Reproducer for a Swift bug that corrupts data when built with -O. This is a stripped down version of the swift-format code where this bug was discovered.

The bug appears to have been introduced in the development snapshots between 2020.03.26 and 2020.03.27.

1. Build and run the executable in debug mode with either the 2020.03.26 or the 2020.03.27 toolchain and observe the output; specifically, the first two tokens printed:

   TOOLCHAINS=org.swift.50202003261a swift build -c debug && .build/debug/swift-opt-corruption
   
   + Token.contextualBreakingStart
   + Token.contextualBreakingStart
   + Token.syntax("P")
   + Token.contextualBreakingEnd
   + Token.syntax("(")
   + Token.syntax(")")
   + Token.contextualBreakingEnd
   + Token.syntax("")
   ... more output
   

2. Build and run the executable in release mode with the 2020.03.26 toolchain and observe that the output is the same (be patient, SwiftSyntax takes a while to compile when optimized):

   TOOLCHAINS=org.swift.50202003261a swift build -c release && .build/release/swift-opt-corruption
   
   + Token.contextualBreakingStart
   + Token.contextualBreakingStart
   + Token.syntax("P")
   + Token.contextualBreakingEnd
   + Token.syntax("(")
   + Token.syntax(")")
   + Token.contextualBreakingEnd
   + Token.syntax("")
   ... more output
   

3. Build and run the executable in release mode with the 2020.03.27 toolchain and observe that the output is different and the program segfaults:

   TOOLCHAINS=org.swift.50202003261a swift build -c release && .build/release/swift-opt-corruption
   
   + Token.contextualBreakingStart
   + Token.contextualBreakingEnd
   + Token.syntax("P")
   + Token.contextualBreakingEnd
   + Token.syntax("(")
   + Token.syntax(")")
   + Token.syntax("")
   [1]    10986 segmentation fault  .build/release/swift-opt-corruption
   

4. Navigate to line 87 of main.swift and make one of the two modifications there, then build again with the 2020.03.27 toolchain and observe that the bug goes away.