#1759 cases 30% slowdown when generating diagnostics

[ahoppen]

When changing swift-parser-cli performance-test to also fold operators and generate diagnostics (like the SourceKit stress tester does), then #1759 causes a 30% slowdown.

Steps to Reproduce

1. Apply the following diff to SwiftSyntax

diff --git a/Sources/swift-parser-cli/Commands/PerformanceTest.swift b/Sources/swift-parser-cli/Commands/PerformanceTest.swift
index 5b3c0afa..844fdaaa 100644
--- a/Sources/swift-parser-cli/Commands/PerformanceTest.swift
+++ b/Sources/swift-parser-cli/Commands/PerformanceTest.swift
@@ -14,6 +14,9 @@ import _InstructionCounter
 import ArgumentParser
 import Foundation
 import SwiftParser
+import SwiftOperators
+import SwiftSyntax
+import SwiftParserDiagnostics
 
 struct PerformanceTest: ParsableCommand {
   static var configuration = CommandConfiguration(
@@ -42,7 +45,11 @@ struct PerformanceTest: ParsableCommand {
     for _ in 0..<self.iterations {
       for file in files {
         file.withUnsafeBytes { buf in
-          _ = Parser.parse(source: buf.bindMemory(to: UInt8.self))
+          var tree = Parser.parse(source: buf.bindMemory(to: UInt8.self))
+          if let foldedTree = OperatorTable.standardOperators.foldAll(tree, errorHandler: { _ in }).as(SourceFileSyntax.self) {
+            tree = foldedTree
+          }
+          _ = ParseDiagnosticsGenerator.diagnostics(for: tree)
         }
       }
     }

2. Save the following file to /tmp/i/input.swift

//  Copyright (c) 2015 Rob Rix. All rights reserved.

#if swift(>=4.2)
#if compiler(>=5)

// Use Swift.Result
extension Result {
	// ResultProtocol
	public

extension Result {
	/// The compatibility alias for the Swift 5's `Result` in the standard library.
	///
	/// See https://github.com/apple/swift-evolution/blob/master/proposals/0235-add-result.md
	/// and https://forums.swift.org/t/accepted-with-modifications-se-0235-add-result-to-the-standard-library/18603
	/// for the details.
	public typealias Success = Value
	/// The compatibility alias for the Swift 5's `Result` in the standard library.
	///
	/// See https://github.com/apple/swift-evolution/blob/master/proposals/0235-add-result.md
	/// and https://forums.swift.org/t/accepted-with-modifications-se-0235-add-result-to-the-standard-library/18603
	/// for the details.
	public typealias Failure = Error
}

extension Result {
	// MARK: Constructors

	/// Constructs a result from an `Optional`, failing with `Error` if `nil`.
	public init(_ value: Value?, failWith: @

extension Result: CustomStringConvertible {
	public var description: String {
		switch self {
		case 

extension Result: CustomDebugStringConvertible {
	public var debugDescription: String {
		return description
	}
}

extension Result: ResultProtocol {
	/// Constructs a success wrapping a `value`.
	public init(value: Value) {
		self = 

extension Result where Result.Failure == AnyError {
	/// Constructs a result from an expression that uses `throw`, failing with `AnyError` if throws.
	public init(_ f: 

// MARK: - Equatable

#if swift(>=4.2)
#if !compiler(>=5)
	extension Result

#if swift(>=4.2)
	// Conformance to `Equatable` will be automatically synthesized.
#else
	extension Result where Result.Success: Equatable

// MARK: - Derive result from failable closure

@available(*, deprecated, renamed: "Result.init(attempt:)")
public func 

@available(*, deprecated, renamed: "Result.init(_:)")
public func 

// MARK: - ErrorConvertible conformance
	
extension NSError: ErrorConvertible {
	public static func error(from error: Swift.

// MARK: - migration support

@available(*, unavailable, message: "Use the overload which returns `Result<T, AnyError>` instead")
public func 

@available(*, unavailable, message: "Use the overload which returns `Result<T, AnyError>` instead")
public func 

#if os(macOS) || os(iOS) || os(tvOS) 

// MARK: -

import Foundation

3. Run swift-parser-cli performance-test --directory /tmp/i --iterations 1000

On my local machine I’m seeing an instruction count of ~27.000.000 before #1759 and ~36.000.000 after that PR. That’s a >30% slowdown.

[ahoppen]

Tracked in Apple’s issue tracker as rdar://111332992