[cfe-dev] [RFC] C++20 modules dependency discovery

[Michael Spencer via cfe-dev]

C++20 is coming and we need to decide how clang will handle dependency
discovery for modules.  In the following, module means compiled C++20
module interface unit, and I will use header unit to refer to the thing
generated by a clang module map.

There are two different modes we care about when it comes to module
dependencies:  implicit and explicit.

Implicit Modules
================

For implicit modules the build system doesn’t know anything about them, and
thus can’t care about any intermediate files.  It needs to know about all
source files that if changed should cause a rebuild of this translation
unit.

For this case clang needs to output the full transitive set of
dependencies, excluding any intermediate temporaries.  This also means that
we can’t get the full set of dependencies without actually at least
preprocessing every module transitively referenced.  This means that `-E
-MD` should fail if it can’t find a module or header unit.

Explicit Modules
================

For explicit modules we only need to know the direct dependencies, as the
build system will handle the transitive set.

For preprocessing we still need to import header units (but only their
preprocessor state), but not normal modules.  For this case it’s ok if `-E
-MD` fails to find a module.  But it does still need to be able to find
header units and module maps.  Additionally the normal Make output syntax
is not sufficient to represent the needed information unless the driver
decides how modules and header units should be built and where intermediate
files should go.  There’s currently a json format working its way through
the tooling subgroup of the standards committee that I think we should
adopt for this.

I think we need separate modes in clang for these along with support for
scanning through header units without actually building a clang module for
them. clang-scan-deps will make use of the explicit mode.  The question I
have is how should we select this mode, and what clang options do we need
to add?

Proposal
========

As a rough idea I propose the following:

* `-M?` means output the json format which can correctly represent
dependencies on a module for which we don’t know what the final file path
will be.
* `clang++ -std=c++20 -E -MD -fimplicit-header-units` should implicitly
find header unit sources, but not modules (as we've not given it any way to
look up how to build modules).
    * This means that the dep file will contain a bunch of `.h`s,
`.modulemap`s, and any `.pcm`s explicitly listed on the command line.
    * This also means erroring on unknown imported modules as we don't know
what to put in the dep file for them.
* `clang++ -std=c++20 -E -MD -fimplicit-header-units
-fimplicit-module-lookup=?`  should do the same as the above, except that
it does know how to find modules, and should list all of the transitive
dependencies of any modules it finds.
* `clang++ -std=c++20 -E -MD` should fail if it hits a module or header
unit, and should never do implicit lookup.
* `clang++ -std=c++20 -E -M?` should scan through header units without
actually building clang modules for them (to get the macros it needs), and
should note all module imports.
    * This means that the dep file will contain only `.h`s that it
includes, and use the json representation of header units and modules.
    * It will also be shallow, with only direct dependencies.

Additionally, we should (eventually) make:

`$ clang++ -std=c++20 a.cpp b.cpp c.cpp a.cppm -o program`

Work without a build system, even in the presence of modules.  To do this
we will need to prescan the files to determine the module dependencies
between them and then build them in dependency order.  This does mean
adding a (simple) build system to the driver (maybe [llbuild](
https://github.com/apple/swift-llbuild)?), but I think it’s worth it to
make simple cases simple.  It may also make sense to actually push this
work out to a real build system.  For example have clang write a temporary
ninja file and invoke ninja to perform the build.

- Michael Spencer
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