encoding/xml: embedded interface doesn't get marshaled correctly

[otaxhu]

Go version

go version go1.22.4 linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/albian/.cache/go-build'
GOENV='/home/albian/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/albian/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/albian/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/opt/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.22.4'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/albian/dev/problem/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2437414976=/tmp/go-build -gno-record-gcc-switches'

What did you do?

Embedding interface type in a struct, pass struct that implements that interface, then marshal it:
https://go.dev/play/p/SBfpU8nOB2S

Embedding concrete struct type in a struct, then marshal it:
https://go.dev/play/p/Wwf-DztRIlO

What did you see happen?

Output of first Playground link (https://go.dev/play/p/SBfpU8nOB2S):

<animal xmlns="urn:whatever">
    <Animal>
        <sound>Woof!</sound>
    </Animal>
</animal>

Output of second Playground link (https://go.dev/play/p/Wwf-DztRIlO):

<animal xmlns="urn:whatever">
    <sound>Woof!</sound>
</animal>

What did you expect to see?

I expected to get second output in both first and second Playground links.

According to the encoding/xml documentation, it says this two things:

https://pkg.go.dev/encoding/xml#Marshal

• an anonymous struct field is handled as if the fields of its value were part of the outer struct.

Marshal handles an interface value by marshaling the value it contains or, if the interface value is nil, by writing nothing.

So it should know that the interface value is a struct and is embedded, so it should treat the inner struct values as if it were part of the outer struct (basically accomplishing the second output)

[gabyhelp]

Related Issues and Documentation

• encoding/xml: fail to roundtrip with interface inside #46432
• encoding/xml: serialization of `interface{}` attribute is unsupported, even when dynamic type implements relevant interfaces #48624
• encoding/xml: exported non-struct anonymous fields are not marshaled #3803 (closed)
• encoding/xml: not properly unmarshaling through interface{} #6836 (closed)
• encoding/xml: XMLName value of embedded structs type not used in Marshaller #38260
• encoding/xml: embedded reference to substruct causes XML marshaller to panic on encoding #50164 (closed)
• encoding/json: Marshaling embedded structures which supports custom json.Marshaler interface #29336 (closed)
• encoding/json: Embedded structs and embedded interfaces treated differently when marshaling #54322 (closed)
• encoding/json: difference in behavior if embed a concrete type or interface for the same value #8386 (closed)
• encoding/xml: cannot unmarshal into without struct tags #65099

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[seankhliao]

this is the same behavior as json, perhaps we just need to document this, ref #54322

[otaxhu]

I don't think it should be like that, we are embedding the interface, so it should follow the semantics of embedding (outer struct contains all of the fields of inner struct).

[seankhliao]

interfaces don't have fields though, they have methods. and (most) methods of structs aren't called when marshaling

[gopherbot]

Change https://go.dev/cl/621076 mentions this issue: encoding/xml: document that embedded interfaces look like non-embedded ones

[otaxhu]

Ok that makes sense, sorry.

Updating the docs would be good.