cmd/compile: go:linkname no longer works for type's method

[fumeboy]

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

1.20.1

Does this issue reproduce with the latest release?

yes

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

GO111MODULE="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/bytedance/Library/Caches/go-build"
GOENV="/Users/bytedance/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/bytedance/go/pkg/mod"
GONOPROXY="*.byted.org,*.everphoto.cn,git.smartisan.com"
GONOSUMDB="*.byted.org,*.everphoto.cn,git.smartisan.com"
GOOS="darwin"
GOPATH="/Users/bytedance/go"
GOPRIVATE="*.byted.org,*.everphoto.cn,git.smartisan.com"
GOPROXY="https://go-mod-proxy.byted.org,https://goproxy.cn,https://proxy.golang.org,direct"
GOROOT="/Users/bytedance/x/go/sdk/go1.20.1"
GOSUMDB="sum.golang.google.cn"
GOTMPDIR=""
GOTOOLDIR="/Users/bytedance/x/go/sdk/go1.20.1/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.20.1"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/bytedance/x/go/sdk/go1.20.1/src/cmd/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 -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/b1/75_q2h8x6_zcbd_746_lvht40000gn/T/go-build2405814180=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

i write this code to explain:

package a

import _ "unsafe"

type A string

//go:linkname A.Fmt fmt.Sprintf
func (A) Fmt(a ...any) string

func main() {
	println(A("%d").Fmt(1))
}

What did you expect to see?

at go1.19, i could run this program without compile error, the go:linkname works fine

What did you see instead?

./a.go:7:3: //go:linkname must refer to declared function or variable

and i print the context before the compile bad exit , i found the scope just contains funcdecl and typedecl, no type method decl

// cmd/compile/internal/noder/writer.go
switch obj := pw.curpkg.Scope().Lookup(l.local).(type) {
case *types2.Func, *types2.Var:
	if _, ok := pw.linknames[obj]; !ok {
		pw.linknames[obj] = l.remote
	} else {
		pw.errorf(l.pos, "duplicate //go:linkname for %s", l.local)
	}

default:

// print here
	for k,e := range pw.curpkg.Scope().Elems() {
		println(1, k,e.String()) // just funcdecl and typedecl, no type method decl
	}

	if types.AllowsGoVersion(1, 18) {
		pw.errorf(l.pos, "//go:linkname must refer to declared function or variable")
	}
}

[mauri870]

The go:linkname directive is meant to be used internally by the go runtime, and I believe it is not officially supported outside of this use case.

/cc @golang/compiler

[thanm]

Thanks for the report.

As @mauri870 remarks, this is not a supported use case for //go:linkname ; the linkname directive is intended for use in the runtime and in a few hand-picked cases in the standard library.

I should also add that //go:linkname is very definitely not covered by the Go 1 compatibility guarantee. We can (and do) make changes to the way //go:linkname works in new releases, and uses outside the Go runtime and standard library can easily break when moving from one release to the next.

Closing out this issue.