Pedantic lint against `unreachable!()`

[Luro02]

It would be nice to have a lint, that detects any use of the unreachable! macro without arguments.

This is useful for procedural macros, where only the following is shown

error: proc-macro derive panicked
 --> tests/test_union.rs:3:10
  |
3 | #[derive(ShortHand)]
  |          ^^^^^^^^^
  |
  = help: message: internal error: entered unreachable code

error: aborting due to previous error

error: could not compile `shorthand`.

To learn more, run the command again with --verbose.

This is fine, if you only have one or two uses of unreachable, but it becomes very difficult in larger codebases, because you have to check every occurrence of unreachable in the codebase.

Adding some text to the macro will be a lot more helpful!
unreachable!("ident is neither enable nor disable")

error: proc-macro derive panicked
 --> tests/test_union.rs:3:10
  |
3 | #[derive(ShortHand)]
  |          ^^^^^^^^^
  |
  = help: message: internal error: entered unreachable code: ident is neither enable nor disable

error: aborting due to previous error

error: could not compile `shorthand`.

To learn more, run the command again with --verbose.

I think panic! in a proc-macro has the same issue.

[flip1995]

Easy extension of #4657

[chansuke]

I'd like to work on this issue

[flip1995]

I just realized, that this lint already exists, but is allow-by-default (and will stay that way). So you have to enable it with #[warn(clippy::unreachable)]. The only thing, I can think of on how to improve this lint, is to add a configuration option to disable the lint if an explanation is provided. Would that help?

[Luro02]

I think the opposite would be better, add a lint, that will warn if unreachable! has no explanation?

[flip1995]

That lint already exists. But it warns regardless of if unreachable! has an explanation or not. Playground

[flip1995]

Just realized, that an issue exists for this (the lint doesn't trigger, when a message is passed)

Duplicate of #3528