Rollup merge of #95179 - b-naber:eval-in-try-unify, r=lcnr

Try to evaluate in try unify and postpone resolution of constants that contain inference variables

We want code like that in [`ui/const-generics/generic_const_exprs/eval-try-unify.rs`](https://github.com/rust-lang/rust/compare/master...b-naber:eval-in-try-unify?expand=1#diff-8027038201cf07a6c96abf3cbf0b0f4fdd8a64ce6292435f01c8ed995b87fe9b) to compile. To do that we need to try to evaluate constants in `try_unify_abstract_consts`, this requires us to be more careful about what constants we try to resolve, specifically we cannot try to resolve constants that still contain inference variables.

r? `@lcnr`

3 files changed, 16 insertions, 5 deletions

diff --git a/compiler/rustc_middle/src/mir/interpret/queries.rs b/compiler/rustc_middle/src/mir/interpret/queries.rs
index 8b0f92d23d7..7e5989b4112 100644
--- a/compiler/rustc_middle/src/mir/interpret/queries.rs
+++ b/compiler/rustc_middle/src/mir/interpret/queries.rs
@@ -1,6 +1,7 @@
 use super::{ErrorHandled, EvalToConstValueResult, GlobalId};
 
 use crate::mir;
+use crate::ty::fold::TypeFoldable;
 use crate::ty::subst::InternalSubsts;
 use crate::ty::{self, TyCtxt};
 use rustc_hir::def_id::DefId;
@@ -38,6 +39,16 @@ impl<'tcx> TyCtxt<'tcx> {
         ct: ty::Unevaluated<'tcx>,
         span: Option<Span>,
     ) -> EvalToConstValueResult<'tcx> {
+        // Cannot resolve `Unevaluated` constants that contain inference
+        // variables. We reject those here since `resolve_opt_const_arg`
+        // would fail otherwise.
+        //
+        // When trying to evaluate constants containing inference variables,
+        // use `Infcx::const_eval_resolve` instead.
+        if ct.substs.has_infer_types_or_consts() {
+            bug!("did not expect inference variables here");
+        }
+
         match ty::Instance::resolve_opt_const_arg(self, param_env, ct.def, ct.substs) {
             Ok(Some(instance)) => {
                 let cid = GlobalId { instance, promoted: ct.promoted };
diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 87af90d059b..cd6ff8254ad 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -331,12 +331,12 @@ rustc_queries! {
         }
     }
 
-    query try_unify_abstract_consts(key: (
-        ty::Unevaluated<'tcx, ()>, ty::Unevaluated<'tcx, ()>
-    )) -> bool {
+    query try_unify_abstract_consts(key:
+        ty::ParamEnvAnd<'tcx, (ty::Unevaluated<'tcx, ()>, ty::Unevaluated<'tcx, ()>
+    )>) -> bool {
         desc {
             |tcx| "trying to unify the generic constants {} and {}",
-            tcx.def_path_str(key.0.def.did), tcx.def_path_str(key.1.def.did)
+            tcx.def_path_str(key.value.0.def.did), tcx.def_path_str(key.value.1.def.did)
         }
     }
 
diff --git a/compiler/rustc_middle/src/ty/relate.rs b/compiler/rustc_middle/src/ty/relate.rs
index 81ee7942c4d..5d6cbcf6907 100644
--- a/compiler/rustc_middle/src/ty/relate.rs
+++ b/compiler/rustc_middle/src/ty/relate.rs
@@ -585,7 +585,7 @@ pub fn super_relate_consts<'tcx, R: TypeRelation<'tcx>>(
         (ty::ConstKind::Unevaluated(au), ty::ConstKind::Unevaluated(bu))
             if tcx.features().generic_const_exprs =>
         {
-            tcx.try_unify_abstract_consts((au.shrink(), bu.shrink()))
+            tcx.try_unify_abstract_consts(relation.param_env().and((au.shrink(), bu.shrink())))
         }
 
         // While this is slightly incorrect, it shouldn't matter for `min_const_generics`