Rollup merge of #56348 - matklad:todo-macro, r=withoutboats

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>

1 files changed, 2 insertions, 1 deletions

diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index fc8ac9a0b3e..296c4c88727 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -301,6 +301,7 @@
 #![feature(stmt_expr_attributes)]
 #![feature(str_internals)]
 #![feature(thread_local)]
+#![feature(todo_macro)]
 #![feature(toowned_clone_into)]
 #![feature(try_reserve)]
 #![feature(unboxed_closures)]
@@ -323,7 +324,7 @@ use prelude::v1::*;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::{assert_eq, assert_ne, debug_assert, debug_assert_eq, debug_assert_ne};
 #[stable(feature = "rust1", since = "1.0.0")]
-pub use core::{unreachable, unimplemented, write, writeln, r#try};
+pub use core::{unreachable, unimplemented, write, writeln, r#try, todo};
 
 #[allow(unused_imports)] // macros from `alloc` are not used on all platforms
 #[macro_use]