pre-expansion gate type_ascription

src/libsyntax/feature_gate/check.rs

@@ -498,21 +498,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         }
     }
 
-    fn visit_expr(&mut self, e: &'a ast::Expr) {
-        match e.kind {
-            ast::ExprKind::Type(..) => {
-                // To avoid noise about type ascription in common syntax errors, only emit if it
-                // is the *only* error.
-                if self.parse_sess.span_diagnostic.err_count() == 0 {
-                    gate_feature_post!(&self, type_ascription, e.span,
-                                       "type ascription is experimental");
-                }
-            }
-            _ => {}
-        }
-        visit::walk_expr(self, e)
-    }
-
     fn visit_pat(&mut self, pattern: &'a ast::Pat) {
         match &pattern.kind {
             PatKind::Slice(pats) => {
@@ -805,6 +790,12 @@ pub fn check_crate(krate: &ast::Crate,
     gate_all!(label_break_value, "labels on blocks are unstable");
     gate_all!(box_syntax, "box expression syntax is experimental; you can call `Box::new` instead");
 
+    // To avoid noise about type ascription in common syntax errors,
+    // only emit if it is the *only* error. (Also check it last.)
+    if parse_sess.span_diagnostic.err_count() == 0 {
+        gate_all!(type_ascription, "type ascription is experimental");
+    }
+
     visit::walk_crate(&mut visitor, krate);
 }
 

src/libsyntax/parse/parser/expr.rs

@@ -252,6 +252,7 @@ impl<'a> Parser<'a> {
                 self.last_type_ascription = Some((self.prev_span, maybe_path));
 
                 lhs = self.parse_assoc_op_cast(lhs, lhs_span, ExprKind::Type)?;
+                self.sess.gated_spans.type_ascription.borrow_mut().push(lhs.span);
                 continue
             } else if op == AssocOp::DotDot || op == AssocOp::DotDotEq {
                 // If we didn’t have to handle `x..`/`x..=`, it would be pretty easy to

src/libsyntax/sess.rs

@@ -50,6 +50,8 @@ crate struct GatedSpans {
     pub label_break_value: Lock<Vec<Span>>,
     /// Spans collected for gating `box_syntax`, e.g. `box $expr`.
     pub box_syntax: Lock<Vec<Span>>,
+    /// Spans collected for gating `type_ascription`, e.g. `42: usize`.
+    pub type_ascription: Lock<Vec<Span>>,
 }
 
 /// Info about a parsing session.

src/test/ui/feature-gates/feature-gate-type_ascription.rs

@@ -1,5 +1,8 @@
 // Type ascription is unstable
 
-fn main() {
+#[cfg(FALSE)]
+fn foo() {
     let a = 10: u8; //~ ERROR type ascription is experimental
 }
+
+fn main() {}

src/test/ui/feature-gates/feature-gate-type_ascription.stderr

@@ -1,5 +1,5 @@
 error[E0658]: type ascription is experimental
-  --> $DIR/feature-gate-type_ascription.rs:4:13
+  --> $DIR/feature-gate-type_ascription.rs:5:13
    |
 LL |     let a = 10: u8;
    |             ^^^^^^