encoding/json/v2: inline struct fields don't work with `MarshalToFunc` but do work with `MarshalerTo`

[danvolchek]

Go version

go version go1.25.4 X:nodwarf5 linux/amd64

Output of go env in your module/workspace:

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/dan/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/dan/.config/go/env'
GOEXE=''
GOEXPERIMENT='nodwarf5'
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build3030282131=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/dan/Desktop/Stuff/development/go-rw2/go.mod'
GOMODCACHE='/home/dan/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/dan/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/dan/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/lib/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.25.4 X:nodwarf5'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

Ran this program: https://go.dev/play/p/Ha2umnCEcq8

What did you see happen?

a1: "custom marshaler for A"
b1: {}

What did you expect to see?

a1: "custom marshaler for A"
b1: "custom marshaler for B"

More specifically, both A1 and B1 embed another struct. Each of the embedded structs defines a custom marshaler: A by implementing the MarshalJSONTo interface and B by implementing a MarshalToFunc.

I expect both of these to be used, but the MarshalToFunc is not, I assume because B is embedded/inlined in B1.

I noticed that assigning a name to B so it doesn't get inlined (https://go.dev/play/p/OcY38RyE7dr) invokes the MarshalToFunc, but the result isn't inlined anymore so the output is different:

a1: "custom marshaler for A"
b1: {"b":"custom marshaler for B"}

If this is a known limitation, I don't see it documented at https://pkg.go.dev/encoding/json/v2.