FunctionSignatureOptimization: don't convert indirect @in to @in_guaranteed if the argument is mutated

This fixes a verifier error because an `@in_guaranteed` argument must not be mutated.

#81444
rdar://151147971

Author: eeckstein

lib/SILOptimizer/FunctionSignatureTransforms/OwnedToGuaranteedTransform.cpp

@@ -88,6 +88,15 @@ bool FunctionSignatureTransform::OwnedToGuaranteedAnalyzeParameters() {
       continue;
     }
 
+    // Make sure that an @in argument is not mutated otherwise than destroyed,
+    // because an @in_guaranteed argument must not be mutated.
+    if (A.hasConvention(SILArgumentConvention::Indirect_In) &&
+        // With opaque values, @in arguments can have non-address types.
+        A.Arg->getType().isAddress() &&
+        isIndirectArgumentMutated(A.Arg, /*ignoreDestroys=*/ true, /*defaultIsMutating=*/true)) {
+      continue;
+    }
+  
     // See if we can find a ref count equivalent strong_release or release_value
     // at the end of this function if our argument is an @owned parameter.
     // See if we can find a destroy_addr at the end of this function if our

test/SILOptimizer/functionsigopts_crash.swift

@@ -35,3 +35,21 @@ func testit(_ p: P) {
 public func callit(s: S) {
   testit(s)
 }
+
+// Check that FSO does not trigger a verifier error caused by a mutated @in argument which is 
+// converted to an @in_guaranteed argument (which must not be mutated).
+
+public protocol IP<Element> {
+  associatedtype Element
+
+  init<Iterator>(iterator: consuming Iterator) where Iterator: IteratorProtocol, Iterator.Element == Element
+}
+
+extension Array: IP {
+  public init<Iterator>(iterator: consuming Iterator) where Iterator: IteratorProtocol, Iterator.Element == Element {
+    self.init()
+    while let next = iterator.next() {
+      append(next)
+    }
+  }
+}