x/tools/go/analysis/passes/nilness: no position information with error

[reillywatson]

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

$ go version
1.12

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
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/reilly/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/reilly/league/services:/Users/reilly/go:"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/gz/4ytqr0r12q1fbx8ct_w3k_880000gn/T/go-build175674008=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Ran the nilness analyzer on this (simplified) package:

package foo

import (
	"fmt"
)

func Foo() {
	var x []int
	for _, a := range x {
		fmt.Println(a)
	}
}

What did you expect to see?

Ideally: a different error about how you're iterating over a nil slice
Less ideally: the same error, but with information telling me where the error is

What did you see instead?

-: nil dereference in index operation

This kind of error is pretty hard to track down if I'm running the analyzer over a large codebase, because it doesn't say which file or package is failing.

[julieqiu]

/cc @matloob

[acestronautical]

This is a major problem that deserves attention. I work for a medium sized tech company looking to adopt golangci-lint and this issue is holding me back.

[gopherbot]

Change https://golang.org/cl/208599 mentions this issue: go/ssa: add position data in indexed range loops

[findleyr]

@vovapi's CL above adds position information to the error message, but I think to improve this further we'd have alter the way go/ssa tracks position information. Something similar was done for staticcheck: dominikh/go-tools@c14c261

[uhthomas]

Thought it might be worth adding a case where the nilness analyzer fails to provide position information where it seemingly should do so.

func do(a interface{}) {
	switch a.(type) {
	case nil:
		return
	}
	switch a.(type) {
	case nil:
		return
	}
}

https://go.dev/play/p/3K_9FFGOWzd

❯ go run golang.org/x/tools/go/analysis/passes/nilness/cmd/nilness@latest test.go
-: impossible condition: non-nil == nil
exit status 3

[gopherbot]

Change https://go.dev/cl/394694 mentions this issue: go/ssa: add position information for switch case conditions

[zpavlinovic]

Thought it might be worth adding a case where the nilness analyzer fails to provide position information where it seemingly should do so.

...

Change https://go.dev/cl/394694 should have now addressed the issue with positions in case conditions.