Document behavior of ! with MbE

Author: Nadrieril

tests/ui/rfcs/rfc-0000-never_patterns/macros.rs

@@ -0,0 +1,40 @@
+// run-pass
+#![feature(never_patterns)]
+#![allow(incomplete_features)]
+
+use ParsedAs::*;
+#[derive(Debug, PartialEq, Eq)]
+enum ParsedAs {
+    Pattern,
+    Never,
+    Other,
+}
+
+macro_rules! detect_pat {
+    ($p:pat) => {
+        Pattern
+    };
+    (!) => {
+        Never
+    };
+    ($($x:tt)*) => {
+        Other
+    };
+}
+
+fn main() {
+    // For backwards compatibility this does not match `$p:pat`. This is similar to how or-patterns
+    // without parens don't match `$p:pat` either.
+    assert_eq!(detect_pat!(!), Never);
+    assert_eq!(detect_pat!(! | true), Other);
+    assert_eq!(detect_pat!(true | !), Other);
+
+    // These are never patterns; they take no body when they're in a match arm.
+    assert_eq!(detect_pat!((!)), Pattern);
+    assert_eq!(detect_pat!((true, !)), Pattern);
+    assert_eq!(detect_pat!(Some(!)), Pattern);
+
+    // These count as normal patterns.
+    assert_eq!(detect_pat!((! | true)), Pattern);
+    assert_eq!(detect_pat!((Ok(x) | Err(&!))), Pattern);
+}

tests/ui/rfcs/rfc-0000-never_patterns/parse.rs

@@ -68,4 +68,9 @@ fn parse(x: Void) {
     //~^ ERROR top-level or-patterns are not allowed in `let` bindings
     let (Ok(_) | Err(!)) = &res;
     let (Ok(_) | Err(&!)) = res.as_ref();
+
+    let ! = x;
+    let y @ ! = x;
 }
+
+fn foo(!: Void) {}