Add try to syntax_pos as an edition-2018-only keyword

Author: scottmcm

src/libsyntax_pos/symbol.rs

@@ -413,23 +413,30 @@ declare_keywords! {
     (49, Virtual,            "virtual")
     (50, Yield,              "yield")
 
+    // Edition-specific keywords currently in use.
+    (51, Try,                "try") // >= 2018 Edition Only
+
     // Edition-specific keywords reserved for future use.
-    (51, Async,              "async") // >= 2018 Edition Only
+    (52, Async,              "async") // >= 2018 Edition Only
 
     // Special lifetime names
-    (52, UnderscoreLifetime, "'_")
-    (53, StaticLifetime,     "'static")
+    (53, UnderscoreLifetime, "'_")
+    (54, StaticLifetime,     "'static")
 
     // Weak keywords, have special meaning only in specific contexts.
-    (54, Auto,               "auto")
-    (55, Catch,              "catch")
-    (56, Default,            "default")
-    (57, Dyn,                "dyn")
-    (58, Union,              "union")
-    (59, Existential,        "existential")
+    (55, Auto,               "auto")
+    (56, Catch,              "catch")
+    (57, Default,            "default")
+    (58, Dyn,                "dyn")
+    (59, Union,              "union")
+    (60, Existential,        "existential")
 }
 
 impl Symbol {
+    fn is_used_keyword_2018(self) -> bool {
+        self == keywords::Try.name()
+    }
+
     fn is_unused_keyword_2018(self) -> bool {
         self == keywords::Async.name()
     }
@@ -444,7 +451,9 @@ impl Ident {
 
     /// Returns `true` if the token is a keyword used in the language.
     pub fn is_used_keyword(self) -> bool {
-        self.name >= keywords::As.name() && self.name <= keywords::While.name()
+        // Note: `span.edition()` is relatively expensive, don't call it unless necessary.
+        self.name >= keywords::As.name() && self.name <= keywords::While.name() ||
+        self.name.is_used_keyword_2018() && self.span.edition() == Edition::Edition2018
     }
 
     /// Returns `true` if the token is a keyword reserved for possible future use.

src/test/parse-fail/keyword-try-as-identifier-edition2018.rs

@@ -0,0 +1,15 @@
+// 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.
+
+// compile-flags: -Z parse-only --edition 2018
+
+fn main() {
+    let try = "foo"; //~ error: expected pattern, found keyword `try`
+}

src/test/run-pass/try-is-identifier-edition2015.rs

@@ -0,0 +1,18 @@
+// 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.
+
+// compile-flags: --edition 2015
+
+fn main() {
+    let try = 2;
+    struct try { try: u32 };
+    let try: try = try { try };
+    assert_eq!(try.try, 2);
+}