cmd/go: go test -fullpath should include full paths for build errors too

[rogpeppe]

What version of Go are you using (go version)?

f90b4cd6554f4f20280aa5229cf42650ed47221d

Does this issue reproduce with the latest release?

N/A

What did you do?

I ran this testscript:

! exec go test -C ./x -fullpath .
! stderr '\./x.go'

-- x/x.go --
package foo

var x = mistake

-- go.mod --
module test

What did you expect to see?

A passing test.

What did you see instead?

> ! exec go test -C ./x -fullpath .
[stdout]
FAIL
[stderr]
# test/x
./x.go:3:9: undefined: mistake
[exit status 1]
> ! stderr '\./x.go'
FAIL: /tmp/testscript142857279/x.txtar/script.txtar:2: unexpected match for `\./x.go` found in stderr: ./x.go

It seems to me that the intent of -fullpath is to cause all filenames to print in a way that's absolute, even when those errors come from the compiler rather than the test binary. I'd expect the compiler error to print an absolute path there too.

[ianlancetaylor]

This means moving -fullpath from being a test option to being a build option, and then using it to pass something to cmd/compile. We would presumably still have the -test.fullpath option, which would be different, but -fullpath would imply -test.fullpath.

CC @bcmills @matloob

[rogpeppe]

Yup, that sounds about right. I think that would be worth doing.

[bcmills]

Sounds reasonable. This is essentially a revision on proposal #37708, so it should probably be a proposal itself.