On Mon, Feb 11, 2019 at 19:39:39 -0500, Matthew Woehlke wrote:
> ...but how does the build system know? If I write 'import foo', and
> 'foo' is supposed to be a source that I haven't written yet, how does
> the build system tell the difference between that and a 'foo' that came
> from system packages?

There are 3 possibilities that I see:

  - It comes from a source that is supposed to be scanned during the
    build already. If that isn't scanned, not finding the `foo.mod` is a
    compiler error. This is exactly the same as a generated header the
    build system doesn't know about missing at compile time. The error
    is in not telling the build tool about the source containing the
    module.
  - It is external and implicitly compiled via compiler magic. The
    compiler must tell the build system where this is being done during
    `scan` and the resulting output file of the implicit module. The
    `collate` step in this case handles hooking up dependencies
    properly.
  - It is the build tools's job to be told about modules from external
    dependencies. Then it is on the author of the build tool input to
    add this rule (whether done manually, via CMake's usage
    requirements, pkg-config entries, etc. is an implementation detail).

None of these require silencing errors about missing inputs from the
`scan` step. Are there other cases that I'm missing that still results
in a "correct" build?

--Ben