Analyzer should warn when patterns are used but not required

[bwilkerson]

Given the following code:

void f(int x) {
  var (num a) = x;
}

the analyzer should warn that the variable pattern is not required and the code could be rewritten as

void f(int x) {
  num a = x;
}

[srawlins]

Good feature. CC @keertip @pq

@bwilkerson "warn when patterns are used but not required" sounds like a very broad concept. Would this require a fairly thorough review of the spec to identify trivial/unnecessary patterns?

[bwilkerson]

That's what I was originally thinking. An alternative would be to find common patterns over time and add support as users run into these cases.

[srawlins]

As far as adding them over time, each such change could be expensive to add, as it could require cleaning the Dart SDK, google3, flutter/flutter, flutter/engine, flutter/plugins, flutter/packages, flutter/cocoon, and the Flutter customer test repos. The ROI is not good there.

Alternatively, writing quick assists to convert trivial patterns into non-patterns can be done incrementally without the clean-up cost, and could be pretty friendly to users.

Maybe it wouldn't be that much work to walk through the spec and dream up as many "trivial pattern" examples as we can early on, to create a warning.

[vsmenon]

@bwilkerson @srawlins - do we need this on the stable milestone? Can we move to cleanup?

[srawlins]

I vote 'cleanup' milestone.