[swift-users] stack trace of Swift executable

[Nick Snyder]

I have a simple main.swift that looks like this:

```
print("Hello, world!")
let x: String? = nil

func say(message: String?) {
    print(x!) // intentional crash
}

say(message: x)
```

Running this results in the crash

```
$ swift build && ./.build/debug/hello
Hello, world!
fatal error: unexpectedly found nil while unwrapping an Optional value
Current stack trace:
0    libswiftCore.dylib                 0x0000000110120130
swift_reportError + 129
1    libswiftCore.dylib                 0x000000011013cb50
_swift_stdlib_reportFatalError + 60
2    libswiftCore.dylib                 0x000000010ff2d250 specialized
specialized StaticString.withUTF8Buffer<A> ((UnsafeBufferPointer<UInt8>) ->
A) -> A + 342
3    libswiftCore.dylib                 0x00000001100a7e90 partial apply
for (_fatalErrorMessage(StaticString, StaticString, file : StaticString,
line : UInt, flags : UInt32) -> Never).(closure #2) + 109
4    libswiftCore.dylib                 0x000000010ff2d250 specialized
specialized StaticString.withUTF8Buffer<A> ((UnsafeBufferPointer<UInt8>) ->
A) -> A + 342
5    libswiftCore.dylib                 0x000000011005a9a0 specialized
_fatalErrorMessage(StaticString, StaticString, file : StaticString, line :
UInt, flags : UInt32) -> Never + 96
6    hello                              0x000000010fee6200 main + 345
7    libdyld.dylib                      0x00007fffe0375234 start + 1
Illegal instruction: 4
```

>From the crash, how do I map back line 6 to an actual line in main.swift?
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