cmd/go: internal cgo types not being rewritten in errors

[rsc]

% go build
# rsc.io/tmp/ftoa/dmg
./dmg.go:171:9: cannot use (_Cfunc_loopdmgstrtod)(_Ctype_int(which), _Ctype_longlong(n), p) (value of float64 type _Ctype_double) as float64 value in return statement

The go command used to post-process these compiler errors to replace _Cfunc_loopdmgstrtod with C.loopdmgstrtod, etc. Something has broken and it no longer does.

[ianlancetaylor]

Looks like this stopped working in the Go 1.10 release. It was broken by https://go.dev/cl/77090.

As far as I can tell it really has been broken since then. The code that does the rewriting is looking for the old error message format which looks like ./foo.go:11[/tmp/go-build717086523/command-line-arguments/_obj/foo.cgo1.go:12:17]. Since CL 77090 the error message is just ./foo.go:11:19, with nothing in square brackets.

The tests for this in cmd/cgo/internal/testerrors pass because that test is explicitly passing -L to the compiler. That directs the compiler to pass the complete file names, which causes the rewriting to kick in. Changing the test to pass -L was also in CL 77090.

@rsc in CL 77090 you said:

Right. FWIW, I just remembered that cmd/go is actively stripping those [] out for cgo. I adjusted the regexp to strip them out for any file name, so that's not on any important critical paths anymore.

I'm not sure when or if that happened.

In any case I am testing a CL.

[gopherbot]

Change https://go.dev/cl/721821 mentions this issue: cmd/go: rewrite cgo names to "C." in compiler error messages