The scanner's interpretation of a //line directive that lacks a filename, such as this:

//line :1

is very surprising.  I would have expected it to mean either "line 1 of the current
file", or "line 1 of the file specified by the previous //line
directive", but instead it means "line 1 of the directory containing the
current file".  This is obviously nonsensical since directories don't have lines,
but this is actually seems to be the intended behaviour, if
go/scanner/scanner_test.go:497 is to be believed:

497: {"\n//line :1\n  line1", "dir", 1},

I think this is a bug.  I would prefer either of the two interpretations I suggested
over the current one.

(Why do I care?  Because the cgo preprocessor emits files containing "//line
:1" prior to each field of a struct type.  I suspect that is probably unintended
too, but that's another story.)