Works around some poor performance in XCBBuildService due to slow reflection machinery (https://twitter.com/_saagarjha/status/1499599539795234816)

README.md

To build this:

$ swiftc -emit-object XCBBuildServiceFix.swift -O
$ clang XCBBuildServiceFix.o XCBBuildServiceFix.s -shared -o libXCBBuildServiceFix.dylib -O -L /usr/lib/swift/ -lswiftCore -F /Applications/Xcode.app/Contents/SharedFrameworks/XCBuild.framework/Versions/Current/PlugIns/XCBBuildService.bundle/Contents/Frameworks/ -framework XCBUtil -framework XCBProtocol

Then inject the result into XCBBuildService, which launches from /Applications/Xcode.app/Contents/SharedFrameworks/XCBuild.framework/Versions/Current/PlugIns/XCBBuildService.bundle/Contents/MacOS/XCBBuildService, using your injection tool of choice.

XCBBuildServiceFix.s

.section __DATA,__interpose
.p2align 3
	.quad _String_init_describing
	.quad _$sSS10describingSSx_tclufC

XCBBuildServiceFix.swift

import Foundation

protocol OptionalProtocol {
	var isNonNil: Bool { get }
}

extension Optional: OptionalProtocol {
	var isNonNil: Bool {
		self != nil
	}

	var _description: String {
		"\(describe(value: self))"
	}
}

extension Bool {
	var _description: String {
		description
	}
}

extension String {
	var _description: String {
		"\"\(description)\""
	}
}

// NOTE: _internal layout probably doesn't really match actual layout.
// This is mostly fine because it's to keep the structs ~appropriately sized.

public struct Path {
	var _internal: (String)

	@_silgen_name("$s7XCBUtil4PathV3strSSvg")
	func str() -> String
}

extension Path {
	var _description: String {
		"XCBUtil.Path(str: \(str()._description))"
	}
}

public struct ArenaInfo {
	var _internal: (Path, Path, Path, Path, Path, Path?, Bool)

	@_silgen_name("$s11XCBProtocol9ArenaInfoV15derivedDataPath7XCBUtil0F0Vvg")
	func derivedDataPath() -> Path

	@_silgen_name("$s11XCBProtocol9ArenaInfoV17buildProductsPath7XCBUtil0F0Vvg")
	func buildProductsPath() -> Path

	@_silgen_name("$s11XCBProtocol9ArenaInfoV22buildIntermediatesPath7XCBUtil0F0Vvg")
	func buildIntermediatesPath() -> Path

	@_silgen_name("$s11XCBProtocol9ArenaInfoV7pchPath7XCBUtil0E0Vvg")
	func pchPath() -> Path

	@_silgen_name("$s11XCBProtocol9ArenaInfoV12indexPCHPath7XCBUtil4PathVvg")
	func indexPCHPath() -> Path

	@_silgen_name("$s11XCBProtocol9ArenaInfoV24indexDataStoreFolderPath7XCBUtil0H0VSgvg")
	func indexDataStoreFolderPath() -> Path?

	@_silgen_name("$s11XCBProtocol9ArenaInfoV20indexEnableDataStoreSbvg")
	func indexEnableDataStore() -> Bool
}

extension ArenaInfo {
	var _description: String {
		"XCBProtocol.ArenaInfo(derivedDataPath: \(derivedDataPath()._description), buildProductsPath: \(buildProductsPath()._description), buildIntermediatesPath: \(buildIntermediatesPath()._description), pchPath: \(pchPath()._description), indexPCHPath: \(indexPCHPath()._description), indexDataStoreFolderPath: \(indexDataStoreFolderPath()._description), indexEnableDataStore: \(indexEnableDataStore()._description))"
	}
}

public struct OrderedSet<T: Hashable> {
	var _internal: ([T], Set<T>)

	@_silgen_name("$s7XCBUtil10OrderedSetV11descriptionSSvg")
	func description() -> String
}

extension OrderedSet {
	var _description: String {
		description()
	}
}

public struct RunDestinationInfo {
	var _internal: (String, String, String?, String, OrderedSet<String>, Bool)

	@_silgen_name("$s11XCBProtocol18RunDestinationInfoV8platformSSvg")
	func platform() -> String

	@_silgen_name("$s11XCBProtocol18RunDestinationInfoV3sdkSSvg")
	func sdk() -> String

	@_silgen_name("$s11XCBProtocol18RunDestinationInfoV10sdkVariantSSSgvg")
	func sdkVariant() -> String?

	@_silgen_name("$s11XCBProtocol18RunDestinationInfoV18targetArchitectureSSvg")
	func targetArchitecture() -> String

	@_silgen_name("$s11XCBProtocol18RunDestinationInfoV22supportedArchitectures7XCBUtil10OrderedSetVySSGvg")
	func supportedArchitectures() -> OrderedSet<String>

	@_silgen_name("$s11XCBProtocol18RunDestinationInfoV21disableOnlyActiveArchSbvg")
	func disableOnlyActiveArch() -> Bool
}

extension RunDestinationInfo {
	var _description: String {
		"XCBProtocol.RunDestinationInfo(platform: \(platform()._description), sdk: \(sdk()._description), sdkVariant: \(sdkVariant()._description), targetArchitecture: \(targetArchitecture()._description), supportedArchitectures: \(supportedArchitectures()._description), disableOnlyActiveArch: \(disableOnlyActiveArch()._description))"
	}
}

func describe<T>(value: T) -> String {
	// Ideally this would use _getTypeName but it doesn't quite work
	if _typeName(T.self) == "Swift.Optional<XCBProtocol.ArenaInfo>", ((value as? OptionalProtocol)?.isNonNil)! {
		return withUnsafePointer(to: value) {
			"Optional(\(UnsafeRawPointer($0).assumingMemoryBound(to: ArenaInfo.self).pointee._description))"
		}
	} else if _typeName(T.self) == "Swift.Optional<XCBProtocol.RunDestinationInfo>", ((value as? OptionalProtocol)?.isNonNil)! {
		return withUnsafePointer(to: value) {
			"Optional(\(UnsafeRawPointer($0).assumingMemoryBound(to: RunDestinationInfo.self).pointee._description))"
		}
	} else if T.self == Path?.self, ((value as? OptionalProtocol)?.isNonNil)! {
		return withUnsafePointer(to: value) {
			"Optional(\(UnsafeRawPointer($0).assumingMemoryBound(to: Path.self).pointee._description))"
		}
	} else {
		return String(describing: value)
	}
}

@_silgen_name("String_init_describing")
func String_init_describing<T>(describing value: T) -> String {
	describe(value: value)
}