Remove "unsafe" keyword from expressions when printing inlinable code

To help older compilers that don't yet support the unsafe expression
deal with the Swift interface files we produce, remove the "unsafe" from
expressions in inlinable code.

Author: DougGregor

lib/ASTGen/Sources/ASTGen/CompilerBuildConfiguration.swift

@@ -13,8 +13,8 @@
 import ASTBridging
 import SwiftDiagnostics
 @_spi(Compiler) import SwiftIfConfig
-import SwiftParser
-import SwiftSyntax
+@_spi(ExperimentalLanguageFeatures) import SwiftParser
+@_spi(ExperimentalLanguageFeatures) import SwiftSyntax
 
 /// A build configuration that uses the compiler's ASTContext to answer
 /// queries.
@@ -517,7 +517,11 @@ public func extractInlinableText(
   sourceText: BridgedStringRef
 ) -> BridgedStringRef {
   let textBuffer = UnsafeBufferPointer<UInt8>(start: sourceText.data, count: sourceText.count)
-  var parser = Parser(textBuffer)
+  var parser = Parser(
+    textBuffer,
+    swiftVersion: Parser.SwiftVersion(from: astContext),
+    experimentalFeatures: Parser.ExperimentalFeatures(from: astContext)
+  )
   let syntax = SourceFileSyntax.parse(from: &parser)
 
   let configuration = CompilerBuildConfiguration(
@@ -531,6 +535,24 @@ public func extractInlinableText(
     retainFeatureCheckIfConfigs: true
   ).result
 
+  // Remove "unsafe" expressions.
+  let inlineableSyntax = syntaxWithoutInactive.withoutUnsafeExpressions
+
   // Remove comments and return the result.
-  return allocateBridgedString(syntaxWithoutInactive.descriptionWithoutCommentsAndSourceLocations)
+  return allocateBridgedString(inlineableSyntax.descriptionWithoutCommentsAndSourceLocations)
+}
+
+/// Used by withoutUnsafeExpressions to remove "unsafe" expressions from
+/// a syntax tree.
+fileprivate class RemoveUnsafeExprSyntaxRewriter: SyntaxRewriter {
+  override func visit(_ node: UnsafeExprSyntax) -> ExprSyntax {
+    return node.expression.with(\.leadingTrivia, node.leadingTrivia)
+  }
+}
+
+extension SyntaxProtocol {
+  /// Return a new syntax tree with all "unsafe" expressions removed.
+  var withoutUnsafeExpressions: Syntax {
+    RemoveUnsafeExprSyntaxRewriter().rewrite(self)
+  }
 }

test/Unsafe/module-interface.swift

@@ -0,0 +1,34 @@
+// RUN: %target-swift-emit-module-interface(%t.swiftinterface) %s -module-name UserModule -enable-experimental-feature AllowUnsafeAttribute  -enable-experimental-feature WarnUnsafe
+// RUN: %target-swift-typecheck-module-from-interface(%t.swiftinterface) -module-name UserModule
+// RUN: %FileCheck %s < %t.swiftinterface
+
+// REQUIRES: swift_feature_AllowUnsafeAttribute
+// REQUIRES: swift_feature_WarnUnsafe
+
+// CHECK: #if compiler(>=5.3) && $AllowUnsafeAttribute
+// CHECK: @unsafe public func getIntUnsafely() -> Swift.Int
+// CHECK: #else
+// CHECK: public func getIntUnsafely() -> Swift.Int
+// CHECK: #endif
+@unsafe public func getIntUnsafely() -> Int { 0 }
+
+// CHECK: @inlinable public func useUnsafeCode()
+@inlinable public func useUnsafeCode() {
+  // CHECK-NOT: unsafe
+  print( unsafe getIntUnsafely())
+}
+
+// CHECK: public protocol P
+public protocol P {
+  func f()
+}
+
+// CHECK: public struct X : @unsafe UserModule.P
+public struct X: @unsafe P {
+// CHECK:  #if compiler(>=5.3) && $AllowUnsafeAttribute
+// CHECK:  @unsafe public func f()
+// CHECK:  #else
+// CHECK:  public func f()
+// CHECK:  #endif
+  @unsafe public func f() { }
+}