Go: declared and not used: x

declared and not used: x is a Go compile error, not a warning: a function declares the local variable x and never reads it. Go treats unused variables as errors on purpose, because they often point to a bug. Use the variable, delete it, or (temporarily) discard it with _ = x. Go 1.19 and earlier printed the same error as x declared but not used.

Example

package main

import "fmt"

func main() {
	x := 42                // declared and not used: x
	result, err := compute() // err is never read
	fmt.Println(result)
}

func compute() (int, error) { return 1, nil }

go build (or go vet, go run) stops with the file, line and column of each declaration, such as ./main.go:6:2: declared and not used: x. Both x and err are reported.

Fixes

// Use it
result, err := compute()
if err != nil {
	return err
}

// Discard values you do not need
result, _ := compute()

// Keep a variable around while debugging
x := 42
_ = x

Shadowing: the subtle case

var err error
if cond {
	data, err := load() // new err, shadows the outer one
	use(data)
}                       // inner err is never read: declared and not used: err
return err

Declare data first and use =: var data []byte; data, err = load().

Unused imports

Imports follow the same rule: an import that is not used fails with "os" imported and not used. Remove it, or use import _ "pkg" when you only need the package's side effects (its init functions). goimports and gopls add and remove imports automatically.