When a `var` is not explicitly given an initializer, initialize it in
one of two ways:
* If its type implements the new interface `Core.Default`, call
`Core.Default.Op` to initialize it.
* Otherwise, if its type implements `UnformedInit`, leave it in an
unformed state. For now, this is always an uninitialized state, but that
will change in the future.
* If neither of those apply, the `var` declaration is ill-formed.
This is a step towards implementing leads decision #6739 and proposals
#257 and #5913.
Assisted-by: Gemini 3.1 Pro via Antigravity
---------
Co-authored-by: Geoff Romer <gromer@google.com>
We can now cast directly from `T*` to `U*`; stop going via `void*`. Also
remove the conversion impl from `void*` as it's now subsumed by the
general impl.
Mostly generated by Gemini; TODO annotations added for cases where we
should support a better way of doing various parts of this.
Assisted-by: Gemini 3 Pro
---------
Co-authored-by: Carbon Infra Bot <carbon-external-infra@google.com>
Richard Smith
