Auto merge of #44691 - cramertj:underscore-lifetimes, r=nikomatsakis

Implement underscore lifetimes

Part of https://github.com/rust-lang/rust/issues/44524

1 files changed, 11 insertions, 0 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 6560943a932..1fef382c83a 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -389,6 +389,9 @@ declare_features! (
     // Copy/Clone closures (RFC 2132)
     (active, clone_closures, "1.22.0", Some(44490)),
     (active, copy_closures, "1.22.0", Some(44490)),
+
+    // allow `'_` placeholder lifetimes
+    (active, underscore_lifetimes, "1.22.0", Some(44524)),
 );
 
 declare_features! (
@@ -1572,6 +1575,14 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         }
         visit::walk_lifetime_def(self, lifetime_def)
     }
+
+    fn visit_lifetime(&mut self, lt: &'a ast::Lifetime) {
+        if lt.ident.name == "'_" {
+            gate_feature_post!(&self, underscore_lifetimes, lt.span,
+                               "underscore lifetimes are unstable");
+        }
+        visit::walk_lifetime(self, lt)
+    }
 }
 
 pub fn get_features(span_handler: &Handler, krate_attrs: &[ast::Attribute]) -> Features {