Stabilise irrefutable if-let and while-let patterns

This stabilises RFC 2086 (https://github.com/rust-lang/rust/issues/44495).

Co-Authored-By: Sebastian Malton <sebastian@malton.name>

src/test/ui/pattern/deny-irrefutable-let-patterns.rs

@@ -0,0 +1,9 @@
+#![deny(irrefutable_let_patterns)]
+
+fn main() {
+    if let _ = 5 {} //~ ERROR irrefutable if-let pattern
+
+    while let _ = 5 { //~ ERROR irrefutable while-let pattern
+        break;
+    }
+}

src/test/ui/pattern/deny-irrefutable-let-patterns.stderr

@@ -0,0 +1,22 @@
+error: irrefutable if-let pattern
+  --> $DIR/deny-irrefutable-let-patterns.rs:4:5
+   |
+LL |     if let _ = 5 {} //~ ERROR irrefutable if-let pattern
+   |     ^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/deny-irrefutable-let-patterns.rs:1:9
+   |
+LL | #![deny(irrefutable_let_patterns)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: irrefutable while-let pattern
+  --> $DIR/deny-irrefutable-let-patterns.rs:6:5
+   |
+LL | /     while let _ = 5 { //~ ERROR irrefutable while-let pattern
+LL | |         break;
+LL | |     }
+   | |_____^
+
+error: aborting due to 2 previous errors
+

src/test/ui/pattern/enum-variant-generic-args.rs

@@ -0,0 +1,43 @@
+// run-pass
+
+#![feature(type_alias_enum_variants)]
+
+#![allow(irrefutable_let_patterns)]
+
+#[allow(dead_code)]
+enum Enum<T> { TSVariant(T), SVariant { v: T } }
+type Alias<T> = Enum<T>;
+type AliasFixed = Enum<()>;
+
+macro_rules! is_variant {
+    (TSVariant, $expr:expr) => (is_variant!(@check TSVariant, (_), $expr));
+    (SVariant, $expr:expr) => (is_variant!(@check SVariant, { v: _ }, $expr));
+    (@check $variant:ident, $matcher:tt, $expr:expr) => (
+        assert!(if let Enum::$variant::<()> $matcher = $expr { true } else { false },
+                "expr does not have correct type");
+    );
+}
+
+fn main() {
+    // Tuple struct variant
+
+    is_variant!(TSVariant, Enum::TSVariant(()));
+    is_variant!(TSVariant, Enum::TSVariant::<()>(()));
+    is_variant!(TSVariant, Enum::<()>::TSVariant(()));
+
+    is_variant!(TSVariant, Alias::TSVariant(()));
+    is_variant!(TSVariant, Alias::<()>::TSVariant(()));
+
+    is_variant!(TSVariant, AliasFixed::TSVariant(()));
+
+    // Struct variant
+
+    is_variant!(SVariant, Enum::SVariant { v: () });
+    is_variant!(SVariant, Enum::SVariant::<()> { v: () });
+    is_variant!(SVariant, Enum::<()>::SVariant { v: () });
+
+    is_variant!(SVariant, Alias::SVariant { v: () });
+    is_variant!(SVariant, Alias::<()>::SVariant { v: () });
+
+    is_variant!(SVariant, AliasFixed::SVariant { v: () });
+}

src/test/ui/pattern/irrefutable-let-patterns.rs

@@ -0,0 +1,11 @@
+// run-pass
+
+#![allow(irrefutable_let_patterns)]
+
+fn main() {
+    if let _ = 5 {}
+
+    while let _ = 5 {
+        break;
+    }
+}