This preserves the constant values of the arguments to the thunk, which
is important if the thunk requires conversion of an `IntLiteral` to some
other type. This should become unnecessary once we have form support,
but avoiding the indirection through a thunk function seems valuable
even once that support is in place.
To support this, track whether a function is a thunk on the Function
object, and if so, what the callee of the thunk is. This information is
also included in formatted SemIR when dumping the thunk.
Instead of using somewhat different approaches for defining in-line
methods at the end of the enclosing scope and defining thunks at the end
of the enclosing scope, we now use the same worklist for both.
This fixes a bug where we would crash when defining thunks if there
happens to be nothing else on the deferred definition worklist, leading
to our leaving the enclosing impl scope before we try to define the
pending thunk. That would only happen if the impl contains no in-line
member function bodies, so only if the impl has only a forward
declaration or a builtin declaration for every method. The latter
case happens (a lot) if we start using thunks in the prelude impls.
One complicating factor here is that this means the deferred definition
worklist moves from the layer containing `check/handle*` and
`check/check_unit.cpp` into the layer containing `check/context.cpp`.
Allowing that required moving a couple of other things that it depends
on -- notably `SuspendedFunction` and `HandleSuspendedFunction` --
around.
Build a `SpecificConstant` (if needed) and `NameRef` instruction when
referencing the thunk target from a thunk. The former is necessary if
the impl is generic in order to call the right version of the thunk
target. This previously caused a crash in lowering.
Also add some more check testing for the interaction of thunks and
generics. This testing uncovered an unrelated bug with thunks for
generic interface functions for which I've added a TODO.
People seemed receptive [on
#toolchain](https://discord.com/channels/655572317891461132/655578254970716160/1379196447827689553),
so proceeding.
For the two name conflicts, I've set it up so that there's a
"foo.carbon" and "foo_with_prelude.carbon", to indicate that the
no-prelude approach is preferred (with a shorter name).
Note min_prelude will require a little more work/thought, I want to
avoid adding `--custom-core` etc.
Richard Smith
Jon Ross-Perkins