Ensures top-level functions after guard statements are typechecked

unless skipping all function bodies

Changes in this commit ensure that all top-level functions appearing
after a `guard` statement are always typechecked, unless
`-experimental-skip-all-function-bodies` has been specified.

This means that such functions should be typechecked and SILGen-ed in
the presence of `-experimental-skip-non-inlinable-function-bodies` and
`-experimental-skip-non-inlinable-function-bodies-without-types` flags.

Fixes #57646

Author: jkshtj

lib/AST/Decl.cpp

@@ -9384,6 +9384,17 @@ bool IsFunctionBodySkippedRequest::evaluate(
       skippingMode == FunctionBodySkipping::NonInlinableWithoutTypes)
     return false;
 
+  // If we have a top-level function with local captures, we can't skip it.
+  // A top-level function can have local captures if it appears after a `guard`
+  // statement.
+  if (auto *fd = dyn_cast<FuncDecl>(afd)) {
+    if (fd->hasTopLevelLocalContextCaptures() &&
+        (skippingMode == FunctionBodySkipping::NonInlinable ||
+         skippingMode == FunctionBodySkipping::NonInlinableWithoutTypes)) {
+      return false;
+    }
+  }
+
   // Skip functions that don't need to be serialized.
   return afd->getResilienceExpansion() != ResilienceExpansion::Minimal;
 }

lib/SILOptimizer/UtilityPasses/SILSkippingChecker.cpp

@@ -75,6 +75,15 @@ static bool shouldHaveSkippedFunction(const SILFunction &F) {
       return false;
   }
 
+  // Top-level functions with local captures, should not be skipped.
+  // A top-level function can have local captures if it appears after a
+  // `guard` statement.
+  if (auto *FD = dyn_cast<FuncDecl>(func)) {
+    if (FD->hasTopLevelLocalContextCaptures()) {
+      return false;
+    }
+  }
+
   // Functions with @backDeployed may be copied into the client, so they
   // shouldn't be skipped. The SILFunction that may be copied into the client
   // should be serialized and therefore is already handled above. However, a

test/SILGen/top_level_captures.swift

@@ -1,5 +1,6 @@
 // RUN: %target-swift-frontend -emit-silgen %s | %FileCheck %s
 // RUN: %target-swift-frontend -enable-experimental-async-top-level -emit-silgen %s | %FileCheck %s
+// RUN: %target-swift-frontend -experimental-skip-non-inlinable-function-bodies -experimental-skip-non-inlinable-function-bodies-without-types -emit-silgen %s | %FileCheck %s
 
 guard let x: Int = nil else { while true { } }