mobile: add flag to build Swift with `-enable-experimental-cxx-intero…

…p` (envoyproxy#25971)

The Swift project has had experimental C++ interoperability support for
a few years now. For example, a
[workgroup](https://forums.swift.org/t/swift-and-c-interoperability-workgroup-announcement/54998/1)
was started in early 2022. C++ interop is considered experimental
because it requires passing an additional flag to the Swift compiler
(`-enable-experimental-cxx-interop`) and the feature has no source or
ABI stability guarantees.

Envoy Mobile in particular would benefit greatly from Swift / C++
interop because it would allow us to shed the intermediate Objective-C
layer that only exists to bridge Envoy's C++ APIs with the consumer-
facing Swift API.

This change proposes a build flag to optionally build the Swift parts of
Envoy Mobile with C++ interop enabled, and enables it by default.

Future changes will gradually add Swift code that interacts directly
with C++ APIs.

Please read the [C++ Interoperability Status](https://github.com/apple/swift/blob/main/docs/CppInteroperability/CppInteroperabilityStatus.md)
to learn more about the current state of things.

Signed-off-by: JP Simard <[email protected]>

.github/workflows/compile_time_options.yml

@@ -67,6 +67,7 @@ jobs:
             --remote_header="Authorization=Bearer $GITHUB_TOKEN" \
             --define=admin_html=enabled \
             --define=envoy_mobile_request_compression=disabled \
+            --define=envoy_mobile_swift_cxx_interop=disabled \
             --@envoy//bazel:http3=False \
             --@com_envoyproxy_protoc_gen_validate//bazel:template-flavor= \
             //library/swift:ios_framework

mobile/.bazelrc

@@ -9,6 +9,7 @@ build --define=admin_html=disabled
 build --define=static_extension_registration=disabled
 build --define=admin_functionality=disabled
 build --define=library_autolink=disabled
+build --define=envoy_mobile_swift_cxx_interop=enabled
 build --experimental_inmemory_dotd_files
 build --experimental_inmemory_jdeps_files
 build --features=debug_prefix_map_pwd_is_dot

mobile/bazel/BUILD

@@ -5,6 +5,11 @@ licenses(["notice"])  # Apache 2
 
 envoy_package()
 
+config_setting(
+    name = "envoy_mobile_swift_cxx_interop",
+    values = {"define": "envoy_mobile_swift_cxx_interop=enabled"},
+)
+
 kt_jvm_library(
     name = "envoy_mobile_test_suite",
     srcs = [

mobile/library/swift/BUILD

@@ -76,6 +76,14 @@ swift_library(
         "stats/Tags.swift",
         "stats/TagsBuilder.swift",
     ] + ["@envoy_mobile_extra_swift_sources//:extra_swift_srcs"],
+    copts = select({
+        "//bazel:envoy_mobile_swift_cxx_interop": [
+            "-enable-experimental-cxx-interop",
+            "-Xcc",
+            "-std=c++17",
+        ],
+        "//conditions:default": [],
+    }),
     defines = envoy_mobile_defines("@envoy"),
     features = [
         "swift.emit_symbol_graph",

mobile/library/swift/HeadersBuilder.swift

@@ -3,10 +3,9 @@ import Foundation
 private let kRestrictedPrefixes = [":", "x-envoy-mobile"]
 
 private func isRestrictedHeader(name: String) -> Bool {
-  let isHostHeader = name.caseInsensitiveCompare("host") == .orderedSame
-  lazy var hasRestrictedPrefix = kRestrictedPrefixes
-    .contains { name.range(of: $0, options: [.caseInsensitive, .anchored]) != nil }
-  return isHostHeader || hasRestrictedPrefix
+  let lowercasedName = name.lowercased()
+  let isHostHeader = lowercasedName == "host"
+  return isHostHeader || kRestrictedPrefixes.contains(where: lowercasedName.starts(with:))
 }
 
 /// Base builder class used to construct `Headers` instances.

mobile/library/swift/Stream.swift

@@ -37,7 +37,7 @@ public class Stream: NSObject {
   ///                        callback.
   ///
   /// - returns: This stream, for chaining syntax.
-  public func readData(_ byteCount: size_t) -> Stream {
+  public func readData(_ byteCount: Int) -> Stream {
     self.underlyingStream.readData(byteCount)
     return self
   }

mobile/library/swift/stats/Element.swift

@@ -9,13 +9,30 @@ public final class Element: NSObject, ExpressibleByStringLiteral {
   internal let value: String
 
   public init(stringLiteral value: String) {
-    guard value.range(of: kPattern, options: .regularExpression) != nil else {
-      preconditionFailure("Element values must conform to the regex '\(kPattern)'.")
-    }
+    precondition(
+      value.matchesStatsElementPattern,
+      "Element values must conform to the regex '\(kPattern)'."
+    )
     self.value = value
   }
 
   public override func isEqual(_ object: Any?) -> Bool {
     return (object as? Element)?.value == self.value
   }
 }
+
+extension String {
+  var matchesStatsElementPattern: Bool {
+    if #available(iOS 16.0, macOS 13.0, *) {
+      return self.contains(/^[A-Za-z_]+$/)
+    }
+
+    // `std` is the name of the C++ stdlib when importing it into Swift.
+    // So effectively this checks if we're compiling with `-enable-experimental-cxx-interop`.
+#if canImport(std)
+    return (self as NSString).range(of: kPattern, options: regularExpression).location != NSNotFound
+#else
+    return self.range(of: kPattern, options: .regularExpression) != nil
+#endif
+  }
+}