unsafe: document a difference in behavior between unsafe.Sizeof(x) and reflect.TypeOf(x).Size() #67465
Description
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