go: cmd/vet: go vet -vettool=$(which shadow) errors in go1.13 only (flag provided but not defined: -unsafeptr)

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

$ go version
go version go1.13 linux/amd64

Does this issue reproduce with the latest release?

Yes

Does this issue reproduce with the previous release?

No

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN="/home/leighmcculloch/local/bin"
GOCACHE="/home/leighmcculloch/.cache/go-build"
GOENV="/home/leighmcculloch/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/leighmcculloch/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/leighmcculloch/local/bin/go/1.13"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/leighmcculloch/local/bin/go/1.13/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/leighmcculloch/devel/stellar-go/go.mod"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build252896639=/tmp/go-build -gno-record-gcc-switches"

What did you do?

$ mkdir test
$ cd test
$ go mod init test
$ cat > main.go <<-EOF
package main

import "fmt"

func main() {
        v := "bye"
        for {
                v := "hello"
                fmt.Println(v)
                break
        }
        fmt.Println(v)
}
EOF
$ go get -u golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow
$ go vet -vettool=$(which shadow)

What did you expect to see?

I expected to see the output indicating the shadow variable:

$ go vet -vettool=$(which shadow)
# test
./main.go:8:3: declaration of "v" shadows declaration at line 6

What did you see instead?

The output was huge, 1541 lines of output. It was repeatedly dumping out an error flag provided but not defined: -unsafeptr once for some number of standard library packages along with the usage documentation for the shadow command. At the very end was the output I expected.

$ go vet -vettool=$(which shadow)
# internal/cpu
flag provided but not defined: -unsafeptr
shadow: check for possible unintended shadowing of variables

Usage: shadow [-flag] [package]

This analyzer check for shadowed variables.
...
# test
./main.go:8:3: declaration of "v" shadows declaration at line 6

To condense the output down a bit we can see the packages it is displaying the error about:

$ go vet -vettool=$(which shadow) 2>&1 | grep '^#'
# internal/cpu
# runtime/internal/sys
# internal/bytealg
# runtime/internal/math
# unicode/utf8
# internal/race
# sync/atomic
# unicode
# math/bits
# internal/testlog
# runtime/internal/atomic
# math
# runtime
# internal/reflectlite
# sync
# sort
# errors
# io
# internal/oserror
# strconv
# syscall
# reflect
# internal/syscall/unix
# time
# internal/fmtsort
# internal/poll
# os
# fmt
# test

Does this issue reproduce with the previous release? (details)

No. This doesn’t happen with Go 1.12.9. If I run the above commands with that version of Go, it behaves as expected and outputs the shadow errors. Example:

$ go1.12.9 vet -vettool=$(which shadow)           
# test
./main.go:8:3: declaration of "v" shadows declaration at line 6