looppointer

An analyzer that finds pointers for loop variables.

What's this?

Sample problem code from: https://github.com/kyoh86/looppointer/blob/main/testdata/simple/simple.go

package main

func main() {
	var intSlice []*int

	println("loop expecting 10, 11, 12, 13")
	for _, p := range []int{10, 11, 12, 13} {
		intSlice = append(intSlice, &p) // want "taking a pointer for the loop variable p"
	}

	println(`slice expecting "10, 11, 12, 13" but "13, 13, 13, 13"`)
	for _, p := range intSlice {
		printp(p)
	}
}

func printp(p *int) {
	println(*p)
}

In Go, the p variable in the above loops is actually a single variable. So in many case (like the above), using it makes for us annoying bugs.

You can find them with looppointer, and fix it.

package main

func main() {
	var intSlice []*int

	println("loop expecting 10, 11, 12, 13")
	for i, p := range []int{10, 11, 12, 13} {
    p := p                          // FIX variable into the local variable
		intSlice = append(intSlice, &p) 
	}

	println(`slice expecting "10, 11, 12, 13"`)
	for _, p := range intSlice {
		printp(p)
	}
}

func printp(p *int) {
	println(*p)
}

ref: https://github.com/kyoh86/looppointer/blob/main/testdata/fixed/fixed.go

Sensing policy

I want to make looppointer as nervous as possible. So some false-positves will be reported.

e.g.

func TestSample(t *testing.T) {
  for _, p := []int{10, 11, 12, 13} {
    t.Run(func(t *testing.T) {
      s = &p // t.Run always called instantly, so it will not be bug.
      ...
    })
  }
}

They can be escaped with pining-variable:

func TestSample(t *testing.T) {
  for _, p := []int{10, 11, 12, 13} {
    p := p // pin a variable to local in the loop
    t.Run(func(t *testing.T) {
      s = &p
      ...
    })
  }
}

If you want to ignore false-positives (with some lints ignored), you should use exportloopref.

Nolint

Diagnostics by looppointer can be suppress with the line comment // nolint:looppointer.

func TestSample(t *testing.T) {
  for _, p := []int{10, 11, 12, 13} {
    t.Run(func(t *testing.T) {
      s = &p // nolint
      ...
    })
  }
}

Install

go:

$ go install github.com/kyoh86/looppointer/cmd/looppointer@latest

homebrew:

$ brew install kyoh86/tap/looppointer

gordon:

$ gordon install kyoh86/looppointer

Usage

looppointer [-flag] [package]

Flags

Flag	Description
-V	print version and exit
-all	no effect (deprecated)
-c int	display offending line with this many lines of context (default -1)
-cpuprofile string	write CPU profile to this file
-debug string	debug flags, any subset of "fpstv"
-fix	apply all suggested fixes
-flags	print analyzer flags in JSON
-json	emit JSON output
-memprofile string	write memory profile to this file
-source	no effect (deprecated)
-tags string	no effect (deprecated)
-trace string	write trace log to this file
-v	no effect (deprecated)

LICENSE

This is distributed under the MIT License.