proposal: cmd/vet: add check for unchecked error return values

[lolbinarycat]

if you are using a lot of functions that return an error value, one mistake you might make is this:

val1, err := someFunc()
if err != nil {
    return err
}
someStruct.someField, err = someOtherFunc()
someOtherStruct.someField, err = Func3()
if err != nil {
    return err
}

It may look simple in the example, but if you have a bunch of loops and if statements, it could be much harder to see, e.g.

// ...
for someCondition {
    strct.Field, err = GetFoo()
}
if err != nil {
    return err
}

This will catch an error if it is the last call is an error, but will let it past otherwise. It is also hard to track down as you don't know what you're looking for, or where you're looking for it, as the function will likely continue running but produce no output, or segfault due to derefrencing a nil pointer (that was supposed to be set by a function that returned an error).
this is worsened by #337.

[bcmills]

See previously #20803, #20148, #19727.

[lolbinarycat]

See previously #20803, #20148, #19727.

Thanks, I will note that none of these address the exact pattern I'm talking about.
namely, #20148 is about forgetting to assign to err, while this issue is about assigning to err, but then forgetting to check it.
Although, they are similar enough to perhaps be parts of the same go vet rule if implemented.

[ianlancetaylor]

If I'm reading this right, I think this is actually a dup of #23142.

[lolbinarycat]

If I'm reading this right, I think this is actually a dup of #23142.

They both come from the same underlying cause, but the person who originally opened that issue didn't open it with that title, and it reads more like someone asking for help than proposing a change.

This proposal also is more focused on error values (i.e. variables of type error).

I also would be willing to try to create a prototype for this.

[rsc]

Turned #23142 into a proposal; closing this one as a duplicate of that.