check if the token is a lifetime before parsing

src/libsyntax/ext/tt/macro_parser.rs

@@ -835,7 +835,13 @@ fn parse_nt<'a>(p: &mut Parser<'a>, sp: Span, name: &str) -> Nonterminal {
         "path" => token::NtPath(panictry!(p.parse_path_common(PathStyle::Type, false))),
         "meta" => token::NtMeta(panictry!(p.parse_meta_item())),
         "vis" => token::NtVis(panictry!(p.parse_visibility(true))),
-        "lifetime" => token::NtLifetime(p.expect_lifetime().ident),
+        "lifetime" => if p.check_lifetime() {
+            token::NtLifetime(p.expect_lifetime().ident)
+        } else {
+            let token_str = pprust::token_to_string(&p.token);
+            p.fatal(&format!("expected a lifetime, found `{}`", &token_str)).emit();
+            FatalError.raise();
+        }
         // this is not supposed to happen, since it has been checked
         // when compiling the macro.
         _ => p.span_bug(sp, "invalid fragment specifier"),

src/libsyntax/parse/parser.rs

@@ -2042,7 +2042,7 @@ impl<'a> Parser<'a> {
         })
     }
 
-    fn check_lifetime(&mut self) -> bool {
+    pub fn check_lifetime(&mut self) -> bool {
         self.expected_tokens.push(TokenType::Lifetime);
         self.token.is_lifetime()
     }

src/test/compile-fail/macro-non-lifetime.rs

@@ -0,0 +1,20 @@
+// Copyright 2018 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.
+
+// Test for issue #50381: non-lifetime passed to :lifetime.
+
+#![feature(macro_lifetime_matcher)]
+
+macro_rules! m { ($x:lifetime) => { } }
+
+fn main() {
+    m!(a);
+    //~^ ERROR expected a lifetime, found `a`
+}