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Expose `-fignore-asserts` as the CPP macro `__GLASGOW_HASKELL_ASSERTS_IGNORED__`

Marten Wijnja requested to merge trac-qqwy/ghc:qqwy/asserts-flag-cpp into master

Control.Exception.assert allows the user to do assertions in development which disappear when compiled with optimizations enabled.

However, Control.Exception.assert always throws an AssertFailed exception. There are cases in which a user might want to do other kinds of assertions: Either throwing a different kind of exception, or maybe different logic (that is not related to exceptions) all-together.

This MR exposes whether the fignore-asserts flag is enabled (either directly or by using -O1/-O2 without also specifying -fno-ignore-asserts) as the CPP macro __GLASGOW_HASKELL_ASSERTS_IGNORED__, therefore giving users a very straightforward way to define custom assert-like functionality.

This particular design was chosen in collaboration with @bgamari.

Fixes #24967 (closed)

Where is the key part of this patch? That is, what should reviewers look at first?

The only code of this MR is in GHC.SysTools.Cpp.hs.

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Edited by Marten Wijnja

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