Feature gate raw identifiers.

src/libsyntax/feature_gate.rs

@@ -452,6 +452,9 @@ declare_features! (
 
     // `use path as _;` and `extern crate c as _;`
     (active, underscore_imports, "1.26.0", Some(48216), None),
+
+    // Raw identifiers allowing keyword names to be used
+    (active, raw_identifiers, "1.26.0", Some(48589), None),
 );
 
 declare_features! (
@@ -1932,6 +1935,17 @@ pub fn check_crate(krate: &ast::Crate,
         parse_sess: sess,
         plugin_attributes,
     };
+
+    if !features.raw_identifiers {
+        for &span in sess.raw_identifier_spans.borrow().iter() {
+            if !span.allows_unstable() {
+                gate_feature!(&ctx, raw_identifiers, span,
+                    "raw identifiers are experimental and subject to change"
+                );
+            }
+        }
+    }
+
     let visitor = &mut PostExpansionVisitor { context: &ctx };
     visitor.whole_crate_feature_gates(krate);
     visit::walk_crate(visitor, krate);

src/libsyntax/parse/lexer/mod.rs

@@ -1796,6 +1796,7 @@ mod tests {
             included_mod_stack: RefCell::new(Vec::new()),
             code_map: cm,
             missing_fragment_specifiers: RefCell::new(HashSet::new()),
+            raw_identifier_spans: RefCell::new(Vec::new()),
             registered_diagnostics: Lock::new(ErrorMap::new()),
             non_modrs_mods: RefCell::new(vec![]),
         }

src/libsyntax/parse/mod.rs

@@ -48,6 +48,9 @@ pub struct ParseSess {
     pub unstable_features: UnstableFeatures,
     pub config: CrateConfig,
     pub missing_fragment_specifiers: RefCell<HashSet<Span>>,
+    /// Places where raw identifiers were used. This is used for feature gating
+    /// raw identifiers
+    pub raw_identifier_spans: RefCell<Vec<Span>>,
     /// The registered diagnostics codes
     pub registered_diagnostics: Lock<ErrorMap>,
     // Spans where a `mod foo;` statement was included in a non-mod.rs file.
@@ -74,6 +77,7 @@ impl ParseSess {
             unstable_features: UnstableFeatures::from_environment(),
             config: HashSet::new(),
             missing_fragment_specifiers: RefCell::new(HashSet::new()),
+            raw_identifier_spans: RefCell::new(Vec::new()),
             registered_diagnostics: Lock::new(ErrorMap::new()),
             included_mod_stack: RefCell::new(vec![]),
             code_map,

src/libsyntax/parse/parser.rs

@@ -784,7 +784,7 @@ impl<'a> Parser<'a> {
 
     fn parse_ident_common(&mut self, recover: bool) -> PResult<'a, ast::Ident> {
         match self.token {
-            token::Ident(i, _) => {
+            token::Ident(i, is_raw) => {
                 if self.token.is_reserved_ident() {
                     let mut err = self.expected_ident_found();
                     if recover {
@@ -793,6 +793,9 @@ impl<'a> Parser<'a> {
                         return Err(err);
                     }
                 }
+                if is_raw {
+                    self.sess.raw_identifier_spans.borrow_mut().push(self.span);
+                }
                 self.bump();
                 Ok(i)
             }