Weird "missing hi file" problem with a serializable Core patch

[Cheng Shao]

Hi all,

Following a short chat in #ghc last week, I did a first attempt of
reusing existing Iface logic to implement serialization for
codegen-related Core. The implementation is included in the attached
patch (~100 loc). As a quick and dirty validation of whether it works,
I also modified the codegen pipeline logic to do a roundtrip: after
CorePrep, the Core bits are converted to Iface, then we immediately
convert it back and use it for later compiling.

With the patch applied, stage-1 GHC would produce a "missing hi file"
error like:

: Bad interface file: _build/stage1/libraries/ghc-prim/build/GHC/Types.hi
      _build/stage1/libraries/ghc-prim/build/GHC/Types.hi:
openBinaryFile: does not exist (No such file or directory)

The error surprises me, since by the time we perform the Core-to-Core
roundtrip, the .hi file should already have been written to disk. Is
there anything obviously wrong with the implementation? I'd appreciate
any pointers or further questions, thanks a lot!

Best regards,
Cheng
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