How to debug Symbol completion/creation?

[abgruszecki]

I have a Symbol that seems like it shouldn't be created. How do I debug how it is created?

OR : I have a Symbol with a suspicious .info value. How do I debug how that value is calculated?

OR : I have a Symbol that asplodes during completion / isn't completed when it should be. How do I debug that?

[abgruszecki]

Symbols have hash codes that are stable between compiler runs. To find out what creates a Symbol, first find out its hash code, then simply print a stack trace in Symbol's constructor when the hash code matches. The stack trace will probably contain some new* methods in Symbols.scala.

When a symbol is created, its .info is set to a lazy type. This lazy type will later be used to actually calculate the .info for the symbol (see complete and doComplete in SymbolLoader). One way to debug information about the completion of the symbol is to print information when the symbol with the correct hash code is being completed.

[abgruszecki]

/cc @romanowski

[smarter]

Symbols have hash codes

Right, hashCode for a symbol delegates to id, which is shorter to type :). And you can get the compiler to append symbol ids in its pretty-printed output using -uniqid

then simply print a stack trace

Which can be done using Thread.dumpStack

debug information about the completion of the symbol

You can set showCompletions to true in https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/config/Config.scala to get a log of every completion, where nested completions will appear nested in the output (pretty useful for debugging cycles).