metadata: only encode optimized MIR for closures when required

The metadata encoder unconditionally encoded the optimized MIR for closures, including during check builds, whereas this is usually limited to cases of actual codegen.

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1519,8 +1519,20 @@ impl EncodeContext<'a, 'tcx> {
             record!(self.tables.fn_sig[def_id] <- substs.as_closure().sig());
         }
         self.encode_generics(def_id.to_def_id());
-        self.encode_optimized_mir(def_id);
-        self.encode_promoted_mir(def_id);
+
+        let mir = {
+            let generics = self.tcx.generics_of(def_id);
+            let needs_inline = (generics.requires_monomorphization(self.tcx)
+                || self.tcx.codegen_fn_attrs(def_id).requests_inline())
+                && !self.metadata_output_only();
+            let is_generator = ty.is_generator();
+            let always_encode_mir = self.tcx.sess.opts.debugging_opts.always_encode_mir;
+            needs_inline || is_generator || always_encode_mir
+        };
+        if mir {
+            self.encode_optimized_mir(def_id);
+            self.encode_promoted_mir(def_id);
+        }
     }
 
     fn encode_info_for_anon_const(&mut self, def_id: LocalDefId) {