For calling non-`()` functions, the Carbon->Carbon thunk now takes an
extra reference parameter and writes the target function's return value
out to that parameter. (At the SemIR level this is how returns already
work, but adding this extra reference parameter is needed so that the
function is lowered correctly.) The C++ thunk now creates a local
variable to be initialized by the Carbon thunk, and then returns that
value to the original C++ caller.
Instead of exporting a class or namespace each time a new C++ name
lookup discovers it, track that we have exported the entity on its name
scope, and if a new name lookup finds the same entity, produce the same
clang declaration.
We don't yet populate the bases or fields, so the class types show up as
empty classes in C++ for now. But we do allow calls to static member
functions.
This works by generating two thunks, one in C++ and one in Carbon. For
example, given this input:
```c++
// Carbon:
fn Callme(f: f32) {}
// C++:
void F() {
// This will call `Callme__cpp_thunk`
Carbon::Callme(1.0);
}
```
These functions are generated:
```c++
// Carbon:
fn Callme__carbon_thunk(ref f: f32) {
// Call the target function.
Callme(f);
}
// C++:
// C++ declaration for the Carbon thunk.
void Callme__carbon_thunk(float& f);
void Callme__cpp_thunk(float f) {
// Call the Carbon thunk with args passed by reference.
Callme__carbon_thunk(f);
}
```
For now, all arguments are passed by reference, even if they are simple
types like pointers or i32.
Functions with non-void return types are not supported yet.
Start recording the clang::DeclContext* -> InstId mapping for use in
later operations.
The test update includes removing the initial fail_* test because I
hadn't thought about the use of namespace aliases as a way to test for
the presence of a namespace without the failure caused by not finding
the thing inside the namespace.
---------
Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Add a clang::ExternalASTSource to begin exposing Carbon entities to
Clang - initially only a single `Carbon` top level namespace.
Subsequent work will add Carbon entities to this namespace.
Likely this CarbonExternalASTSource will be refactored into another
file, tie into/reference SemIR::File and CppFile, etc eventually - but
that'll wait for future patches.
If there's mechanical problems with the current implementation - how I'm
creating the new NamespaceDecl, etc - I'm all ears. It's very much in
the "it seems to work" state, not much more than that.
This does break Clang Modules (header modules, C++20 modules,
precompiled headers, etc) since they're implemented as an
ExternalASTSource as well, and Clang's ASTContext only supports one
ExternalASTSource at a time. To fix that regression we'll need to
implement some kind of ExternalASTSource multiplexing support - either
in Clang or Carbon (unclear which).
This regression of modules support can be observed by the following:
`A.h`
```
inline void f1() { }
```
`module.modulemap`
```
module A {
header "A.h"
export *
}
```
`test.carbon`
```
import Cpp inline '''
// Hardcode the pragma to ensure this isn't silently falling back to
// textual inclusion.
void f2() {
f1();
}
''';
```
```
carbon compile test.carbon -- -I . -fmodules -fimplicit-modules -fmodules-cache-path=module_cache
```
I wrote a `file_test` test for this, but it doesn't /quite/ work because
`file_test` provides an in-memory filesystem for tests to make them more
hermetic, but Clang's Filesystem abstrtaction is for reading only - so
the module that's written out successfully can't be found when it needs
to be read back in - so the test doesn't pass as a baseline. Clang does
have support for `llvm::vfs::OutputBackend` which allows virtualizing
output - which I guess we could tie together with the InMemoryFilesystem
we use for input to make such a test work. But I guess that's not worth
the effort here?
---------
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Nicholas Bishop
Richard Smith
David Blaikie
