cmd/compile: Incorrect column number in build error for unused variable

[bradleyfalzon]

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

go version go1.9rc1 linux/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/bradleyf/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build365274238=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

package main

import "fmt"

func main() {
        n, err := fmt.Print(1)
}

Run this program with go build

What did you expect to see?

./main.go:6:2: n declared and not used
./main.go:6:5: err declared and not used

Note the column numbers.

What did you see instead?

./main.go:6:22: n declared and not used
./main.go:6:22: err declared and not used

Column 22 for the above program is the argument to the fmt.Print function. There's nothing wrong with the argument, the problem is with the returned arguments.

It appears this may be a known issue, mentioned in 2a5cf48 but it's not clear whether this is the same issue.

At the very least, when the editor is highlighting the argument but the error is elsewhere on the same line, it's confusing.

[mvdan]

CC @mdempsky @griesemer

[bradleyfalzon]

Another example which I most likely the same issue but it is confusing when the editor is highlighting the wrong identifier:

package main

func main() {
        var a int64 = 1
        Foo("SOMEVAR", a)
}

func Foo(_ string, _ int) {}

$ go build
./main.go:5:6: cannot use a (type int64) as type int in argument to Foo

Column 6, in this case, is the first argument to the Foo function call, but the problem is with the second argument.

[griesemer]

Haven't checked if it's a duplicate but this can be improved for sure.

[dmitshur]

This is what a code editor that displays issues inline shows the user:

Note that it visually points to the 3rd parameter, the empty string, very prominently.

However, the actual issue is that the logger variable is declared and not used. There is no issue with the empty string parameter.

Emitting an incorrect column is an issue that is magnified in the case of editors that visualize that information more prominently than what's visible by glancing at terminal output only.

[dmitshur]

@griesemer This is a feature that's new to Go 1.9, and it has a bug. What's the reason this issue is added to Go 1.10 milestone instead of 1.9?

Is it because the bug is considered to be minor and happens only in a few cases, so the feature is still overall helpful and worth being included, despite the incorrect behavior in this particular case?

[griesemer]

@shurcooL It's not a show stopper. Yes, it's a new feature, and we know it has bugs (mostly due to the nature of how the existing code deals with position information). It can be disabled with the -C cmd/compile flag. It's definitively minor, even though it's annoying and perhaps confusing if one is using an IDE and not very familiar with the language. We will fix all these over time and the intention is to have 100% accurate positions.

[odeke-em]

Great news, seems like @mdempsky fixed this issue with commit f22ef70 and CL https://go-review.googlesource.com/c/66810/
Running it on tip 32f994a gives me

$ go version && go run main.go 
go version devel +32f994a Mon Nov 6 07:09:50 2017 +0000 darwin/amd64
# command-line-arguments
./main.go:6:9: n declared and not used
./main.go:6:12: err declared and not used

I'll submit a test to ensure we don't regress.

[gopherbot]

Change https://golang.org/cl/76150 mentions this issue: cmd/compile: lock in test for column numbers in unused error

[odeke-em]

Alright, the regress test addressed by CL https://go-review.googlesource.com/c/go/+/76150 and commit 0c55495. Please get the latest code.