Warning if levels of indentation exceeds, say, 4

[tbodt]

I'd like to have a warning from Clippy if I write code with more than 4 levels of indentation, to incentivize me to split up complicated functions into less complicated ones.

[Manishearth]

Clippy already warns for complicated functions, base on cyclomatic complexity. The threshold is configurable.

[felix91gr]

I think this issue is outdated by now.

Lemme explain. First, existing lints:

1. The Cyclomatic Complexity lint was renamed (and an attempt was made at restructuring it) as Cognitive Complexity, based on a publicity paper by SonarSource. I did that work, and after many months of research, I concluded that such a lint isn't even possible with the technology we have available right now. The work was canned, and the lint later downgraded to the nursery.
2. That lint is available at the nursery category if anyone wants to use it. I have to update its documentation though, to point out the many issues it has.

Besides those facts, regarding indentation in particular:

In Rust, there can be two functions that do exactly the same, step by step (but eg with or without a let chain to reduce indentation), that will have different indentation levels. So indentation isn't even a stable indicator of complexity.

Most of the relevant discussion is at #3793

If anyone needs some help with this, I'm all ears. But I think this issue can be safely closed.

[Centri3]

I agree with closing this, but for a different reason: excessive_nesting has been added way after 2017. This specifically looks for nesting so isn't really based on complexity or formatting (except in the case of blocks with only an expression)