Add new static analyzer: ineffassign

[farazdagi]

What type of PR is this?

Other / QA Toolset

What does this PR do? Why is it needed?

• The ineffassign tool proved to be a valuable asset in detecting subtle issues in variable assignments (see Check err for filterAttestationsForBlockInclusion #7398 and Fix ineffectual assignments #7403). @terencechain suggested that we can benefit from static code analyzer that enforces effectual assignments only. This PR introduces such an analyzer.

Which issues(s) does this PR fix?

N/A

Other notes for review

• ❗ Merge after Enable cryptorand analyzer in slasher #7417

• Here is how output of this analyzer looks like:
  

• The only exceptions are the following constructs:

func (ds *Service) detectIncomingAttestations(ctx context.Context, ch chan *ethpb.IndexedAttestation) {
  ctx, span := trace.StartSpan(ctx, "detection.detectIncomingAttestations")
  defer span.End()
  // ctx is not used at all in this func

Although ctx after re-assigning from trace.StartSpan is not used in a func (making in ineffectual assignment), we seem to be ok with ctx argument not used within funcs (we have many places where ctx argument is defined but not used). Normally, non-used arguments are immediately removed, but with ctx we are lenient in our current code.

As such, replacing ctx, span := trace.StartSpan(ctx, "detection.detectIncomingAttestations") with _, span := trace.StartSpan(ctx, "detection.detectIncomingAttestations") will require next developer to be careful enough, to make sure ctx is overwritten again.

NB: Overall, I think even ctx variable shouldn't be used as placeholder, and if func doesn't rely on it, that argument should be removed and only reintroduced when necessary. If we decide to proceed with that, updating this analyzer will be trivial (just delete two lines of code), moreover, after the deletion this analyzer will help detect cases which need to be updated.

[codecov]

Codecov Report

Merging #7413 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7413      +/-   ##
==========================================
- Coverage   60.48%   60.47%   -0.01%     
==========================================
  Files         419      419              
  Lines       30618    30618              
==========================================
- Hits        18519    18516       -3     
- Misses       9070     9072       +2     
- Partials     3029     3030       +1