Auto merge of #131856 - lcnr:typing-mode, r=<try>

TypingMode 🤔

unsure how and whether to actually get this ready for merge, opening for input + perf for now

#### `TypingMode::from_param_env`

I am using this even in cases where I know that the `param_env` will always be `Reveal::UserFacing`. This is to make it easier to correctly refactor this code in the future, any time we use `Reveal::UserFacing` in a body while not defining its opaque types is incorrect and should use a `TypingMode` which only reveals opaques defined by that body instead, cc #124598

r? `@compiler-errors`

Author: bors

compiler/rustc_borrowck/src/lib.rs

@@ -34,7 +34,7 @@ use rustc_infer::infer::{
 use rustc_middle::mir::tcx::PlaceTy;
 use rustc_middle::mir::*;
 use rustc_middle::query::Providers;
-use rustc_middle::ty::{self, ParamEnv, RegionVid, TyCtxt};
+use rustc_middle::ty::{self, ParamEnv, RegionVid, TyCtxt, TypingMode};
 use rustc_middle::{bug, span_bug};
 use rustc_mir_dataflow::Analysis;
 use rustc_mir_dataflow::impls::{
@@ -440,7 +440,7 @@ pub struct BorrowckInferCtxt<'tcx> {
 
 impl<'tcx> BorrowckInferCtxt<'tcx> {
     pub(crate) fn new(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Self {
-        let infcx = tcx.infer_ctxt().with_opaque_type_inference(def_id).build();
+        let infcx = tcx.infer_ctxt().build(TypingMode::analysis_in_body(tcx, def_id));
         BorrowckInferCtxt { infcx, reg_var_to_origin: RefCell::new(Default::default()) }
     }
 

compiler/rustc_borrowck/src/region_infer/opaque_types.rs

@@ -9,6 +9,7 @@ use rustc_macros::extension;
 use rustc_middle::ty::visit::TypeVisitableExt;
 use rustc_middle::ty::{
     self, GenericArgKind, GenericArgs, OpaqueHiddenType, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable,
+    TypingMode,
 };
 use rustc_span::Span;
 use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
@@ -346,8 +347,7 @@ fn check_opaque_type_well_formed<'tcx>(
     let infcx = tcx
         .infer_ctxt()
         .with_next_trait_solver(next_trait_solver)
-        .with_opaque_type_inference(parent_def_id)
-        .build();
+        .build(TypingMode::analysis_in_body(tcx, parent_def_id));
     let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
     let identity_args = GenericArgs::identity_for_item(tcx, def_id);
 
@@ -517,7 +517,7 @@ impl<'tcx> LazyOpaqueTyEnv<'tcx> {
             },
         );
 
-        let infcx = tcx.infer_ctxt().build();
+        let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
         let ocx = ObligationCtxt::new(&infcx);
 
         let wf_tys = ocx.assumed_wf_types(param_env, parent).unwrap_or_else(|_| {

compiler/rustc_codegen_ssa/src/base.rs

@@ -21,7 +21,7 @@ use rustc_middle::mir::BinOp;
 use rustc_middle::mir::mono::{CodegenUnit, CodegenUnitNameBuilder, MonoItem};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf, TyAndLayout};
-use rustc_middle::ty::{self, Instance, Ty, TyCtxt};
+use rustc_middle::ty::{self, Instance, Ty, TyCtxt, TypingMode};
 use rustc_session::Session;
 use rustc_session::config::{self, CrateType, EntryFnType, OptLevel, OutputType};
 use rustc_span::symbol::sym;
@@ -119,7 +119,7 @@ pub fn validate_trivial_unsize<'tcx>(
 ) -> bool {
     match (source_data.principal(), target_data.principal()) {
         (Some(hr_source_principal), Some(hr_target_principal)) => {
-            let infcx = tcx.infer_ctxt().build();
+            let infcx = tcx.infer_ctxt().build(TypingMode::PostAnalysis);
             let universe = infcx.universe();
             let ocx = ObligationCtxt::new(&infcx);
             infcx.enter_forall(hr_target_principal, |target_principal| {

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -15,7 +15,7 @@ use rustc_middle::mir::visit::Visitor;
 use rustc_middle::mir::*;
 use rustc_middle::span_bug;
 use rustc_middle::ty::adjustment::PointerCoercion;
-use rustc_middle::ty::{self, Instance, InstanceKind, Ty, TypeVisitableExt};
+use rustc_middle::ty::{self, Instance, InstanceKind, Ty, TypeVisitableExt, TypingMode};
 use rustc_mir_dataflow::Analysis;
 use rustc_mir_dataflow::impls::MaybeStorageLive;
 use rustc_mir_dataflow::storage::always_storage_live_locals;
@@ -581,7 +581,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 // Typeck only does a "non-const" check since it operates on HIR and cannot distinguish
                 // which path expressions are getting called on and which path expressions are only used
                 // as function pointers. This is required for correctness.
-                let infcx = tcx.infer_ctxt().build();
+                let infcx = tcx.infer_ctxt().build(TypingMode::from_param_env(param_env));
                 let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
 
                 let predicates = tcx.predicates_of(callee).instantiate(tcx, fn_args);

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -12,7 +12,7 @@ use rustc_middle::mir::CallSource;
 use rustc_middle::span_bug;
 use rustc_middle::ty::print::{PrintTraitRefExt as _, with_no_trimmed_paths};
 use rustc_middle::ty::{
-    self, Closure, FnDef, FnPtr, GenericArgKind, GenericArgsRef, Param, TraitRef, Ty,
+    self, Closure, FnDef, FnPtr, GenericArgKind, GenericArgsRef, Param, TraitRef, Ty, TypingMode,
     suggest_constraining_type_param,
 };
 use rustc_middle::util::{CallDesugaringKind, CallKind, call_kind};
@@ -108,7 +108,7 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
                     let obligation =
                         Obligation::new(tcx, ObligationCause::dummy(), param_env, trait_ref);
 
-                    let infcx = tcx.infer_ctxt().build();
+                    let infcx = tcx.infer_ctxt().build(TypingMode::from_param_env(param_env));
                     let mut selcx = SelectionContext::new(&infcx);
                     let implsrc = selcx.select(&obligation);
 

compiler/rustc_const_eval/src/check_consts/qualifs.rs

@@ -114,11 +114,10 @@ impl Qualif for HasMutInterior {
             ty::TraitRef::new(cx.tcx, freeze_def_id, [ty::GenericArg::from(ty)]),
         );
 
-        let infcx = cx
-            .tcx
-            .infer_ctxt()
-            .with_opaque_type_inference(cx.body.source.def_id().expect_local())
-            .build();
+        let infcx = cx.tcx.infer_ctxt().build(ty::TypingMode::analysis_in_body(
+            cx.tcx,
+            cx.body.source.def_id().expect_local(),
+        ));
         let ocx = ObligationCtxt::new(&infcx);
         ocx.register_obligation(obligation);
         let errors = ocx.select_all_or_error();

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -9,7 +9,9 @@ use rustc_middle::query::TyCtxtAt;
 use rustc_middle::ty::layout::{
     self, FnAbiError, FnAbiOfHelpers, FnAbiRequest, LayoutError, LayoutOfHelpers, TyAndLayout,
 };
-use rustc_middle::ty::{self, GenericArgsRef, ParamEnv, Ty, TyCtxt, TypeFoldable, Variance};
+use rustc_middle::ty::{
+    self, GenericArgsRef, ParamEnv, Ty, TyCtxt, TypeFoldable, TypingMode, Variance,
+};
 use rustc_middle::{mir, span_bug};
 use rustc_session::Limit;
 use rustc_span::Span;
@@ -325,7 +327,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             return true;
         }
         // Slow path: spin up an inference context to check if these traits are sufficiently equal.
-        let infcx = self.tcx.infer_ctxt().build();
+        let infcx = self.tcx.infer_ctxt().build(TypingMode::from_param_env(self.param_env));
         let ocx = ObligationCtxt::new(&infcx);
         let cause = ObligationCause::dummy_with_span(self.cur_span());
         // equate the two trait refs after normalization

compiler/rustc_const_eval/src/util/compare_types.rs

@@ -5,7 +5,7 @@
 
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::traits::ObligationCause;
-use rustc_middle::ty::{ParamEnv, Ty, TyCtxt, Variance};
+use rustc_middle::ty::{ParamEnv, Ty, TyCtxt, TypingMode, Variance};
 use rustc_trait_selection::traits::ObligationCtxt;
 
 /// Returns whether the two types are equal up to subtyping.
@@ -45,7 +45,7 @@ pub fn relate_types<'tcx>(
     }
 
     let mut builder = tcx.infer_ctxt().ignoring_regions();
-    let infcx = builder.build();
+    let infcx = builder.build(TypingMode::non_body_analysis());
     let ocx = ObligationCtxt::new(&infcx);
     let cause = ObligationCause::dummy();
     let src = ocx.normalize(&cause, param_env, src);

compiler/rustc_hir_analysis/src/check/check.rs

@@ -30,6 +30,7 @@ use rustc_trait_selection::traits;
 use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _;
 use rustc_type_ir::fold::TypeFoldable;
 use tracing::{debug, instrument};
+use ty::TypingMode;
 use {rustc_attr as attr, rustc_hir as hir};
 
 use super::compare_impl_item::{check_type_bounds, compare_impl_method, compare_impl_ty};
@@ -276,7 +277,7 @@ fn check_opaque_meets_bounds<'tcx>(
     };
     let param_env = tcx.param_env(defining_use_anchor);
 
-    let infcx = tcx.infer_ctxt().with_opaque_type_inference(defining_use_anchor).build();
+    let infcx = tcx.infer_ctxt().build(TypingMode::analysis_in_body(tcx, defining_use_anchor));
     let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
 
     let args = match *origin {
@@ -1675,8 +1676,7 @@ pub(super) fn check_coroutine_obligations(
         // typeck writeback gives us predicates with their regions erased.
         // As borrowck already has checked lifetimes, we do not need to do it again.
         .ignoring_regions()
-        .with_opaque_type_inference(def_id)
-        .build();
+        .build(TypingMode::analysis_in_body(tcx, def_id));
 
     let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
     for (predicate, cause) in &typeck_results.coroutine_stalled_predicates {

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -17,7 +17,7 @@ use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::util::ExplicitSelf;
 use rustc_middle::ty::{
     self, GenericArgs, GenericParamDefKind, Ty, TyCtxt, TypeFoldable, TypeFolder,
-    TypeSuperFoldable, TypeVisitableExt, Upcast,
+    TypeSuperFoldable, TypeVisitableExt, TypingMode, Upcast,
 };
 use rustc_middle::{bug, span_bug};
 use rustc_span::Span;
@@ -209,7 +209,7 @@ fn compare_method_predicate_entailment<'tcx>(
     let param_env = traits::normalize_param_env_or_error(tcx, param_env, normalize_cause);
     debug!(caller_bounds=?param_env.caller_bounds());
 
-    let infcx = &tcx.infer_ctxt().build();
+    let infcx = &tcx.infer_ctxt().build(TypingMode::non_body_analysis());
     let ocx = ObligationCtxt::new_with_diagnostics(infcx);
 
     // Create obligations for each predicate declared by the impl
@@ -469,7 +469,7 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
         ObligationCause::misc(tcx.def_span(impl_m_def_id), impl_m_def_id),
     );
 
-    let infcx = &tcx.infer_ctxt().build();
+    let infcx = &tcx.infer_ctxt().build(TypingMode::non_body_analysis());
     let ocx = ObligationCtxt::new_with_diagnostics(infcx);
 
     // Normalize the impl signature with fresh variables for lifetime inference.
@@ -1149,7 +1149,7 @@ fn compare_self_type<'tcx>(
         let self_arg_ty = tcx.fn_sig(method.def_id).instantiate_identity().input(0);
         let param_env = ty::ParamEnv::reveal_all();
 
-        let infcx = tcx.infer_ctxt().build();
+        let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
         let self_arg_ty = tcx.liberate_late_bound_regions(method.def_id, self_arg_ty);
         let can_eq_self = |ty| infcx.can_eq(param_env, untransformed_self_ty, ty);
         match ExplicitSelf::determine(self_arg_ty, can_eq_self) {
@@ -1754,7 +1754,7 @@ fn compare_const_predicate_entailment<'tcx>(
         ObligationCause::misc(impl_ct_span, impl_ct_def_id),
     );
 
-    let infcx = tcx.infer_ctxt().build();
+    let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
     let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
 
     let impl_ct_own_bounds = impl_ct_predicates.instantiate_own_identity();
@@ -1885,7 +1885,7 @@ fn compare_type_predicate_entailment<'tcx>(
     let param_env = traits::normalize_param_env_or_error(tcx, param_env, normalize_cause);
     debug!(caller_bounds=?param_env.caller_bounds());
 
-    let infcx = tcx.infer_ctxt().build();
+    let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
     let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
 
     for (predicate, span) in impl_ty_own_bounds {
@@ -1947,7 +1947,7 @@ pub(super) fn check_type_bounds<'tcx>(
     let impl_ty_args = GenericArgs::identity_for_item(tcx, impl_ty.def_id);
     let rebased_args = impl_ty_args.rebase_onto(tcx, container_id, impl_trait_ref.args);
 
-    let infcx = tcx.infer_ctxt().build();
+    let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
     let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
 
     // A synthetic impl Trait for RPITIT desugaring or assoc type for effects desugaring has no HIR,

compiler/rustc_hir_analysis/src/check/compare_impl_item/refine.rs

@@ -8,7 +8,7 @@ use rustc_middle::span_bug;
 use rustc_middle::traits::{ObligationCause, Reveal};
 use rustc_middle::ty::{
     self, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperVisitable, TypeVisitable,
-    TypeVisitableExt, TypeVisitor,
+    TypeVisitableExt, TypeVisitor, TypingMode,
 };
 use rustc_span::Span;
 use rustc_trait_selection::regions::InferCtxtRegionExt;
@@ -132,7 +132,7 @@ pub(super) fn check_refining_return_position_impl_trait_in_trait<'tcx>(
     let param_env = ty::ParamEnv::new(tcx.mk_clauses_from_iter(hybrid_preds), Reveal::UserFacing);
     let param_env = normalize_param_env_or_error(tcx, param_env, ObligationCause::dummy());
 
-    let ref infcx = tcx.infer_ctxt().build();
+    let ref infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
     let ocx = ObligationCtxt::new(infcx);
 
     // Normalize the bounds. This has two purposes:

compiler/rustc_hir_analysis/src/check/dropck.rs

@@ -9,7 +9,7 @@ use rustc_infer::infer::outlives::env::OutlivesEnvironment;
 use rustc_infer::infer::{RegionResolutionError, TyCtxtInferExt};
 use rustc_infer::traits::{ObligationCause, ObligationCauseCode};
 use rustc_middle::ty::util::CheckRegions;
-use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt};
+use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt, TypingMode};
 use rustc_trait_selection::regions::InferCtxtRegionExt;
 use rustc_trait_selection::traits::{self, ObligationCtxt};
 
@@ -124,7 +124,7 @@ fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
     adt_def_id: LocalDefId,
     adt_to_impl_args: GenericArgsRef<'tcx>,
 ) -> Result<(), ErrorGuaranteed> {
-    let infcx = tcx.infer_ctxt().build();
+    let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
     let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
 
     let impl_span = tcx.def_span(drop_impl_def_id.to_def_id());

compiler/rustc_hir_analysis/src/check/entry.rs

@@ -4,7 +4,7 @@ use rustc_hir as hir;
 use rustc_hir::Node;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::span_bug;
-use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_middle::ty::{self, Ty, TyCtxt, TypingMode};
 use rustc_session::config::EntryFnType;
 use rustc_span::Span;
 use rustc_span::def_id::{CRATE_DEF_ID, DefId, LocalDefId};
@@ -128,7 +128,7 @@ fn check_main_fn_ty(tcx: TyCtxt<'_>, main_def_id: DefId) {
             tcx.dcx().emit_err(errors::MainFunctionReturnTypeGeneric { span: return_ty_span });
             return;
         };
-        let infcx = tcx.infer_ctxt().build();
+        let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
         let cause = traits::ObligationCause::new(
             return_ty_span,
             main_diagnostics_def_id,

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -84,7 +84,7 @@ use rustc_infer::infer::{self, TyCtxtInferExt as _};
 use rustc_infer::traits::ObligationCause;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
-use rustc_middle::ty::{self, GenericArgs, GenericArgsRef, Ty, TyCtxt};
+use rustc_middle::ty::{self, GenericArgs, GenericArgsRef, Ty, TyCtxt, TypingMode};
 use rustc_middle::{bug, span_bug};
 use rustc_session::parse::feature_err;
 use rustc_span::def_id::CRATE_DEF_ID;
@@ -530,7 +530,7 @@ fn suggestion_signature<'tcx>(
             let ty = tcx.type_of(assoc.def_id).instantiate_identity();
             let val = tcx
                 .infer_ctxt()
-                .build()
+                .build(TypingMode::non_body_analysis())
                 .err_ctxt()
                 .ty_kind_suggestion(tcx.param_env(assoc.def_id), ty)
                 .unwrap_or_else(|| "value".to_string());
@@ -620,7 +620,7 @@ pub fn check_function_signature<'tcx>(
 
     let param_env = ty::ParamEnv::empty();
 
-    let infcx = &tcx.infer_ctxt().build();
+    let infcx = &tcx.infer_ctxt().build(TypingMode::non_body_analysis());
     let ocx = ObligationCtxt::new_with_diagnostics(infcx);
 
     let actual_sig = tcx.fn_sig(fn_id).instantiate_identity();

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -17,7 +17,7 @@ use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::trait_def::TraitSpecializationKind;
 use rustc_middle::ty::{
     self, AdtKind, GenericArgKind, GenericArgs, GenericParamDefKind, Ty, TyCtxt, TypeFoldable,
-    TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor, Upcast,
+    TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor, TypingMode, Upcast,
 };
 use rustc_middle::{bug, span_bug};
 use rustc_session::parse::feature_err;
@@ -105,7 +105,7 @@ where
     F: for<'a> FnOnce(&WfCheckingCtxt<'a, 'tcx>) -> Result<(), ErrorGuaranteed>,
 {
     let param_env = tcx.param_env(body_def_id);
-    let infcx = &tcx.infer_ctxt().build();
+    let infcx = &tcx.infer_ctxt().build(TypingMode::non_body_analysis());
     let ocx = ObligationCtxt::new_with_diagnostics(infcx);
 
     let mut wfcx = WfCheckingCtxt { ocx, span, body_def_id, param_env };
@@ -764,7 +764,7 @@ fn test_region_obligations<'tcx>(
     // Unfortunately, we have to use a new `InferCtxt` each call, because
     // region constraints get added and solved there and we need to test each
     // call individually.
-    let infcx = tcx.infer_ctxt().build();
+    let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
 
     add_constraints(&infcx);
 

compiler/rustc_hir_analysis/src/coherence/builtin.rs

@@ -15,7 +15,9 @@ use rustc_infer::infer::{self, RegionResolutionError, TyCtxtInferExt};
 use rustc_infer::traits::Obligation;
 use rustc_middle::ty::adjustment::CoerceUnsizedInfo;
 use rustc_middle::ty::print::PrintTraitRefExt as _;
-use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt, suggest_constraining_type_params};
+use rustc_middle::ty::{
+    self, Ty, TyCtxt, TypeVisitableExt, TypingMode, suggest_constraining_type_params,
+};
 use rustc_span::{DUMMY_SP, Span};
 use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
 use rustc_trait_selection::traits::misc::{
@@ -213,7 +215,7 @@ fn visit_implementation_of_dispatch_from_dyn(checker: &Checker<'_>) -> Result<()
 
     let param_env = tcx.param_env(impl_did);
 
-    let infcx = tcx.infer_ctxt().build();
+    let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
     let cause = ObligationCause::misc(span, impl_did);
 
     // Later parts of the compiler rely on all DispatchFromDyn types to be ABI-compatible with raw
@@ -354,7 +356,7 @@ pub(crate) fn coerce_unsized_info<'tcx>(
 
     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
 
-    let infcx = tcx.infer_ctxt().build();
+    let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
     let cause = ObligationCause::misc(span, impl_did);
     let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
                        mt_b: ty::TypeAndMut<'tcx>,

compiler/rustc_hir_analysis/src/coherence/orphan.rs

@@ -7,7 +7,7 @@ use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
 use rustc_lint_defs::builtin::UNCOVERED_PARAM_IN_PROJECTION;
 use rustc_middle::ty::{
     self, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeSuperVisitable,
-    TypeVisitable, TypeVisitableExt, TypeVisitor,
+    TypeVisitable, TypeVisitableExt, TypeVisitor, TypingMode,
 };
 use rustc_middle::{bug, span_bug};
 use rustc_span::def_id::{DefId, LocalDefId};
@@ -302,7 +302,7 @@ fn orphan_check<'tcx>(
     }
 
     // (1)  Instantiate all generic params with fresh inference vars.
-    let infcx = tcx.infer_ctxt().intercrate(true).build();
+    let infcx = tcx.infer_ctxt().build(TypingMode::Coherence);
     let cause = traits::ObligationCause::dummy();
     let args = infcx.fresh_args_for_item(cause.span, impl_def_id.to_def_id());
     let trait_ref = trait_ref.instantiate(tcx, args);