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Add flag to control whether self-recompilation information is written to interface

Matthew Pickering requested to merge wip/22188 into master

This patch adds the flag -fwrite-self-recomp-info which controls whether interface files contain the information necessary to answer the question:

Do I need to recompile myself or is this current interface file suitable?

Why? Most packages are only built once either by a distribution or cabal and then placed into an immutable store, after which we will never ask this question. Therefore we can derive two benefits from omitting this information.

  • Primary motivation: It vastly reduces the surface area for creating non-deterministic interface files. See issue #10424 which motivated a proper fix to that issue. Distributions have long contained versions of GHC which just have broken self-recompilation checking (in order to get deterministic interface files).

  • Secondary motivation: This reduces the size of interface files slightly.. the mi_usages field can be quite big but probably this isn't such a great benefit.

  • Third motivation: Conceptually clarity about which parts of an interface file are used in order to communicate with subsequent packages about the interface for a module. And which parts are used to self-communicate during recompilation checking.

The main tracking issue is #22188 (closed) but fixes issues such as #10424 in a proper way.

Edited by Matthew Pickering

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