When creating the C++ thunk, make the parameters references if the
corresponding callee parameters are `ref`s.
When creating the Carbon thunk, tag the call arguments as `ref` if the
corresponding callee parameters are `ref`s.
The FunctionDecl created for calling the Carbon thunk now takes a `self`
parameter for non-static methods, and the C++ thunk now passes an extra
argument for that `self` parameter when needed.
The CXXMethodDecl thunk created for calling methods now sets the storage
class appropriate depending on whether the method is static or not.
To reduce the number of parameters being passed around to thunk-building
functions, added a `FunctionInfo` struct and pass that around instead.
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.
Allow any type that has a mapping from Carbon to C++ to be exposed to
C++ via name lookup. This also exposes the logic to export Carbon
classes to C++ to apply during type mapping, which gives very slight
support for passing Carbon types to C++ functions from Carbon, but not
really enough to sensibly test yet.
Depends on #7042.
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.
Nicholas Bishop
Richard Smith