Gate more precise generator auto traits behind feature

Author: Aaron1011

src/librustc/query/mod.rs

@@ -931,6 +931,10 @@ rustc_queries! {
         query all_traits(_: CrateNum) -> &'tcx [DefId] {
             desc { "fetching all foreign and local traits" }
         }
+
+        query uses_generator_mir_traits(_: CrateNum) -> bool {
+            desc { "determining whether crate uses generator MIR traits" }
+        }
     }
 
     Linking {

src/librustc/traits/select.rs

@@ -2753,6 +2753,18 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
 
             ty::GeneratorWitness(did, types) => {
+				let use_mir = if did.is_local() {
+                    self.tcx().features().generator_mir_traits
+                } else {
+                    self.tcx().uses_generator_mir_traits(did.krate)
+                };
+
+                if !use_mir {
+					// This is sound because no regions in the witness can refer to
+					// the binder outside the witness. So we'll effectivly reuse
+					// the implicit binder around the witness.
+					return types.skip_binder().to_vec()
+                }
                 // Note that we need to use optimized_mir here,
                 // in order to have the `StateTransform` pass run
                 /*let gen_mir = self.tcx().optimized_mir(did);

src/librustc_feature/active.rs

@@ -526,6 +526,8 @@ declare_features! (
     /// Allows using `&mut` in constant functions.
     (active, const_mut_refs, "1.41.0", Some(57349), None),
 
+    (active, generator_mir_traits, "1.41.0", None, None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

src/librustc_metadata/rmeta/decoder/cstore_impl.rs

@@ -240,6 +240,10 @@ provide! { <'tcx> tcx, def_id, other, cdata,
 
         Arc::new(syms)
     }
+
+    uses_generator_mir_traits => {
+        cdata.root.generator_mir_traits
+    }
 }
 
 pub fn provide(providers: &mut Providers<'_>) {

src/librustc_metadata/rmeta/encoder.rs

@@ -544,6 +544,7 @@ impl<'tcx> EncodeContext<'tcx> {
             exported_symbols,
             interpret_alloc_index,
             per_def,
+            generator_mir_traits: tcx.features().generator_mir_traits
         });
 
         let total_bytes = self.position();

src/librustc_metadata/rmeta/mod.rs

@@ -214,6 +214,7 @@ crate struct CrateRoot<'tcx> {
     profiler_runtime: bool,
     sanitizer_runtime: bool,
     symbol_mangling_version: SymbolManglingVersion,
+    generator_mir_traits: bool
 }
 
 #[derive(RustcEncodable, RustcDecodable)]

src/libsyntax_pos/symbol.rs

@@ -334,6 +334,7 @@ symbols! {
         FxHashSet,
         FxHashMap,
         gen_future,
+        generator_mir_traits,
         generators,
         generic_associated_types,
         generic_param_attrs,