Removing really old event types

[TeofilC]

ghc-events has strong backwards compatibility guarantees, which is good. But we also support parsing certain events from extremely old versions of the eventlog spec. For instance we support a bunch of events for Mercury, which seems to have been a project that used the eventlog 14 years ago.

Pretty much every consumer of the eventlog has to ignore these events and I think getting rid of these from the code would tidy things up at no real cost, since I think no one is using them.

My suggestion is that we deprecate them and then make a new major release that removes them.

If in the future, GHC stops emitting an event, we should also get better at documenting that in the ghc-events docs.

[Mikolaj]

Agreed!

[wenkokke]

Perhaps we could add a single DeprecatedEvent constructor and create pattern synonyms for backwards compatibility? That way we can remove these events but not break any existing code, under the assumption that these events aren't emitted anymore anyway. This could also serve as documentation for reserved tags.