Rodrigo Mesquita authored Jul 08, 2025 and Marge Bot committed Sep 12, 2025

This commit makes GHC read `lib/targets/default.target`, a file with a
serialized value of `ghc-toolchain`'s `GHC.Toolchain.Target`.

Moreover, it removes all the now-redundant entries from `lib/settings`
that are configured as part of a `Target` but were being written into
`settings`.

This makes it easier to support multiple targets from the same compiler
(aka runtime retargetability). `ghc-toolchain` can be re-run many times
standalone to produce a `Target` description for different targets, and,
in the future, GHC will be able to pick at runtime amongst different
`Target` files.

This commit only makes it read the default `Target` configured in-tree
or configured when installing the bindist.

The remaining bits of `settings` need to be moved to `Target` in follow
up commits, but ultimately they all should be moved since they are
per-target relevant.

Fixes #24212

On Windows, the constant overhead of parsing a slightly more complex
data structure causes some small-allocation tests to wiggle around 1 to
2 extra MB (1-2% in these cases).

-------------------------
Metric Increase:
    MultiLayerModulesTH_OneShot
    T10421
    T10547
    T12234
    T12425
    T13035
    T18140
    T18923
    T9198
    TcPlugin_RewritePerf
-------------------------