Don't ICE for postfix match after as

compiler/rustc_parse/src/parser/expr.rs

@@ -860,6 +860,7 @@ impl<'a> Parser<'a> {
                     ExprKind::MethodCall(_) => "a method call",
                     ExprKind::Call(_, _) => "a function call",
                     ExprKind::Await(_, _) => "`.await`",
+                    ExprKind::Match(_, _, MatchKind::Postfix) => "a postfix match",
                     ExprKind::Err(_) => return Ok(with_postfix),
                     _ => unreachable!("parse_dot_or_call_expr_with_ shouldn't produce this"),
                 }

tests/ui/match/postfix-match/match-after-as.rs

@@ -0,0 +1,7 @@
+#![feature(postfix_match)]
+
+fn main() {
+    1 as i32.match {};
+    //~^ ERROR cast cannot be followed by a postfix match
+    //~| ERROR non-exhaustive patterns
+}

tests/ui/match/postfix-match/match-after-as.stderr

@@ -0,0 +1,28 @@
+error: cast cannot be followed by a postfix match
+  --> $DIR/match-after-as.rs:4:5
+   |
+LL |     1 as i32.match {};
+   |     ^^^^^^^^
+   |
+help: try surrounding the expression in parentheses
+   |
+LL |     (1 as i32).match {};
+   |     +        +
+
+error[E0004]: non-exhaustive patterns: type `i32` is non-empty
+  --> $DIR/match-after-as.rs:4:5
+   |
+LL |     1 as i32.match {};
+   |     ^^^^^^^^
+   |
+   = note: the matched value is of type `i32`
+help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern as shown
+   |
+LL ~     1 as i32 {
+LL +         _ => todo!(),
+LL ~     };
+   |
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0004`.