Golang cmd/go, encoding/json/v2: setting GOEXPERIMENT=jsonv2 causes "go env -json" to crash

Go version

go version go1.25.0 darwin/arm64

Output of `go env` in your module/workspace:

AR='ar'
CC='clang'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='clang++'
GCCGO='gccgo'
GO111MODULE=''
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/redacted/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/redacted/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/m4/sr146z197b1df6lgmh2gkqbw0000gq/T/go-build2415365221=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/Users/redacted/projects/demos/jsondump/go.mod'
GOMODCACHE='/Users/redacted/go/pkg/mod'
GONOPROXY='redacted'
GONOSUMDB='redacted'
GOOS='darwin'
GOPATH='/Users/redacted/go'
GOPRIVATE='redacted'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/Users/redacted/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25.0.darwin-arm64'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/redacted/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/redacted/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25.0.darwin-arm64/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.25.0'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

Run:

GOEXPERIMENT=jsonv2 /usr/local/go/bin/go env -json

In a folder with the following go.mod

module something

go 1.25

In the absence of the above go.mod file, go version returns go version go1.24.1 darwin/arm64.

As for why I'm doing this:

I am trying to experiment with the new encoding/json/v2 and related packages, so I've configured VS Code to have the following settings.json, so that I don't get red-squiggles in my IDE and so auto-complete works:

{
    "go.toolsEnvVars": {
        "GOEXPERIMENT": "jsonv2"
    },
    "go.testEnvVars": {
        "GOEXPERIMEN": "jsonv2"
    }
}

During started, gopls runs go env -json GOPATH GOROOT GOPROXY GOBIN GOMODCACHE, but crashes because of the GOEXPERIMENT env-var.

(I don't actually know if the above settings are enough to make my IDE do what I want - it's what I tried and it crashed)

What did you see happen?

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x3 pc=0x10316aa40]

goroutine 1 [running]:
cmd/go/internal/fips140.Init()
        cmd/go/internal/fips140/fips140.go:117 +0x120
cmd/go/internal/modload.Init()
        cmd/go/internal/modload/init.go:419 +0x38
cmd/go/internal/toolchain.Exec({0x140000aca68, 0x8})
        cmd/go/internal/toolchain/select.go:356 +0x46c
cmd/go/internal/toolchain.Select()
        cmd/go/internal/toolchain/select.go:284 +0xf28
main.main()
        cmd/go/main.go:107 +0x58

I'm not sure exactly which go-executable broke (1.24.1 or 1.25.0). I'm guessing 1.25.0, otherwise the GOEXPERIMENT=jsonv2 shouldn't have done anything?

What did you expect to see?

Something like:

{
        "AR": "ar",
        "CC": "clang",
        "CGO_CFLAGS": "-O2 -g",
        "CGO_CPPFLAGS": "",
        "CGO_CXXFLAGS": "-O2 -g",
        "CGO_ENABLED": "1",
        "CGO_FFLAGS": "-O2 -g",
        "CGO_LDFLAGS": "-O2 -g",
        "CXX": "clang++",
        "GCCGO": "gccgo",
        "GO111MODULE": "",
        "GOARCH": "arm64",
        "GOARM64": "v8.0",
       // ...
}

Comment From: dmitshur

CC @dsnet, @neild.

Comment From: seankhliao

I think we need more info on reproducing this: cmd/go should return an error of go: unknown GOEXPERIMENT jsonv2 if it doesn't know about an experiment (e.g. 1.24 doesn't know about jsonv2), but that shouldn't prevent it from doing a version switch to 1.25 and returning an answer.

Comment From: aslatter

What more info would you be looking for?

Here's a minimal repro (modulo everything else on my machine, I guess):

mkdir tmp
cd tmp
go version
printf "module something\ngo 1.25\n"  > go.mod
GOEXPERIMENT=jsonv2 go env -json

The output is:

go version go1.24.1 darwin/arm64
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x3 pc=0x100dd2a40]

goroutine 1 [running]:
cmd/go/internal/fips140.Init()
    cmd/go/internal/fips140/fips140.go:117 +0x120
cmd/go/internal/modload.Init()
    cmd/go/internal/modload/init.go:419 +0x38
cmd/go/internal/toolchain.Exec({0x140000aca58, 0x8})
    cmd/go/internal/toolchain/select.go:356 +0x46c
cmd/go/internal/toolchain.Select()
    cmd/go/internal/toolchain/select.go:284 +0xf28
main.main()
    cmd/go/main.go:107 +0x58

Comment From: dmitshur

It sounds this happens when the toolchain selection is involved, as it doesn't happen with a local go1.25.0 toolchain. Can you try it again with GODEBUG=toolchaintrace=1 and see if that adds helpful information for seeing at which point this happens? Well, while typing the above it turned out quick enough for me to try:

$ GOEXPERIMENT=jsonv2 GODEBUG=toolchaintrace=1 go env -json
go: upgrading toolchain to go1.25.0 (required by go line in go.mod; upgrade allowed by GOTOOLCHAIN=auto)
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x3 pc=0x102ae2a40]

goroutine 1 [running]:
cmd/go/internal/fips140.Init()
    cmd/go/internal/fips140/fips140.go:117 +0x120
cmd/go/internal/modload.Init()
    cmd/go/internal/modload/init.go:419 +0x38
cmd/go/internal/toolchain.Exec({0x14000010a50, 0x8})
    cmd/go/internal/toolchain/select.go:356 +0x46c
cmd/go/internal/toolchain.Select()
    cmd/go/internal/toolchain/select.go:284 +0xf28
main.main()
    cmd/go/main.go:107 +0x58

I also see it happens with go version, go env, too, not just go env -json.

This is likely similar to past issues like #74111 and #61455. CC @golang/command-line, @prattmic.

Comment From: aslatter

FYI if I upgrade my base install to Go 1.24.6, the problem goes away. I can downgrade back to 1.24.1, but if it is a bug that has been fixed there probably isn't anything that can be done.

Comment From: dmitshur

Maybe this issue is exactly the same as #74111. The fix for that was backported to Go 1.24.5. Can you see if it happens with Go 1.24.4 but not 1.24.5?

Comment From: aslatter

I think the JSON flag is a red-herring - I just made assumptions based on the json experiment. I get the crash without the -json flag, and my stack-trace looks the same as what's in #74111, and I get the same error with a nonsense GOEXPERIMENT value.

If your spidey-senses are still giving you reason to worry I can try more versions, but my guess is I won't learn anything.

Comment From: dmitshur

Sounds good, let's close it as a duplicate of that issue. Thanks for the report.

Golang cmd/go, encoding/json/v2: setting GOEXPERIMENT=jsonv2 causes "go env -json" to crash

Go version

Output of go env in your module/workspace:

What did you do?

What did you see happen?

What did you expect to see?

Output of `go env` in your module/workspace: