As reported by @ricarkol in mirage/mirage#847 (comment)

Take a `Configure phase key, such as warn_error: mirage configure --warn-error true, and do a mirage build afterwards. The mirage build step does not take the warn_error as true into account at all. mirage describe or mirage describe --eval presume warn_error to be false as well. The full commandline ends up in .mirage.config, but isn't respected/loaded for the build. Additionally, a let warn_error = true appears in the generated main.ml key_gen.ml - my intuition is that this shouldn't be there since it is a `Configure-only key.

I tried and specified a custom my_string key - also using phase:`Configure - which didn't get transported over to the mirage build.

The only `Configure phase key which is properly transported to build seems to be --target.

I tried to find the issue within functoria and mirage but without any success. Someone more familiar with the codebase (@Drup ? @samoht ? ;) may have some ideas what is wrong here.

It is pretty annoying and a showstopper for the --gdb support (mirage/mirage#847) @ricarkol implemented.