Auto merge of #49469 - Nokel81:allow-irrefutable-let-patterns, r=nikomatsakis

Implementation of RFC 2086 - Allow Irrefutable Let patterns

This is the set of changes for RFC2086. Tracking issue #44495. Rendered [here](https://github.com/rust-lang/rfcs/pull/2086)

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+# `irrefutable_let_patterns`
+
+The tracking issue for this feature is: [#44495]
+
+[#44495]: https://github.com/rust-lang/rust/issues/44495
+
+------------------------
+
+This feature changes the way that "irrefutable patterns" are handled
+in the `if let` and `while let` forms. An *irrefutable pattern* is one
+that cannot fail to match -- for example, the `_` pattern matches any
+value, and hence it is "irrefutable". Without this feature, using an
+irrefutable pattern in an `if let` gives a hard error (since often
+this indicates programmer error). But when the feature is enabled, the
+error becomes a lint (since in some cases irrefutable patterns are
+expected). This means you can use `#[allow]` to silence the lint:
+
+```rust
+#![feature(irrefutable_let_patterns)]
+
+#[allow(irrefutable_let_patterns)]
+fn main() {
+    // These two examples used to be errors, but now they
+    // trigger a lint (that is allowed):
+    if let _ = 5 {}
+    while let _ = 5 { break; }
+}
+```