bug: look like to watch same address when assign to another variable from sub-slice.

[tomoyukikino]

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

$ go version
go version go1.19.3 darwin/amd64

Does this issue reproduce with the latest release?

yes

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

intel MacOS Monterey version 12.6, yes I use go env.

go env Output

$ go env
GO111MODULE="off"
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/tomoyuki/Library/Caches/go-build"
GOENV="/Users/tomoyuki/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/tomoyuki/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/tomoyuki/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.19.3"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/wz/r55pw77x0_1dv15r4w3wtds00000gn/T/go-build1838909510=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I assign y slice variable from x sub-slice.
I try to append 30 to y variable. I expect to append y variable only. but append y and x.
perhaps, y and x is look like watch some address.
In this case, should be pass by value, but it's call by pointer.

package main

import "fmt"

func main() {
	x := []int{1, 2, 3, 4}
	y := x[:2]

	fmt.Println("x:", x) // x: [1 2 3 4]
	fmt.Println("y:", y) // y: [1 2]

	y = append(y, 30)
	fmt.Println("x:", x) // x: [1 2 30 4] ← I think x should be [1 2 3 4].
	fmt.Println("y:", y) // y: [1 2 30]
}

play : https://go.dev/play/p/HWwI5WCjDXm

What did you expect to see?

Of course, I know how to fix this bug, should use copy.

package main

import "fmt"

func main() {
	x := []int{1, 2, 3, 4}
	y := make([]int, len(x[:2]))
	copy(y, x[:2])

	fmt.Println("x:", x) // x: [1 2 3 4]
	fmt.Println("y:", y) // y: [1 2]

	y = append(y, 30)
	fmt.Println("x:", x) // x: [1 2 3 4] ← no problem
	fmt.Println("y:", y) // y: [1 2 30]
}

play : https://go.dev/play/p/IayY-PdoM0l

[ianlancetaylor]

That is how the language works. Please see https://go.dev/blog/slices-intro.