in go/parser/interface.go:

const (
    PackageClauseOnly Mode             = 1 << iota // parsing stops after package clause
    ImportsOnly                                    // parsing stops after import declarations
    ParseComments                                  // parse comments and add them to AST
    Trace                                          // print a trace of parsed productions
    DeclarationErrors                              // report declaration errors
    SpuriousErrors                                 // same as AllErrors, for backward-compatibility
    AllErrors         = SpuriousErrors             // report all (not just the first 10) errors per file
)

One would expect the "=" to line up. The algorithm for that layout has been
tweaked many times, though. Revisit at some point.