cmd/vet: prefix in vet output breaks error file format compatibility

[mirza-s]

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

$ go version
go version go1.13 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN="/Users/mbp/go/bin"
GOCACHE="/Users/mbp/Library/Caches/go-build"
GOENV="/Users/mbp/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY="github.com/mirza-s"
GONOSUMDB="github.com/mirza-s"
GOOS="darwin"
GOPATH="/Users/mbp/go"
GOPRIVATE="github.com/mirza-s"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/nt/0hzfppd92jvbk99fhvnhy8ph0000gn/T/go-build729502798=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Run

go vet cmd/main.go

What did you expect to see?

cmd/main.go:187:62: undefined: a

What did you see instead?

vet: cmd/main.go:187:62: undeclared name: a

This appears to be introduced in 38431f1

[toothrot]

Hi @mirza-s,

I can confirm this output change locally:

# vet.go
package main

import "fmt"

func main() {
	fmt.Println(a)
}

$ go1.12 vet ./vet.go
# command-line-arguments [command-line-arguments.test]
./vet.go:6:14: undefined: a

$ go vet ./vet.go
# command-line-arguments
vet: ./vet.go:6:14: undeclared name: a

$ go build ~/vet.go 
# command-line-arguments
./vet.go:6:14: undefined: a

I'm not sure if this is a feature or a bug.

/cc @rsc, @josharian and @bradfitz, who have context on the CLs for #31916