[SILGen] Add a new CaptureEmission kind specifically for emitting

captured local variables for the assign_by_wrapper setter.

Since assign_by_wrapper will always be re-written to initialization
if the captured local variable is uninitialized, it's unnecessary
to mark the capture as an escape. This lets us support out-of-line
initialization for local property wrappers.

Author: hborla

lib/SILGen/SILGenFunction.cpp

@@ -187,13 +187,18 @@ void SILGenFunction::emitCaptures(SILLocation loc,
   // Partial applications take ownership of the context parameters, so we'll
   // need to pass ownership rather than merely guaranteeing parameters.
   bool canGuarantee;
+  bool captureCanEscape = true;
   switch (purpose) {
   case CaptureEmission::PartialApplication:
     canGuarantee = false;
     break;
   case CaptureEmission::ImmediateApplication:
     canGuarantee = true;
     break;
+  case CaptureEmission::AssignByWrapper:
+    canGuarantee = false;
+    captureCanEscape = false;
+    break;
   }
 
   auto expansion = getTypeExpansionContext();
@@ -381,7 +386,8 @@ void SILGenFunction::emitCaptures(SILLocation loc,
         } else {
           capturedArgs.push_back(emitManagedRetain(loc, Entry.box));
         }
-        escapesToMark.push_back(entryValue);
+        if (captureCanEscape)
+          escapesToMark.push_back(entryValue);
       } else {
         // Address only 'let' values are passed by box.  This isn't great, in
         // that a variable captured by multiple closures will be boxed for each

lib/SILGen/SILGenFunction.h

@@ -115,6 +115,11 @@ enum class CaptureEmission {
   /// Captures are being emitted for partial application to form a closure
   /// value.
   PartialApplication,
+  /// Captures are being emitted for partial application of a local property
+  /// wrapper setter for assign_by_wrapper. Captures are guaranteed to not
+  /// escape, because assign_by_wrapper will not use the setter if the captured
+  /// variable is not initialized.
+  AssignByWrapper,
 };
 
 /// Different ways in which an l-value can be emitted.

lib/SILGen/SILGenLValue.cpp

@@ -1490,7 +1490,7 @@ namespace {
         auto captureInfo = SGF.SGM.Types.getLoweredLocalCaptures(setter);
         if (!captureInfo.getCaptures().empty()) {
           SmallVector<ManagedValue, 4> captures;
-          SGF.emitCaptures(loc, setter, CaptureEmission::PartialApplication, captures);
+          SGF.emitCaptures(loc, setter, CaptureEmission::AssignByWrapper, captures);
 
           for (auto capture : captures)
             capturedArgs.push_back(capture.forward(SGF));