Add UseLet style rule

[tresni]

I asked a question about my rule in the #detekt channel, and there seemed to be some appetite for the rule. This is what my rule currently looks like.

I found that in a code base I was working with, there were a lot of instances of if (x != null) { do_something(x) } else null and wanted to encourage the use of ?.let in this specific case. The plugin also looks for if (x == null) { null } else do_something(x) as we had some of that too.

[detekt-ci]

	Messages
📖	Thanks for adding a new rule to detekt ❤️
✅	We detekted that you added tests, to your rule, that's awesome!
✅	We detekted that you updated the default-detekt-config.yml file, that's awesome!

Generated by 🚫 dangerJS against 49a5a00

[codecov]

Codecov Report

Merging #6091 (49a5a00) into main (a8936c1) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main    #6091      +/-   ##
============================================
+ Coverage     84.83%   84.86%   +0.02%     
- Complexity     3997     4011      +14     
============================================
  Files           567      568       +1     
  Lines         13400    13422      +22     
  Branches       2362     2368       +6     
============================================
+ Hits          11368    11390      +22     
  Misses          871      871              
  Partials       1161     1161              

Impacted Files	Coverage Δ
...rturbosch/detekt/rules/style/StyleGuideProvider.kt	100.00% <100.00%> (ø)
.../io/gitlab/arturbosch/detekt/rules/style/UseLet.kt	100.00% <100.00%> (ø)

[BraisGabin]

LGTM. Just remove the documentation update so we can talk about it in other pr because I'm not 100% sure about it. But that shouldn't block the rest of the pr.

[marschwar]

Thank you for the rule. It looks really good.

I know this style of tests is really popular but to me it feels harder to read than necessary and hard to maintain. But I may be the only one to see it that way.

I agree with @BraisGabin that the documentation change should be its own pr.

[BraisGabin]

I agree they are hard to read. But test all the cases possible cases is really nice. All those tests would be a nightmare to maintain too if they are duplicated time over time... It's a difficult compromise.

[marschwar]

I think I might have found a false positive.

    fun testCallToCreateTempFile(s: String?) {
        val x = if (s == null) {
            println("foo")
            null
        } else {
            "x"
        }
    }

In my opinion this cannot be easily rewritten using s?.let.

[tresni]

@marschwar I agree that that is the exception case. We are using that as the times to use @Suppress as I don't know a meaningful way to exclude that. In our code base, we found only a single time that we were doing that vs ~100 cases where we should have just used let. That hit rate was acceptable for us.

It was pointed out that you could rewrite that as s?.let { "X" } ?: run { println("foo") } but internally we decided that wasn't really better.

[atulgpt]

@marschwar I agree that that is the exception case. We are using that as the times to use @Suppress as I don't know a meaningful way to exclude that. In our code base, we found only a single time that we were doing that vs ~100 cases where we should have just used let. That hit rate was acceptable for us.

It was pointed out that you could rewrite that as s?.let { "X" } ?: run { println("foo") } but internally we decided that wasn't really better.

Hi, @tresni thanks for contributing towards the detekt rule. Also regarding the above false positive you can ignore that if else branch has more statements other than returning null

[BraisGabin]

Also regarding the above false positive you can ignore that if else branch has more statements other than returning null

Yes. And I think that with that, and a test to demonstrate it, this rule is ready to merge.

[marschwar]

Great work.

@detekt/maintainers We could enable it for the detekt codebase right away. There are no findings.

[marschwar]

@tresni Could you please activate your rule in the config/detekt/detekt.yml.