Rollup merge of rust-lang#42162 - est31:closure-to-fn-coercion, r=aturon

Stabilize non capturing closure to fn coercion

Stabilisation PR for non capturing closure to fn coercion.

closes rust-lang#39817

src/doc/unstable-book/src/SUMMARY.md

@@ -25,7 +25,6 @@
     - [cfg_target_has_atomic](language-features/cfg-target-has-atomic.md)
     - [cfg_target_thread_local](language-features/cfg-target-thread-local.md)
     - [cfg_target_vendor](language-features/cfg-target-vendor.md)
-    - [closure_to_fn_coercion](language-features/closure-to-fn-coercion.md)
     - [compiler_builtins](language-features/compiler-builtins.md)
     - [concat_idents](language-features/concat-idents.md)
     - [conservative_impl_trait](language-features/conservative-impl-trait.md)

src/librustc_typeck/check/coercion.rs

@@ -76,7 +76,6 @@ use rustc::ty::relate::RelateResult;
 use rustc::ty::subst::Subst;
 use errors::DiagnosticBuilder;
 use syntax::abi;
-use syntax::feature_gate;
 use syntax::ptr::P;
 use syntax_pos;
 
@@ -614,14 +613,6 @@ impl<'f, 'gcx, 'tcx> Coerce<'f, 'gcx, 'tcx> {
         let node_id_a = self.tcx.hir.as_local_node_id(def_id_a).unwrap();
         match b.sty {
             ty::TyFnPtr(_) if self.tcx.with_freevars(node_id_a, |v| v.is_empty()) => {
-                if !self.tcx.sess.features.borrow().closure_to_fn_coercion {
-                    feature_gate::emit_feature_err(&self.tcx.sess.parse_sess,
-                                                   "closure_to_fn_coercion",
-                                                   self.cause.span,
-                                                   feature_gate::GateIssue::Language,
-                                                   feature_gate::CLOSURE_TO_FN_COERCION);
-                    return self.unify_and(a, b, identity());
-                }
                 // We coerce the closure, which has fn type
                 //     `extern "rust-call" fn((arg0,arg1,...)) -> _`
                 // to

src/libsyntax/feature_gate.rs

@@ -321,9 +321,6 @@ declare_features! (
     // `extern "msp430-interrupt" fn()`
     (active, abi_msp430_interrupt, "1.16.0", Some(38487)),
 
-    // Coerces non capturing closures to function pointers
-    (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),
@@ -427,6 +424,8 @@ declare_features! (
     (accepted, loop_break_value, "1.19.0", Some(37339)),
     // Permits numeric fields in struct expressions and patterns.
     (accepted, relaxed_adts, "1.19.0", Some(35626)),
+    // Coerces non capturing closures to function pointers
+    (accepted, closure_to_fn_coercion, "1.19.0", Some(39817)),
 );
 
 // If you change this, please modify src/doc/unstable-book as well. You must
@@ -1026,9 +1025,6 @@ pub const EXPLAIN_VIS_MATCHER: &'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>,
 }

src/test/compile-fail/issue-40000.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(closure_to_fn_coercion)]
-
 fn main() {
     let bar: fn(&mut u32) = |_| {};
 

src/test/run-pass/closure-to-fn-coercion.rs

@@ -8,10 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// ignore-stage0: new feature, remove this when SNAP
-
-#![feature(closure_to_fn_coercion)]
-
 const FOO: fn(u8) -> u8 = |v: u8| { v };
 
 const BAR: [fn(&mut u32); 5] = [

src/test/run-pass/closure_to_fn_coercion-expected-types.rs

@@ -9,7 +9,6 @@
 // except according to those terms.
 // Ensure that we deduce expected argument types when a `fn()` type is expected (#41755)
 
-#![feature(closure_to_fn_coercion)]
 fn foo(f: fn(Vec<u32>) -> usize) { }
 
 fn main() {