Implementation of RFC 2086 - Allow Irrefutable Let patterns

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diff --git a/src/doc/unstable-book/src/language-features/irrefutable-let-pattern.md b/src/doc/unstable-book/src/language-features/irrefutable-let-pattern.md
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+# `irrefutable_let_pattern`
+
+The tracking issue for this feature is: [#44495]
+
+[#44495]: https://github.com/rust-lang/rust/issues/44495
+
+------------------------
+
+This feature changes the way that the irrefutable pattern is handled
+in the `if let` and `while let` forms. The old way was to always error
+but now with a tag the error-by-default lint can be switched off.
+
+```rust
+#![feature(irrefutable_let_pattern)]
+
+fn main() {
+    #[allow(irrefutable_let_pattern)]
+    if let _ = 5 {}
+
+    #[allow(irrefutable_let_pattern)]
+    while let _ = 5 {}
+}
+```