File.contents will crash for files that cannot be read later, when initialised with init(pathDeferringReading:)

[mildm8nnered]

This can be seen by running SwiftLint against swift

brew install swiftlint
git clone https://github.com/apple/swift
cd swift
swiftlint

The problem file is ./validation-test/IDE/crashers_fixed/033-swift-identifier-isoperatorslow.swift , which cannot be interpreted as UTF-8

% swiftlint  ./validation-test/IDE/crashers_fixed/033-swift-identifier-isoperatorslow.swift          
Linting Swift files at paths ./validation-test/IDE/crashers_fixed/033-swift-identifier-isoperatorslow.swift
Linting '033-swift-identifier-isoperatorslow.swift' (1/1)
SourceKittenFramework/File.swift:20: Fatal error: 'try!' expression unexpectedly raised an error: Error Domain=NSCocoaErrorDomain Code=261 "The file “033-swift-identifier-isoperatorslow.swift” couldn’t be opened using text encoding Unicode (UTF-8)." UserInfo={NSFilePath=/Users/martin.redington/Documents/Hudl/Source/swift/validation-test/IDE/crashers_fixed/033-swift-identifier-isoperatorslow.swift, NSStringEncoding=4}
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace.
zsh: trace trap  ../Sportscode64/Pods/SwiftLint/swiftlint 

See realm/SwiftLint#4821

But actually, if the file cannot be read for any reason, then this will cause a crash

touch t.swift
chmod -r t.swift
swiftlint  t.swift
Linting Swift files at paths t.swift
Linting 't.swift' (1/1)
SourceKittenFramework/File.swift:20: Fatal error: 'try!' expression unexpectedly raised an error: Error Domain=NSCocoaErrorDomain Code=257 "The file “t.swift” couldn’t be opened because you don’t have permission to view it." UserInfo={NSFilePath=/Users/martin.redington/Documents/Hudl/Source/SourceKitten/t.swift, NSUnderlyingError=0x600002a48210 {Error Domain=NSPOSIXErrorDomain Code=13 "Permission denied"}}
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace.
zsh: trace trap  ../Sportscode64/Pods/SwiftLint/swiftlint t.swift