[SILOpt] Fix mutable state in EagerSpecializer (#70083)

* [SILOpt] Fix mutable state in EagerSpecializer

rdar://118554892

On the first occurance of a `@_specialize` attribute with `target:` argument, `onlyCreatePrespecializations` would be set to `true`, preventing any subsequent pre-specializations with `exported: false`

* Fix test on non-darwin platforms

Author: drexin

lib/SILOptimizer/Transforms/EagerSpecializer.cpp

@@ -746,7 +746,7 @@ SILValue EagerDispatch::emitArgumentConversion(
 
 namespace {
 class EagerSpecializerTransform : public SILFunctionTransform {
-  bool onlyCreatePrespecializations;
+  const bool onlyCreatePrespecializations;
 public:
   EagerSpecializerTransform(bool onlyPrespecialize)
       : onlyCreatePrespecializations(onlyPrespecialize) {}
@@ -834,6 +834,8 @@ void EagerSpecializerTransform::run() {
   // performed.
   SmallVector<SILSpecializeAttr *, 8> attrsToRemove;
 
+  bool onlyCreatePrespecializations = this->onlyCreatePrespecializations;
+
   for (auto *SA : F.getSpecializeAttrs()) {
     if (onlyCreatePrespecializations && !SA->isExported()) {
       attrsToRemove.push_back(SA);

test/SILOptimizer/pre_specialize_rdar118554892.swift

@@ -0,0 +1,39 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -parse-stdlib -O -Xllvm -sil-disable-pass=function-signature-opts -emit-sil %s | %FileCheck %s
+
+// Regression test for rdar://118554892
+
+import Swift
+
+public struct Foo {}
+
+@_specializeExtension
+extension Array {
+  // The `target:` here triggered the bug we are testing for here. So while we don't test this
+  // specialization itself, it has to remain here for the test to properly function.
+  @_specialize(exported: true,
+               target: _endMutation(),
+               where Element == Foo)
+  @usableFromInline
+  mutating func rdar118554892__specialize_class__endMutation(){ Builtin.unreachable() }
+}
+
+extension Sequence where Element: StringProtocol {
+  // CHECK-DAG: sil shared [noinline] @$sST28pre_specialize_rdar118554892Sy7ElementRpzrlE3xxxSSyFSaySSG_Tg5 : $@convention(method) (@guaranteed Array<String>)
+  // CHECK-DAG: sil shared [noinline] @$sST28pre_specialize_rdar118554892Sy7ElementRpzrlE3xxxSSyFSaySsG_Tg5 : $@convention(method) (@guaranteed Array<Substring>)
+  @_specialize(where Self == Array<Substring>)
+  @_specialize(where Self == Array<String>)
+  @inline(never) // prevent inlining the specialization into the function body
+  public func xxx() -> String {
+    return _xxx()
+  }
+
+  @inline(__always)
+  internal func _xxx() -> String {
+    var result = ""
+    for x in self {
+      result.append(x._ephemeralString)
+    }
+    return result
+  }
+}