unsafe: document a difference in behavior between unsafe.Sizeof(x) and reflect.TypeOf(x).Size()

[Gandem]

Go version

go version go1.22.3 darwin/arm64

Output of go env in your module/workspace:

```shell GO111MODULE='on' GOARCH='arm64' GOBIN='' GOCACHE='/Users/nayef.ghattas/Library/Caches/go-build' GOENV='/Users/nayef.ghattas/Library/Application Support/go/env' GOEXE='' GOEXPERIMENT='' GOFLAGS='' GOHOSTARCH='arm64' GOHOSTOS='darwin' GOINSECURE='' GOMODCACHE='/Users/nayef.ghattas/go/pkg/mod' GONOPROXY='' GOOS='darwin' GOPATH='/Users/nayef.ghattas/go' GOPRIVATE='' GOPROXY='https://proxy.golang.org|direct' GOROOT='/opt/homebrew/Cellar/go/1.22.3/libexec' GOSUMDB='sum.golang.org' GOTMPDIR='' GOTOOLCHAIN='auto' GOTOOLDIR='/opt/homebrew/Cellar/go/1.22.3/libexec/pkg/tool/darwin_arm64' GOVCS='' GOVERSION='go1.22.3' GCCGO='gccgo' AR='ar' CC='cc' CXX='c++' CGO_ENABLED='1' GOMOD='/dev/null' 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 arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/z1/9gbm9wxx6rl1l71csb2w18hc0000gq/T/go-build3531211748=/tmp/go-build -gno-record-gcc-switches -fno-common' ```

What did you do?

go doc unsafe.Sizeof and go doc reflect.Type

What did you see happen?

Per https://go.dev/play/p/ovkwhK0eSWV for a variable of interface static type:

• unsafe.Sizeof() returns the size of the static type of the interface (which is 2*WordCount = 16 on most architectures)
• reflect.TypeOf().Size() returns the size of the dynamic type of the interface (which depends on the number of fields and underlying padding of the struct implementing that interface)

What did you expect to see?

A statement either in go doc unsafe.Sizeof or in go doc reflect.Type that explains the difference in behavior between both functions for variables of interface static type.

As a matter of fact go doc reflect.Type mentions:

	// Size returns the number of bytes needed to store
	// a value of the given type; it is analogous to unsafe.Sizeof.
	Size() uintptr

Which can be slightly misleading, as unsafe.Sizeof(i) will return a different result than reflect.TypeOf(i).Size() if i is of interface static type (as reflect.TypeOf() returns the dynamic type of the interface as already documented in go doc reflect.TypeOf)

I've submitted the following CL as a documentation suggestion: https://go-review.googlesource.com/c/go/+/586275

[gopherbot]

Change https://go.dev/cl/586275 mentions this issue: unsafe: document the behavior of Sizeof on an interface

[dmitshur]

CC @griesemer.

[itstarsun]

What you want is
reflect.TypeOf(&testAsInterface).Elem().Size() not reflect.TypeOf(testAsInterface).Size(), because reflect.TypeOf take an interface to the type you want.

See reflect.TypeFor for correct implementation:

go/src/reflect/type.go

Lines 3026 to 3033 in 1667dbd

	// TypeFor returns the [Type] that represents the type argument T.
	func TypeFor[T any]() Type {
	var v T
	if t := TypeOf(v); t != nil {
	return t // optimize for T being a non-interface kind
	}
	return TypeOf((*T)(nil)).Elem() // only for an interface kind
	}

[Gandem]

What you want is
reflect.TypeOf(&testAsInterface).Elem().Size() not reflect.TypeOf(testAsInterface).Size(), because reflect.TypeOf take an interface to the type you want.

While reflect.TypeOf(&testAsInterface).Elem().Size() is indeed a way to get the size of the static type of the interface through reflect (which in that case will be equivalent to unsafe.Sizeof), I believe it's still worth documenting the slight difference in behavior between reflect.TypeOf(x).Size() and unsafe.Sizeof(x) when x is of interface static type.