cmd/gofmt: Slice access not being formatted to reflect operator precedence

[richgrov]

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

$ go version
go version go1.20.4 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GO111MODULE=""                                                                                                                                                                                                                                           
GOARCH="amd64"                                                                                                                                                                                                                                                
GOBIN=""
GOCACHE="~/.cache/go-build"
GOENV="~/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="~/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="~/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.20.4"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="~/Desktop/oneworld/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 -fdebug-prefix-map=/tmp/go-build41908708=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://go.dev/play/p/7EnkFFdEfKt

gofmt (or clicking the "Format" button on the link) will insert spaces between operators to indicate their precedence (line 7). However, this does not apply to a slice accessor (line 10).

This can make code hard to read in more complex operations, such as converting cartesian coordinates to an index (example below.)

What did you expect to see?

func (arr *Array3d) getByte(x, y, z int) byte {
  return arr.data[x*arr.width*arr.height + z*arr.height + y)
}

What did you see instead?

func (arr *Array3d) getByte(x, y, z int) byte {
  return arr.data[x*arr.width*arr.height+z*arr.height+y]
}

[dr2chase]

@griesemer @mvdan

[griesemer]

The precedence emphasis depends on nesting level. It's not just slices, it also happens in array indexing, and I think a few other cases. I agree it's not very consistent and has been brought up before. It's also trivial to change.

But changing this would cause a huge amount of churn millions of .go files so we have refrained from doing so in the past.
Putting on hold for now.

cc: @rsc who originally wrote the emphasis algorithm.