $Optional<@callee_guaranteed () -> ()>
+// CHECK:       } // end sil function '$s19objc_preconcurrency21testMainHandlerWritesyyAA8OldWorldCF'
+func testMainHandlerWrites(_ oldWorld: OldWorld) {
+  oldWorld.handler = oldWorld.mainHandler
+  oldWorld.mainHandler = oldWorld.handler
+}
+
+// CHECK-LABEL: sil hidden [ossa] @$s19objc_preconcurrency32testMainHandlerNonOptionalWritesyyAA8OldWorldCF
+// CHECK:         = convert_function {{.*}} : $@callee_guaranteed () -> () to $@Sendable @callee_guaranteed () -> ()
+// CHECK:         = convert_function {{.*}} : $@Sendable @callee_guaranteed () -> () to $@callee_guaranteed () -> ()
+// CHECK:       } // end sil function '$s19objc_preconcurrency32testMainHandlerNonOptionalWritesyyAA8OldWorldCF'
+func testMainHandlerNonOptionalWrites(_ oldWorld: OldWorld) {
+  oldWorld.nonOptionalHandler = oldWorld.nonOptionalMainHandler
+  oldWorld.nonOptionalMainHandler = oldWorld.nonOptionalHandler
+}
+
+// CHECK-LABEL: sil hidden [ossa] @$s19objc_preconcurrency15testMixedWritesyyAA8OldWorldCF
+//
+//                << sendable conversions should be here >>
+// CHECK:         = unchecked_bitwise_cast {{.*}} : $Optional<@Sendable @callee_guaranteed () -> ()> to $Optional<@callee_guaranteed () -> ()>
+// CHECK:         = convert_function {{.*}} : $@callee_guaranteed () -> () to $@Sendable @callee_guaranteed () -> ()
+//
+//                << but main actor type mismatches are accepted by SIL >>
+// CHECK:         [[NO_MAIN_ACTOR:%[0-9]+]] = partial_apply {{.*}} : $@convention(thin) (@guaranteed @callee_guaranteed () -> @out ()) -> ()
+// CHECK:         [[SETTER:%[0-9]+]] = class_method {{.*}} : $OldWorld, #OldWorld.nonOptionalMainHandler!setter : (OldWorld) -> (@escaping @MainActor () -> ()) -> (), $@convention(method) (@owned @callee_guaranteed () -> (), @guaranteed OldWorld) -> ()
+// CHECK:         apply [[SETTER]]([[NO_MAIN_ACTOR]]
+// CHECK:       } // end sil function '$s19objc_preconcurrency15testMixedWritesyyAA8OldWorldCF'
+func testMixedWrites(_ oldWorld: OldWorld) {
+  oldWorld.nonOptionalHandler = oldWorld.handler ?? {}
+  oldWorld.nonOptionalMainHandler = oldWorld.mainHandler ?? {}
+}
+
 func modify(_ v: inout () -> Void) {
   v = {}
 }
 
 // CHECK-LABEL: sil hidden [ossa] @$s19objc_preconcurrency15testInoutAccessyySo12NSTouchGrassCF
-// CHECK:         unchecked_addr_cast {{.*}} : $*@Sendable @callee_guaranteed () -> () to $*@callee_guaranteed () -> ()
+// CHECK:         [[BEFORE_MODIFY:%[0-9]+]] = convert_function {{.*}} : $@Sendable @callee_guaranteed () -> () to $@callee_guaranteed () -> ()
+// CHECK:         store [[BEFORE_MODIFY]] to [init] [[INOUT_ALLOC:%[0-9]+]] : $*@callee_guaranteed () -> ()
+// CHECK:         [[MODIFY_FN:%[0-9]+]] = function_ref @$s19objc_preconcurrency6modifyyyyyczF : $@convention(thin) (@inout @callee_guaranteed () -> ()) -> ()
+// CHECK:         = apply [[MODIFY_FN]]([[INOUT_ALLOC]])
+// CHECK:         [[AFTER_MODIFY:%[0-9]+]] = load [take] [[INOUT_ALLOC]] : $*@callee_guaranteed () -> ()
+// CHECK:         convert_function [[AFTER_MODIFY]] : $@callee_guaranteed () -> () to $@Sendable @callee_guaranteed () -> ()
 // CHECK:       } // end sil function '$s19objc_preconcurrency15testInoutAccessyySo12NSTouchGrassCF'
 func testInoutAccess(_ grass: NSTouchGrass) {
   modify(&grass.exceptionHandler)
@@ -38,7 +148,12 @@ func testInoutAccess(_ grass: NSTouchGrass) {
 
 
 // CHECK-LABEL: sil hidden [ossa] @$s19objc_preconcurrency21testProtocolVarAccess1pyAA1P_p_tF
-// CHECK:         unchecked_addr_cast {{.*}} : $*@Sendable @callee_guaranteed () -> () to $*@callee_guaranteed () -> ()
+// CHECK:         [[BEFORE_MODIFY:%[0-9]+]] = convert_function {{.*}} : $@Sendable @callee_guaranteed () -> () to $@callee_guaranteed () -> ()
+// CHECK:         store [[BEFORE_MODIFY]] to [init] [[INOUT_ALLOC:%[0-9]+]] : $*@callee_guaranteed () -> ()
+// CHECK:         [[MODIFY_FN:%[0-9]+]] = function_ref @$s19objc_preconcurrency6modifyyyyyczF : $@convention(thin) (@inout @callee_guaranteed () -> ()) -> ()
+// CHECK:         = apply [[MODIFY_FN]]([[INOUT_ALLOC]])
+// CHECK:         [[AFTER_MODIFY:%[0-9]+]] = load [take] [[INOUT_ALLOC]] : $*@callee_guaranteed () -> ()
+// CHECK:         convert_function [[AFTER_MODIFY]] : $@callee_guaranteed () -> () to $@Sendable @callee_guaranteed () -> ()
 // CHECK:       } // end sil function '$s19objc_preconcurrency21testProtocolVarAccess1pyAA1P_p_tF'
 func testProtocolVarAccess(p: P) {
   modify(&p.sendyHandler)