Generalize get_nullable_type to accept types that have an all-ones bit
pattern as their sentry "null" value.

This will allow [`OwnedFd`], [`BorrowedFd`], [`OwnedSocket`], and
[`BorrowedSocket`] to be marked with
`#[rustc_nonnull_optimization_guaranteed]`, which will allow
`Option<OwnedFd>`, `Option<BorrowedFd>`, `Option<OwnedSocket>`, and
`Option<BorrowedSocket>` to be used in FFI declarations, as described
in the [I/O safety RFC].

For example, it will allow a function like `open` on Unix and `WSASocketW`
on Windows to be declared using `Option<OwnedFd>` and `Option<OwnedSocket>`
return types, respectively.

The actual change to add `#[rustc_nonnull_optimization_guaranteed]`
to the abovementioned types will be a separate PR, as it'll depend on
having this patch in the stage0 compiler.

Also, update the diagnostics to mention that "niche optimizations" are
used in libstd as well as libcore, as `rustc_layout_scalar_valid_range_start`
and `rustc_layout_scalar_valid_range_end` are already in use in libstd.

[`OwnedFd`]: https://github.com/rust-lang/rust/blob/c9dc44be24c58ff13ce46416c4b97ab5c1bd8429/library/std/src/os/fd/owned.rs#L49
[`BorrowedFd`]: https://github.com/rust-lang/rust/blob/c9dc44be24c58ff13ce46416c4b97ab5c1bd8429/library/std/src/os/fd/owned.rs#L29
[`OwnedSocket`]: https://github.com/rust-lang/rust/blob/c9dc44be24c58ff13ce46416c4b97ab5c1bd8429/library/std/src/os/windows/io/socket.rs#L51
[`BorrowedSocket`]: https://github.com/rust-lang/rust/blob/c9dc44be24c58ff13ce46416c4b97ab5c1bd8429/library/std/src/os/windows/io/socket.rs#L29
[I/O safety RFC]: https://github.com/rust-lang/rfcs/blob/master/text/3128-io-safety.md#ownedfd-and-borrowedfdfd-1

Co-authored-by: Daniel Henry-Mantilla <daniel.henry.mantilla@gmail.com>

Fixes rust-lang#94646

Co-authored-by: Mark Rousskov <mark.simulacrum@gmail.com>

Given

```rust
match Some(42) {}
```

suggest

```rust
match Some(42) { None | Some(_) => todo!(), }
```

…st it

Given

```rust
match Some(42) {
    Some(0) => {}
}
```

suggest

```rust
match Some(42) {
    Some(0) => {}
    None | Some(_) => todo!(),
}
```

…est it

Given

```rust
match Some(42) {
    Some(0) => {}
    Some(1) => {}
}
```

suggest

```rust
match Some(42) {
    Some(0) => {}
    Some(1) => {}
    None | Some(_) => todo!(),
}
```

… `span_label`

This makes the order of the output always consistent:

1. Place of the `match` missing arms
2. The `enum` definition span
3. The structured suggestion to add a fallthrough arm

…li-obk

Tweak output for non-exhaustive `match` expression

* Provide structured suggestion when missing `match` arms
* Move pointing at the missing variants *after* the main error

<img width="1164" alt="" src="https://user-images.githubusercontent.com/1606434/146312085-b57ef4a3-6e96-4f32-aa2a-803637d9eeba.png">

Add Result::{ok, err, and, or, unwrap_or} as const

Already opened tracking issue rust-lang#92384.

I don't think that this should actually cause any issues as long as the constness is unstable, but we may want to double-check that this doesn't get interpreted as a weird `Drop` bound even for non-const usages.

…without-arg, r=Mark-Simulacrum

Remove argument from closure in thread::Scope::spawn.

This implements ```@danielhenrymantilla's``` [suggestion](rust-lang#93203 (comment)) for improving the scoped threads interface.

Summary:

The `Scope` type gets an extra lifetime argument, which represents basically its own lifetime that will be used in `&'scope Scope<'scope, 'env>`:

```diff
- pub struct Scope<'env> { .. };
+ pub struct Scope<'scope, 'env: 'scope> { .. }

  pub fn scope<'env, F, T>(f: F) -> T
  where
-     F: FnOnce(&Scope<'env>) -> T;
+     F: for<'scope> FnOnce(&'scope Scope<'scope, 'env>) -> T;
```

This simplifies the `spawn` function, which now no longer passes an argument to the closure you give it, and now uses the `'scope` lifetime for everything:

```diff
-     pub fn spawn<'scope, F, T>(&'scope self, f: F) -> ScopedJoinHandle<'scope, T>
+     pub fn spawn<F, T>(&'scope self, f: F) -> ScopedJoinHandle<'scope, T>
      where
-         F: FnOnce(&Scope<'env>) -> T + Send + 'env,
+         F: FnOnce() -> T + Send + 'scope,
-         T: Send + 'env;
+         T: Send + 'scope;
```

The only difference the user will notice, is that their closure now takes no arguments anymore, even when spawning threads from spawned threads:

```diff
  thread::scope(|s| {
-     s.spawn(|_| {
+     s.spawn(|| {
          ...
      });
-     s.spawn(|s| {
+     s.spawn(|| {
          ...
-         s.spawn(|_| ...);
+         s.spawn(|| ...);
      });
  });
```

<details><summary>And, as a bonus, errors get <em>slightly</em> better because now any lifetime issues point to the outermost <code>s</code> (since there is only one <code>s</code>), rather than the innermost <code>s</code>, making it clear that the lifetime lasts for the entire <code>thread::scope</code>.

</summary>

```diff
  error[E0373]: closure may outlive the current function, but it borrows `a`, which is owned by the current function
   --> src/main.rs:9:21
    |
- 7 |         s.spawn(|s| {
-   |                  - has type `&Scope<'1>`
+ 6 |     thread::scope(|s| {
+   |                    - lifetime `'1` appears in the type of `s`
  9 |             s.spawn(|| println!("{:?}", a)); // might run after `a` is dropped
    |                     ^^                  - `a` is borrowed here
    |                     |
    |                     may outlive borrowed value `a`
    |
  note: function requires argument type to outlive `'1`
   --> src/main.rs:9:13
    |
  9 |             s.spawn(|| println!("{:?}", a)); // might run after `a` is dropped
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  help: to force the closure to take ownership of `a` (and any other referenced variables), use the `move` keyword
    |
  9 |             s.spawn(move || println!("{:?}", a)); // might run after `a` is dropped
    |                     ++++
"
```
</details>

The downside is that the signature of `scope` and `Scope` gets slightly more complex, but in most cases the user wouldn't need to write those, as they just use the argument provided by `thread::scope` without having to name its type.

Another downside is that this does not work nicely in Rust 2015 and Rust 2018, since in those editions, `s` would be captured by reference and not by copy. In those editions, the user would need to use `move ||` to capture `s` by copy. (Which is what the compiler suggests in the error.)

…attr, r=lcnr

Emit `unused_attributes` if a level attr only has a reason

Fixes a comment from `compiler/rustc_lint/src/levels.rs`. Lint level attributes that only contain a reason will also trigger the `unused_attribute` lint. The lint now also checks for the `expect` lint level.

That's it, have a great rest of the day for everyone reasoning this 🙃

cc: rust-lang#55112

…s-tests, r=davidtwco

Generalize `get_nullable_type` to allow types where null is all-ones.

Generalize get_nullable_type to accept types that have an all-ones bit
pattern as their sentry "null" value.

This will allow [`OwnedFd`], [`BorrowedFd`], [`OwnedSocket`], and
[`BorrowedSocket`] to be marked with
`#[rustc_nonnull_optimization_guaranteed]`, which will allow
`Option<OwnedFd>`, `Option<BorrowedFd>`, `Option<OwnedSocket>`, and
`Option<BorrowedSocket>` to be used in FFI declarations, as described
in the [I/O safety RFC].

For example, it will allow a function like `open` on Unix and `WSASocketW`
on Windows to be declared using `Option<OwnedFd>` and `Option<OwnedSocket>`
return types, respectively.

The actual change to add `#[rustc_nonnull_optimization_guaranteed]`
to the abovementioned types will be a separate PR, as it'll depend on
having this patch in the stage0 compiler.

Also, update the diagnostics to mention that "niche optimizations" are
used in libstd as well as libcore, as `rustc_layout_scalar_valid_range_start`
and `rustc_layout_scalar_valid_range_end` are already in use in libstd.

[`OwnedFd`]: https://github.com/rust-lang/rust/blob/c9dc44be24c58ff13ce46416c4b97ab5c1bd8429/library/std/src/os/fd/owned.rs#L49
[`BorrowedFd`]: https://github.com/rust-lang/rust/blob/c9dc44be24c58ff13ce46416c4b97ab5c1bd8429/library/std/src/os/fd/owned.rs#L29
[`OwnedSocket`]: https://github.com/rust-lang/rust/blob/c9dc44be24c58ff13ce46416c4b97ab5c1bd8429/library/std/src/os/windows/io/socket.rs#L51
[`BorrowedSocket`]: https://github.com/rust-lang/rust/blob/c9dc44be24c58ff13ce46416c4b97ab5c1bd8429/library/std/src/os/windows/io/socket.rs#L29
[I/O safety RFC]: https://github.com/rust-lang/rfcs/blob/master/text/3128-io-safety.md#ownedfd-and-borrowedfdfd-1

…ing, r=lcnr

diagnostics: only talk about `Cargo.toml` if running under Cargo

Fixes rust-lang#94646

…sumption, r=Mark-Simulacrum

promot debug_assert to assert

Fixes rust-lang#94705

…=lnicola

⬆️ rust-analyzer

r? `@ghost`