Encode const mir for closures if they're const

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -888,8 +888,8 @@ fn should_encode_mir(tcx: TyCtxt<'_>, def_id: LocalDefId) -> (bool, bool) {
         | DefKind::AssocConst
         | DefKind::Static(..)
         | DefKind::Const => (true, false),
-        // Full-fledged functions
-        DefKind::AssocFn | DefKind::Fn => {
+        // Full-fledged functions + closures
+        DefKind::AssocFn | DefKind::Fn | DefKind::Closure => {
             let generics = tcx.generics_of(def_id);
             let needs_inline = (generics.requires_monomorphization(tcx)
                 || tcx.codegen_fn_attrs(def_id).requests_inline())
@@ -900,15 +900,6 @@ fn should_encode_mir(tcx: TyCtxt<'_>, def_id: LocalDefId) -> (bool, bool) {
             let always_encode_mir = tcx.sess.opts.unstable_opts.always_encode_mir;
             (is_const_fn, needs_inline || always_encode_mir)
         }
-        // Closures can't be const fn.
-        DefKind::Closure => {
-            let generics = tcx.generics_of(def_id);
-            let needs_inline = (generics.requires_monomorphization(tcx)
-                || tcx.codegen_fn_attrs(def_id).requests_inline())
-                && tcx.sess.opts.output_types.should_codegen();
-            let always_encode_mir = tcx.sess.opts.unstable_opts.always_encode_mir;
-            (false, needs_inline || always_encode_mir)
-        }
         // Generators require optimized MIR to compute layout.
         DefKind::Generator => (false, true),
         // The others don't have MIR.

tests/ui/consts/auxiliary/closure-in-foreign-crate.rs

@@ -0,0 +1,8 @@
+#![crate_type = "lib"]
+#![feature(const_closures, const_trait_impl)]
+#![allow(incomplete_features)]
+
+pub const fn test() {
+    let cl = const || {};
+    cl();
+}

tests/ui/consts/closure-in-foreign-crate.rs

@@ -0,0 +1,8 @@
+// aux-build:closure-in-foreign-crate.rs
+// build-pass
+
+extern crate closure_in_foreign_crate;
+
+const _: () = closure_in_foreign_crate::test();
+
+fn main() {}