AttributedSubstring.== crashes on empty strings

[cobbal]

When comparing 2 empty AttributedSubstrings from different base AttributedStrings, something crashes. Tested on mac, untested on other platforms.

SubstrCrash.swift:

import Foundation

func sub(_ s: AttributedString) -> AttributedSubstring { s[s.startIndex..<s.endIndex] }
print(sub("") == sub(""))

Result:

 $ swift --version
swift-driver version: 1.45.2 Apple Swift version 5.6 (swiftlang-5.6.0.323.62 clang-1316.0.20.8)
Target: x86_64-apple-macosx12.0

$ swift SubstrCrash.swift
Stack dump:
0.	Program arguments: /Applications/Xcode-13.3.1.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend -frontend -interpret SubstrCrash.swift -enable-objc-interop -stack-check -sdk /Applications/Xcode-13.3.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.3.sdk -color-diagnostics -new-driver-path /Applications/Xcode-13.3.1.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-driver -resource-dir /Applications/Xcode-13.3.1.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift -module-name SubstrCrash -target-sdk-version 12.3
1.	Apple Swift version 5.6 (swiftlang-5.6.0.323.62 clang-1316.0.20.8)
2.	Compiling with the current language version
3.	While running user code "SubstrCrash.swift"
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0  swift-frontend           0x0000000110b14de7 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 39
1  swift-frontend           0x0000000110b13e38 llvm::sys::RunSignalHandlers() + 248
2  swift-frontend           0x0000000110b15440 SignalHandler(int) + 288
3  libsystem_platform.dylib 0x00007ff8021b7dfd _sigtramp + 29
4  libsystem_platform.dylib 0x0000000000000001 _sigtramp + 18446603370545578529
5  libswiftFoundation.dylib 0x00007ff813bf2716 function signature specialization <Arg[2] = Dead> of static Foundation.AttributedSubstring.== infix(Foundation.AttributedSubstring, Foundation.AttributedSubstring) -> Swift.Bool + 2982
6  libswiftFoundation.dylib 0x0000000113b9d0ff function signature specialization <Arg[2] = Dead> of static Foundation.AttributedSubstring.== infix(Foundation.AttributedSubstring, Foundation.AttributedSubstring) -> Swift.Bool + 18446603374875555215
7  swift-frontend           0x000000010b927560 llvm::orc::runAsMain(int (*)(int, char**), llvm::ArrayRef<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, llvm::Optional<llvm::StringRef>) + 1792
8  swift-frontend           0x000000010b883dcd swift::RunImmediately(swift::CompilerInstance&, std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&, swift::IRGenOptions const&, swift::SILOptions const&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >&&) + 13613
9  swift-frontend           0x000000010b84b2c3 performCompileStepsPostSILGen(swift::CompilerInstance&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, swift::PrimarySpecificPaths const&, int&, swift::FrontendObserver*) + 2259
10 swift-frontend           0x000000010b83e953 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 21107
11 swift-frontend           0x000000010b7fc5d4 swift::mainEntry(int, char const**) + 1108
12 dyld                     0x0000000113ba652e start + 462
[1]    87164 illegal hardware instruction  swift SubstrCrash.swift

[jmschonfeld]

Thanks for finding this! You're right, it looks a bug in AttributedString.Runs.== surfaced through AttributedSubstring.== - I'll post a fix for this