Push isolated(any) conversions down through multiple levels of conversion.

I'm not sure there's any way to test this right now.  We don't naturally
emit multiple function conversions on a single operand, and the only way
to get a similar effect is to coerce, which interrupts the application of
`@_inheritActorContext`.  So I think this is dead code until we add closure
isolation controls, and even then it might be dead unless we allow coercion
of isolated closures, which maybe we won't.  But it's the right thing to do
in the abstract, and I was thinking of it now.

Author: rjmccall

lib/SILGen/SILGenConvert.cpp

@@ -1546,9 +1546,45 @@ static bool isMatchedAnyToAnyObjectConversion(CanType from, CanType to) {
   return false;
 }
 
+static Conversion withNewInputType(const Conversion &conv,
+                                   AbstractionPattern origType,
+                                   CanType substType,
+                                   SILType loweredType) {
+  switch (conv.getKind()) {
+  case Conversion::Reabstract:
+    return Conversion::getReabstract(origType, substType, loweredType,
+                                     conv.getReabstractionOutputOrigType(),
+                                     conv.getReabstractionOutputSubstType(),
+                                     conv.getReabstractionOutputLoweredType());
+  case Conversion::Subtype:
+    return Conversion::getSubtype(substType, conv.getBridgingResultType(),
+                                  conv.getBridgingLoweredResultType());
+  default:
+    llvm_unreachable("shouldn't be trying to combine these kinds");
+  }
+}
+
+static Conversion withNewOutputType(const Conversion &conv,
+                                    AbstractionPattern origType,
+                                    CanType substType,
+                                    SILType loweredType) {
+  switch (conv.getKind()) {
+  case Conversion::Reabstract:
+    return Conversion::getReabstract(conv.getReabstractionInputOrigType(),
+                                     conv.getReabstractionInputSubstType(),
+                                     conv.getReabstractionInputLoweredType(),
+                                     origType, substType, loweredType);
+  case Conversion::Subtype:
+    return Conversion::getSubtype(conv.getBridgingSourceType(),
+                                  substType, loweredType);
+  default:
+    llvm_unreachable("shouldn't be trying to combine these kinds");
+  }
+}
+
 /// Can a sequence of conversions from type1 -> type2 -> type3 be represented
 /// as a conversion from type1 -> type3, or does that lose critical information?
-static bool isPeepholeableConversionImpl(CanType type1,
+static bool isCombinableConversionImpl(CanType type1,
                                          CanType type2,
                                          CanType type3) {
   if (type1 == type2 || type2 == type3) return true;
@@ -1563,12 +1599,12 @@ static bool isPeepholeableConversionImpl(CanType type1,
       // If we have optional -> optional conversions at both stages,
       // look through them all.
       if (auto object1 = type1.getOptionalObjectType()) {
-        return isPeepholeableConversionImpl(object1, object2, object3);
+        return isCombinableConversionImpl(object1, object2, object3);
 
       // If we have an injection in the first stage, we'll still know we have
       // an injection in the overall conversion.
       } else {
-        return isPeepholeableConversionImpl(type1, object2, object3);
+        return isCombinableConversionImpl(type1, object2, object3);
       }
 
     // We have an injection in the second stage.  If we lose optionality
@@ -1580,7 +1616,7 @@ static bool isPeepholeableConversionImpl(CanType type1,
 
     // Otherwise, we're preserving that we have an injection overall.
     } else {
-      return isPeepholeableConversionImpl(type1, type2, object3);
+      return isCombinableConversionImpl(type1, type2, object3);
     }
   }
 
@@ -1620,9 +1656,9 @@ static bool isPeepholeableConversionImpl(CanType type1,
     assert(tuple1->getNumElements() == tuple3->getNumElements());
     assert(tuple2->getNumElements() == tuple3->getNumElements());
     for (auto i : range(tuple3->getNumElements())) {
-      if (!isPeepholeableConversionImpl(tuple1.getElementType(i),
-                                        tuple2.getElementType(i),
-                                        tuple3.getElementType(i)))
+      if (!isCombinableConversionImpl(tuple1.getElementType(i),
+                                      tuple2.getElementType(i),
+                                      tuple3.getElementType(i)))
         return false;
     }
     return true;
@@ -1633,15 +1669,15 @@ static bool isPeepholeableConversionImpl(CanType type1,
     auto fn1 = cast<AnyFunctionType>(type1);
     assert(fn1->getNumParams() == fn3->getNumParams());
     assert(fn2->getNumParams() == fn3->getNumParams());
-    if (!isPeepholeableConversionImpl(fn1.getResult(),
-                                      fn2.getResult(),
-                                      fn3.getResult()))
+    if (!isCombinableConversionImpl(fn1.getResult(),
+                                    fn2.getResult(),
+                                    fn3.getResult()))
       return false;
     for (auto i : range(fn3->getNumParams())) {
       // Note the reversal for invariance.
-      if (!isPeepholeableConversionImpl(fn3.getParams()[i].getParameterType(),
-                                        fn2.getParams()[i].getParameterType(),
-                                        fn1.getParams()[i].getParameterType()))
+      if (!isCombinableConversionImpl(fn3.getParams()[i].getParameterType(),
+                                      fn2.getParams()[i].getParameterType(),
+                                      fn1.getParams()[i].getParameterType()))
         return false;
     }
     return true;
@@ -1650,9 +1686,9 @@ static bool isPeepholeableConversionImpl(CanType type1,
   if (auto exp3 = dyn_cast<PackExpansionType>(type3)) {
     auto exp2 = cast<PackExpansionType>(type2);
     auto exp1 = cast<PackExpansionType>(type1);
-    return isPeepholeableConversionImpl(exp1.getPatternType(),
-                                        exp2.getPatternType(),
-                                        exp3.getPatternType());
+    return isCombinableConversionImpl(exp1.getPatternType(),
+                                      exp2.getPatternType(),
+                                      exp3.getPatternType());
   }
 
   // The only remaining types that support subtyping are classes and
@@ -1662,12 +1698,61 @@ static bool isPeepholeableConversionImpl(CanType type1,
 
 /// Can we combine the given conversions so that we go straight from
 /// innerSrcType to outerDestType, or does that lose information?
-static bool isPeepholeableConversion(CanType innerSrcType, CanType innerDestType,
-                                     CanType outerSrcType, CanType outerDestType) {
-  assert(innerDestType == outerSrcType &&
+static bool isCombinableConversion(const Conversion &inner,
+                                   const Conversion &outer) {
+  assert(inner.getResultType() == outer.getSourceType() &&
          "unexpected intermediate conversion");
 
-  return isPeepholeableConversionImpl(innerSrcType, innerDestType, outerDestType);
+  return isCombinableConversionImpl(inner.getSourceType(),
+                                    inner.getResultType(),
+                                    outer.getResultType());
+}
+
+/// Given that we cannot combine the given conversions, at least
+/// "salvage" them to propagate semantically-critical contextual
+/// type information inward.
+static std::optional<CombinedConversions>
+salvageUncombinableConversion(SILGenFunction &SGF,
+                              const Conversion &inner,
+                              const Conversion &outer) {
+  // If the outer type is `@isolated(any)`, and the intermediate type
+  // is non-isolated, propagate the `@isolated(any)` conversion inwards.
+  // We don't want to do this if the intermediate function has some
+  // explicit isolation because we need to honor that conversion even
+  // if it's not the formal isolation of the source function (e.g. if
+  // the user coerces a nonisolated function to a @MainActor function
+  // type).  But if the intermediate function type is non-isolated, the
+  // actual closure might still be isolated, either because we're
+  // type-checking in some mode that doesn't propagate isolation in types
+  // or because the isolation isn't representable in the type system
+  // (e.g. it's isolated to some capture).
+  if (auto outerOutputFnType =
+        dyn_cast<AnyFunctionType>(outer.getResultType())) {
+    auto intermediateFnType = cast<AnyFunctionType>(outer.getSourceType());
+    if (outerOutputFnType->getIsolation().isErased() &&
+        intermediateFnType->getIsolation().isNonIsolated()) {
+      // Construct new intermediate orig/subst/lowered types that are
+      // just the old intermediate type with `@isolated(any)`.
+      auto newIntermediateSubstType = intermediateFnType.withExtInfo(
+        intermediateFnType->getExtInfo().withIsolation(
+          FunctionTypeIsolation::forErased()));
+      auto newIntermediateOrigType =
+        AbstractionPattern(newIntermediateSubstType);
+      auto newIntermediateLoweredType =
+        SGF.getLoweredType(newIntermediateSubstType);
+
+      // Construct the new conversions with the new intermediate type.
+      return CombinedConversions(
+               withNewOutputType(inner, newIntermediateOrigType,
+                                 newIntermediateSubstType,
+                                 newIntermediateLoweredType),
+               withNewInputType(outer, newIntermediateOrigType,
+                                newIntermediateSubstType,
+                                newIntermediateLoweredType));
+    }
+  }
+
+  return std::nullopt;
 }
 
 static std::optional<CombinedConversions>
@@ -1676,11 +1761,8 @@ combineReabstract(SILGenFunction &SGF,
                   const Conversion &inner) {
   // We can never combine conversions in a way that would lose information
   // about the intermediate types.
-  if (!isPeepholeableConversion(inner.getReabstractionInputSubstType(),
-                                inner.getReabstractionOutputSubstType(),
-                                outer.getReabstractionInputSubstType(),
-                                outer.getReabstractionOutputSubstType()))
-    return std::nullopt;
+  if (!isCombinableConversion(inner, outer))
+    return salvageUncombinableConversion(SGF, inner, outer);
 
   // Recognize when the whole conversion is an identity.
   if (inner.getReabstractionInputLoweredType().getObjectType() ==
@@ -1705,11 +1787,8 @@ combineSubtypeIntoReabstract(SILGenFunction &SGF,
                              const Conversion &inner) {
   // We can never combine conversions in a way that would lose information
   // about the intermediate types.
-  if (!isPeepholeableConversion(inner.getBridgingSourceType(),
-                                inner.getBridgingResultType(),
-                                outer.getReabstractionInputSubstType(),
-                                outer.getReabstractionOutputSubstType()))
-    return std::nullopt;
+  if (!isCombinableConversion(inner, outer))
+    return salvageUncombinableConversion(SGF, inner, outer);
 
   auto inputSubstType = inner.getBridgingSourceType();
   auto inputOrigType = AbstractionPattern(inputSubstType);
@@ -1727,11 +1806,8 @@ combineSubtypeIntoReabstract(SILGenFunction &SGF,
 static std::optional<CombinedConversions>
 combineSubtype(SILGenFunction &SGF,
                const Conversion &outer, const Conversion &inner) {
-  if (!isPeepholeableConversion(inner.getBridgingSourceType(),
-                                inner.getBridgingResultType(),
-                                outer.getBridgingSourceType(),
-                                outer.getBridgingResultType()))
-    return std::nullopt;
+  if (!isCombinableConversion(inner, outer))
+    return salvageUncombinableConversion(SGF, inner, outer);
 
   return CombinedConversions(
     Conversion::getSubtype(inner.getBridgingSourceType(),