Added page to unstable book.

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+# `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 and enums. 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. Structs 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;
+```
+