outside of borrowck, do not provide an implicit_region_bound

see comment added to the field in `VerifyBoundCx`.

compiler/rustc_borrowck/src/type_check/constraint_conversion.rs

@@ -23,7 +23,7 @@ pub(crate) struct ConstraintConversion<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     universal_regions: &'a UniversalRegions<'tcx>,
     region_bound_pairs: &'a RegionBoundPairs<'tcx>,
-    implicit_region_bound: Option<ty::Region<'tcx>>,
+    implicit_region_bound: ty::Region<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     locations: Locations,
     span: Span,
@@ -36,7 +36,7 @@ pub(crate) fn new(
         infcx: &'a InferCtxt<'a, 'tcx>,
         universal_regions: &'a UniversalRegions<'tcx>,
         region_bound_pairs: &'a RegionBoundPairs<'tcx>,
-        implicit_region_bound: Option<ty::Region<'tcx>>,
+        implicit_region_bound: ty::Region<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
         locations: Locations,
         span: Span,
@@ -120,7 +120,7 @@ pub(super) fn convert(&mut self, query_constraint: &QueryOutlivesConstraint<'tcx
                     &mut *self,
                     tcx,
                     region_bound_pairs,
-                    implicit_region_bound,
+                    Some(implicit_region_bound),
                     param_env,
                 )
                 .type_must_outlive(origin, t1, r2);

compiler/rustc_borrowck/src/type_check/free_region_relations.rs

@@ -61,7 +61,7 @@ pub(crate) struct CreateResult<'tcx> {
 pub(crate) fn create<'tcx>(
     infcx: &InferCtxt<'_, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
-    implicit_region_bound: Option<ty::Region<'tcx>>,
+    implicit_region_bound: ty::Region<'tcx>,
     universal_regions: &Rc<UniversalRegions<'tcx>>,
     constraints: &mut MirTypeckRegionConstraints<'tcx>,
 ) -> CreateResult<'tcx> {
@@ -223,7 +223,7 @@ struct UniversalRegionRelationsBuilder<'this, 'tcx> {
     infcx: &'this InferCtxt<'this, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     universal_regions: Rc<UniversalRegions<'tcx>>,
-    implicit_region_bound: Option<ty::Region<'tcx>>,
+    implicit_region_bound: ty::Region<'tcx>,
     constraints: &'this mut MirTypeckRegionConstraints<'tcx>,
 
     // outputs:

compiler/rustc_borrowck/src/type_check/input_output.rs

@@ -230,7 +230,7 @@ pub(crate) fn normalize_and_add_constraints(&mut self, t: Ty<'tcx>) -> Fallible<
                 self.infcx,
                 &self.borrowck_context.universal_regions,
                 &self.region_bound_pairs,
-                Some(self.implicit_region_bound),
+                self.implicit_region_bound,
                 self.param_env,
                 Locations::All(DUMMY_SP),
                 DUMMY_SP,

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -157,7 +157,7 @@ pub(crate) fn type_check<'mir, 'tcx>(
     } = free_region_relations::create(
         infcx,
         param_env,
-        Some(implicit_region_bound),
+        implicit_region_bound,
         universal_regions,
         &mut constraints,
     );
@@ -1142,7 +1142,7 @@ fn push_region_constraints(
             self.infcx,
             self.borrowck_context.universal_regions,
             self.region_bound_pairs,
-            Some(self.implicit_region_bound),
+            self.implicit_region_bound,
             self.param_env,
             locations,
             locations.span(self.body),

compiler/rustc_infer/src/infer/outlives/obligations.rs

@@ -141,9 +141,6 @@ pub fn take_registered_region_obligations(&self) -> Vec<(hir::HirId, RegionOblig
     ///   `('a, K)` in this list tells us that the bounds in scope
     ///   indicate that `K: 'a`, where `K` is either a generic
     ///   parameter like `T` or a projection like `T::Item`.
-    /// - `implicit_region_bound`: if some, this is a region bound
-    ///   that is considered to hold for all type parameters (the
-    ///   function body).
     /// - `param_env` is the parameter environment for the enclosing function.
     /// - `body_id` is the body-id whose region obligations are being
     ///   processed.
@@ -151,7 +148,6 @@ pub fn take_registered_region_obligations(&self) -> Vec<(hir::HirId, RegionOblig
     pub fn process_registered_region_obligations(
         &self,
         region_bound_pairs_map: &FxHashMap<hir::HirId, RegionBoundPairs<'tcx>>,
-        implicit_region_bound: Option<ty::Region<'tcx>>,
         param_env: ty::ParamEnv<'tcx>,
     ) {
         assert!(
@@ -170,13 +166,8 @@ pub fn process_registered_region_obligations(
             let sup_type = self.resolve_vars_if_possible(sup_type);
 
             if let Some(region_bound_pairs) = region_bound_pairs_map.get(&body_id) {
-                let outlives = &mut TypeOutlives::new(
-                    self,
-                    self.tcx,
-                    &region_bound_pairs,
-                    implicit_region_bound,
-                    param_env,
-                );
+                let outlives =
+                    &mut TypeOutlives::new(self, self.tcx, &region_bound_pairs, None, param_env);
                 outlives.type_must_outlive(origin, sup_type, sub_region);
             } else {
                 self.tcx.sess.delay_span_bug(

compiler/rustc_infer/src/infer/outlives/verify.rs

@@ -16,6 +16,11 @@
 pub struct VerifyBoundCx<'cx, 'tcx> {
     tcx: TyCtxt<'tcx>,
     region_bound_pairs: &'cx RegionBoundPairs<'tcx>,
+    /// During borrowck, if there are no outlives bounds on a generic
+    /// parameter `T`, we assume that `T: 'in_fn_body` holds.
+    ///
+    /// Outside of borrowck the only way to prove `T: '?0` is by
+    /// setting  `'?0` to `'empty`.
     implicit_region_bound: Option<ty::Region<'tcx>>,
     param_env: ty::ParamEnv<'tcx>,
 }

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -220,7 +220,7 @@ pub fn find_auto_trait_generics<A>(
                 .map(|&(id, _)| (id, vec![]))
                 .collect();
 
-            infcx.process_registered_region_obligations(&body_id_map, None, full_env);
+            infcx.process_registered_region_obligations(&body_id_map, full_env);
 
             let region_data = infcx
                 .inner

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -407,11 +407,7 @@ fn resolve_negative_obligation<'cx, 'tcx>(
     // function bodies with closures).
     outlives_env.save_implied_bounds(CRATE_HIR_ID);
 
-    infcx.process_registered_region_obligations(
-        outlives_env.region_bound_pairs_map(),
-        Some(tcx.lifetimes.re_root_empty),
-        param_env,
-    );
+    infcx.process_registered_region_obligations(outlives_env.region_bound_pairs_map(), param_env);
 
     let errors = infcx.resolve_regions(region_context, &outlives_env);
 

compiler/rustc_typeck/src/check/regionck.rs

@@ -366,7 +366,6 @@ fn visit_region_obligations(&mut self, hir_id: hir::HirId) {
     fn resolve_regions_and_report_errors(&self) {
         self.infcx.process_registered_region_obligations(
             self.outlives_environment.region_bound_pairs_map(),
-            Some(self.tcx.lifetimes.re_root_empty),
             self.param_env,
         );
 

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -596,13 +596,7 @@ fn ty_known_to_outlive<'tcx>(
 ) -> bool {
     resolve_regions_with_wf_tys(tcx, id, param_env, &wf_tys, |infcx, region_bound_pairs| {
         let origin = infer::RelateParamBound(DUMMY_SP, ty, None);
-        let outlives = &mut TypeOutlives::new(
-            infcx,
-            tcx,
-            region_bound_pairs,
-            Some(infcx.tcx.lifetimes.re_root_empty),
-            param_env,
-        );
+        let outlives = &mut TypeOutlives::new(infcx, tcx, region_bound_pairs, None, param_env);
         outlives.type_must_outlive(origin, ty, region);
     })
 }