testing: test errors do not include full path name #21119

Please answer these questions before submitting your issue. Thanks!

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

go version go1.8.3 darwin/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/aron/gofoo"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/ts/s940qvdj5vj1czr9qh07fvtw0000gn/T/go-build353906001=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

Create src/pathing/pathing_test.go with the content:

package pathing

import (
	"testing"
)

func Name() string {
	return "pathing"
}

func TestName(t *testing.T) {
	//var name string
	if name = Name(); name != "Pathing" {
		t.Error("unexpected name")
	}
}

What did you expect to see?

When there is a compilation error (name is not defined):

$ go test pathing
# pathing
src/pathing/pathing_test.go:13: undefined: name
FAIL	pathing [build failed]

When there is a test failure (uncomment the name declaration):

go test pathing
--- FAIL: TestName (0.00s)
	src/pathing/pathing_test.go:14: unexpected name
FAIL
FAIL	pathing	0.007s

What did you see instead?

When there is a test failure:

go test pathing
--- FAIL: TestName (0.00s)
	pathing_test.go:14: unexpected name
FAIL
FAIL	pathing	0.007s

Notice that test failures are always the filename with no pathing. This appears to come from the decorate function: https://github.com/golang/go/blob/release-branch.go1.8/src/testing/testing.go#L303

Test-time build failures contain more context when run outside the package directory. They are always an accurate relative path to the file-under-test. This appears to come from LineHist.LineString https://github.com/golang/go/blob/release-branch.go1.8/src/cmd/internal/obj/line.go#L218

Here is an example build failure when run from some other package:

$ go test pathing
# pathing
../pathing/pathing_test.go:13: undefined: name
FAIL	pathing [build failed]

The way go test failures are reported makes it difficult for tooling that processes compilation/test output (emacs electric compilation buffers) to locate the reported file. The errors that come from the build phase are relative paths; the ones coming from test are filenames-only.

Test failures should include path to the file-under-test.