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)

src/test/compile-fail/feature-gate-without_gate_irrefutable_pattern.rs

@@ -0,0 +1,17 @@
+// gate-test-irrefutable_let_patterns
+
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[allow(irrefutable_let_patterns)]
+fn main() {
+    if let _ = 5 {}
+    //~^ ERROR 15:12: 15:13: irrefutable if-let pattern [E0162]
+}

src/test/compile-fail/should-fail-no_gate_irrefutable_if_let_pattern.rs

@@ -0,0 +1,15 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// should-fail-irrefutable_let_patterns
+fn main() {
+    if let _ = 5 {}
+    //~^ ERROR irrefutable if-let pattern [E0162]
+}

src/test/compile-fail/should-fail-with_gate_irrefutable_pattern_deny.rs

@@ -0,0 +1,17 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(irrefutable_let_patterns)]
+
+// should-fail-irrefutable_let_patterns_with_gate
+fn main() {
+    if let _ = 5 {}
+    //~^ ERROR irrefutable if-let pattern [irrefutable_let_patterns]
+}

src/test/run-pass/allow_irrefutable_let_patterns.rs

@@ -0,0 +1,21 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(irrefutable_let_patterns)]
+
+// must-compile-successfully-irrefutable_let_patterns_with_gate
+#[allow(irrefutable_let_patterns)]
+fn main() {
+    if let _ = 5 {}
+
+    while let _ = 5 {
+        break;
+    }
+}