RFC 2008: Stabilization

This commit stabilizes RFC 2008 (#44109) by removing the feature gate.

Signed-off-by: David Wood <david@davidtw.co>

src/doc/unstable-book/src/language-features/non-exhaustive.md

@@ -1,76 +0,0 @@
-# `non_exhaustive`
-
-The tracking issue for this feature is: [#44109]
-
-[#44109]: https://github.com/rust-lang/rust/issues/44109
-
-------------------------
-
-The `non_exhaustive` gate allows you to use the `#[non_exhaustive]` attribute
-on structs, enums and enum variants. When applied within a crate, users of the
-crate will need to use the `_` pattern when matching enums and use the `..`
-pattern when matching structs. Enum variants cannot be matched against.
-Structs and enum variants marked as `non_exhaustive` will not be able to
-be created normally outside of the defining crate. This is demonstrated
-below:
-
-```rust,ignore (pseudo-Rust)
-use std::error::Error as StdError;
-
-#[non_exhaustive]
-pub enum Error {
-    Message(String),
-    Other,
-}
-impl StdError for Error {
-    fn description(&self) -> &str {
-        // This will not error, despite being marked as non_exhaustive, as this
-        // enum is defined within the current crate, it can be matched
-        // exhaustively.
-        match *self {
-            Message(ref s) => s,
-            Other => "other or unknown error",
-        }
-    }
-}
-```
-
-```rust,ignore (pseudo-Rust)
-use mycrate::Error;
-
-// This will not error as the non_exhaustive Error enum has been matched with
-// a wildcard.
-match error {
-    Message(ref s) => ...,
-    Other => ...,
-    _ => ...,
-}
-```
-
-```rust,ignore (pseudo-Rust)
-#[non_exhaustive]
-pub struct Config {
-    pub window_width: u16,
-    pub window_height: u16,
-}
-
-// We can create structs as normal within the defining crate when marked as
-// non_exhaustive.
-let config = Config { window_width: 640, window_height: 480 };
-
-// We can match structs exhaustively when within the defining crate.
-if let Ok(Config { window_width, window_height }) = load_config() {
-    // ...
-}
-```
-
-```rust,ignore (pseudo-Rust)
-use mycrate::Config;
-
-// We cannot create a struct like normal if it has been marked as
-// non_exhaustive.
-let config = Config { window_width: 640, window_height: 480 };
-// By adding the `..` we can match the config as below outside of the crate
-// when marked non_exhaustive.
-let &Config { window_width, window_height, .. } = config;
-```