Rollup merge of #40025 - est31:master, r=eddyb

Implement non-capturing closure to fn coercion

Implements non capturing closure coercion ([RFC 1558](https://github.com/rust-lang/rfcs/blob/master/text/1558-closure-to-fn-coercion.md)).

cc tracking issue #39817

1 files changed, 7 insertions, 0 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index b7be084fa0b..b66b08f8cd8 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -328,6 +328,10 @@ declare_features! (
 
     // Used to identify crates that contain sanitizer runtimes
     // rustc internal
+    (active, closure_to_fn_coercion, "1.17.0", Some(39817)),
+
+    // Used to identify crates that contain sanitizer runtimes
+    // rustc internal
     (active, sanitizer_runtime, "1.17.0", None),
 );
 
@@ -982,6 +986,9 @@ pub const EXPLAIN_DERIVE_UNDERSCORE: &'static str =
 pub const EXPLAIN_PLACEMENT_IN: &'static str =
     "placement-in expression syntax is experimental and subject to change.";
 
+pub const CLOSURE_TO_FN_COERCION: &'static str =
+    "non-capturing closure to fn coercion is experimental";
+
 struct PostExpansionVisitor<'a> {
     context: &'a Context<'a>,
 }