reduce unreachable-code churn after todo!()

[llogiq]

This was prompted by a rant by /u/matthieum on /r/rust. The idea here is that while writing the code, either rust-analyzer or ourselves may insert todo!() at places we intend to implement later. Unfortunately, that marks all following code as unreachable, which will prompt some lint churn.

So to combat that, I modify the lint to check whether the unreachability stems from a todo!() macro and in this case omit the lint (by setting diverges to Diverges::WarnedAlways instead of Always(..)). Hopefully that makes the unreachable code lint less churn-y during development and improve the developer experience.

I inserted the check after when we already found some unreachable code, so perf shouldn't suffer too badly.

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[Kivooeo]

Could you please also add a link to the Zulip discussion? It would help provide context.

Also, could you please add a test to demonstrate that it works?

[llogiq]

If we find that this is too heavy on perf (which it would only be in macro-heavy crates where macros produce a lot of exprs of type !), we could first check that the expression has ExprKind::Call(path, [txt]) where path refers to core::panic::panic.

[iago-lito]

Here is the zulip thread.

[llogiq]

Also, could you please add a test to demonstrate that it works?

I added some code to the respective UI test. E.g. current rustc warns on todo!(); let _ = false.