Don't run resolve_vars_if_possible in normalize_erasing_regions

NOTE: `needs_infer()` needs to come after ignoring generic parameters

Author: jyn514

compiler/rustc_traits/src/normalize_erasing_regions.rs

@@ -2,7 +2,7 @@ use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::traits::query::NoSolution;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::subst::GenericArg;
-use rustc_middle::ty::{self, ParamEnvAnd, TyCtxt};
+use rustc_middle::ty::{self, ParamEnvAnd, TyCtxt, TypeFoldable};
 use rustc_trait_selection::traits::query::normalize::AtExt;
 use rustc_trait_selection::traits::{Normalized, ObligationCause};
 use std::sync::atomic::Ordering;
@@ -31,8 +31,14 @@ fn normalize_generic_arg_after_erasing_regions<'tcx>(
                     None,
                 );
 
-                let normalized_value = infcx.resolve_vars_if_possible(normalized_value);
-                infcx.tcx.erase_regions(normalized_value)
+                let resolved_value = infcx.resolve_vars_if_possible(normalized_value);
+                // It's unclear when `resolve_vars` would have an effect in a
+                // fresh `InferCtxt`. If this assert does trigger, it will give
+                // us a test case.
+                debug_assert_eq!(normalized_value, resolved_value);
+                let erased = infcx.tcx.erase_regions(resolved_value);
+                debug_assert!(!erased.needs_infer(), "{:?}", erased);
+                erased
             }
             Err(NoSolution) => bug!("could not fully normalize `{:?}`", value),
         }