cmd/go/internal/work: panic for a repeated //go:debug directive for the same setting

[AndrewWPhillips]

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

$ go version                                 
go version go1.21.0 windows/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
set GO111MODULE=on
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\Andrew\AppData\Local\go-build
set GOENV=C:\Users\Andrew\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\Andrew\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\Andrew\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org
set GOROOT=C:/Users/Andrew/go/go1.21.0
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLCHAIN=auto
set GOTOOLDIR=C:\Users\Andrew\go\go1.21.0\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.21.0
set GCCGO=gccgo
set GOAMD64=v1
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=NUL
set GOWORK=
set CGO_CFLAGS=-O2 -g
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-O2 -g
set CGO_FFLAGS=-O2 -g
set CGO_LDFLAGS=-O2 -g
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=C:\Users\Andrew\AppData\Local\Temp\go-build1748980101=/tmp/go-build -gno-record-gcc-switches

What did you do?

https://go.dev/play/p/k6MIftXQ3q7

//go:debug panicnil=1
//go:debug panicnil=1

package main

func main() {
}

What did you expect to see?

Build or compiler error message.

What did you see instead?

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x1 addr=0x20 pc=0x7ab2f7]

goroutine 36 [running]:
bytes.(*Buffer).Write(0xc00019a0d0?, {0xc000132120?, 0x38?, 0xc0001891f8?})
bytes/buffer.go:176 +0x17
fmt.Fprintf({0xe22e80, 0x0}, {0xd3c601, 0x38}, {0xc0001891f8, 0x3, 0x3})
fmt/print.go:225 +0x97
cmd/go/internal/work.(*Builder).checkDirectives(0xc0000e2000, 0x497d5c6d09af60fa?)
cmd/go/internal/work/exec.go:978 +0x4ed
cmd/go/internal/work.(*Builder).build(0xc0000e2000, {0xe28230?, 0x11f3e40?}, 0xc0003cde40)
cmd/go/internal/work/exec.go:536 +0x71d
cmd/go/internal/work.ActorFunc.Act(0xe28230?, 0x11f3e40?, {0xe28230?, 0x11f3e40?}, 0x7c206e6977726164?)
cmd/go/internal/work/action.go:78 +0x2d
cmd/go/internal/work.(*Builder).Do.func3({0xe28230, 0x11f3e40}, 0xc0003cde40)
cmd/go/internal/work/exec.go:151 +0x7e4
cmd/go/internal/work.(*Builder).Do.func4()
cmd/go/internal/work/exec.go:219 +0xa8
created by cmd/go/internal/work.(*Builder).Do in goroutine 1
cmd/go/internal/work/exec.go:205 +0x405

[gopherbot]

Change https://go.dev/cl/523836 mentions this issue: cmd/go/internal/work: avoid panic for a repeated //go:debug setting