extra testing of how NLL handles wildcard type `_`
test that wildcard type `_` is not duplicated by `type Foo<X> = (X, X);` and potentially instantiated at different types when used in type ascriptions in let bindings.
(NLL's handling of this for the type ascription *expression form* is currently broken, or at least differs from what AST-borrowck does. I'll file a separate bug about that. Its not something critical to address since that expression is guarded by `#![feature(type_ascription)]`.)
cc #55748
Add todo!() macro
The primary use-case of `todo!()` macro is to be a much easier to type
alternative to `unimplemented!()` macro.
EDIT: hide unpopular proposal about re-purposing unimplemented
<details>
However, instead of just replacing `unimplemented!()`, it gives it a
more nuanced meaning: a thing which is intentionally left
unimplemented and which should not be called at runtime. Usually,
you'd like to prevent such cases statically, but sometimes you, for
example, have to implement a trait only some methods of which are
applicable. There are examples in the wild of code doing this thing,
and in this case, the current message of `unimplemented`, "not *yet*
implemented" is slightly misleading.
With the addition of TODO, you have three nuanced choices for a
`!`-returning macro (in addition to a good-old panic we all love):
* todo!()
* unreachable!()
* unimplemented!()
Here's a rough guideline what each one means:
- `todo`: use it during development, as a "hole" or placeholder. It
might be a good idea to add a pre-commit hook which checks that
`todo` is not accidentally committed.
- `unreachable!()`: use it when your code can statically guarantee
that some situation can not happen. If you use a library and hit
`unreachable!()` in the library's code, it's definitely a bug in the
library. It's OK to have `unreachable!()` in the code base,
although, if possible, it's better to replace it with
compiler-verified exhaustive checks.
- `unimplemented!()`: use it when the type checker forces you to
handle some situation, but there's a contract that a callee must not
actually call the code. If you use a library and hit
`unimplemented!()`, it's probably a bug in your code, though
it *could* be a bug in the library (or library docs) as well. It is
ok-ish to see an `unimplemented!()` in real code, but it usually
signifies a clunky, eyebrow-rising API.
</details>
