Go-tools: staticcheck: detect ineffective assignments that do not involve function calls

[dominikh]

Currently, SA4006 only detects unused values if the rhs of the assignment is a function call, but not for other expressions. That is because in the SSA form, those assignments are never represented in the first place.

Example:

func fn1(arg int) {
    x := 1
    x = 2
    x = 3
    fmt.Println(x)
}

func fn2(arg int) {
    x := gen()
    x = gen()
    x = gen()
    fmt.Println(x)
}

func gen() int { return 0 }

/*
foo.go:13:2: this value of x is never used (SA4006)
foo.go:14:2: this value of x is never used (SA4006)
*/

[mvdan]

If this information is removed from the SSA representation, how are you going to retrieve it?

[dominikh]

Also we don't seem to be matching post increment/decrement or <binop>= even when they show up in SSA.

[dgryski]

How does ineffassign do it?

[dominikh]

It probably just looks at the AST differently than we do.

[ainar-g]

This one is really annoying and bites me all the time. Just recently, on a hobby project of mine, I've spent ~15 minutes pondering, why do I have a panic. The code used to be:

func NewFoo() (f Foo) {
    return foo{}
}

(Foo is an interface.) Then foo got some pointer fields, and instead of rewriting it into:

func NewFoo(b *Bar) (f Foo) {
    f = foo{bar: b}
    // Some stuff.
    return f
}

I accidentally miss-rewrote it into:

func NewFoo(b *Bar) (f Foo) {
    f = foo{bar: b}
    // Some stuff.
    return foo{} // N.B. Should be "f" here.
}

I guess ineffassign it is for now :-/

[dominikh]

@ainar-g I'm working on it.

[vearutop]

I had a buggy code with forgotten pointer, that hopefully could be flagged as ineffective assignment.

func foo(r Row /* this should have been a pointer */, postcode region.Postcode) {
    if r.Postcode != postcode {
        r.Postcode = postcode
    }
}