By using the `rust.detach` pragma on a property, we make that property
not appear in the generated struct as a field, and provide instead
a `generated::Class::emit_property` function that can be used to emit
the corresponding TRAP entry independently.
This generates test source files from code blocks in class docstrings.
By default the test code is generated as is, but it can optionally:
* be wrapped in a function providing an adequate context using
`@rust.doc_test_function(name, *, lifetimes=(), return_type="()", **kwargs)`,
with `kwargs` providing both generic and normal params depending on
capitalization
* be skipped altogether using `@rust.skip_doc_test`
So for example an annotation like
```python
@rust.doc_test_function("foo",
lifetimes=("a",),
T="Eq",
x="&'a T",
y="&'a T",
return_type="&'a T")
```
will result in the following wrapper:
```rust
fn foo<'a, T: Eq>(x: &'a T, y: &'a T) -> &'a T {
// example code here
}
```
This simplifies several instances of metaprogramming by leveraging
[constraints and concepts from C++20][1]. This:
* gets rid of `std::enable_if` by usage of `requires`, making it more
readable and yield better compiler messages.
* uses `requires` instead of `static_assert` to enforce `TrapLabel`
typing
* simplifies all compile-time tests for validity of a given expression
* uses some standard library concepts where possible
* generalizes and simplifies `SwiftLocationExtractor`
Notice that in order to use the `std::derived_from` concept, `virtual`
inheritance had to be added to the label tags, because diamond
inheritance is a problem otherwise. That's because
`std::derived_from<T, U>` requires that `T*` be convertible to `U*`,
which is false if there are multiple non-virtual inheritance paths from
`U` to `T`. As tags never get actually instantiated, there is no runtime
performance penalty in using `virtual` inheritance.
[1]: https://en.cppreference.com/w/cpp/language/constraints
