Auto merge of #107940 - BoxyUwU:const_ty_assertion_use_semantic_equality, r=compiler-errors

use semantic equality for const param type equality assertion

Fixes #107898

See added test for what caused this ICE

---

The current in assertion in `relate.rs` is rather inadequate when keeping in mind future expansions to const generics:
- it will ICE when there are infer vars in a projection in a const param ty
- it will spurriously return false when either ty has infer vars because of using `==` instead of `infcx.at(..).eq`
- i am also unsure if it would be possible with `adt_const_params` to craft a situation where the const param type is not wf causing `normalize_erasing_regions` to `bug!` when we would have emitted a diagnostic.

This impl feels pretty Not Great to me  although i am not sure what a better idea would be.

- We have to have the logic behind a query because neither `relate.rs` or `combine.rs` have access to trait solving machinery (without evaluating nested obligations this assert will become _far_ less useful under lazy norm, which consts are already doing)
- `relate.rs` does not have access to canonicalization machinery which is necessary in order to have types potentially containing infer vars in query arguments.

We could possible add a method to `TypeRelation` to do this assertion rather than a query but to avoid implementing the same logic over and over we'd probably end up with the logic in a free function somewhere in `rustc_trait_selection` _anyway_ so I don't think that would be much better.

We could also just remove this assertion, it should not actually be necessary for it to be present. It has caught some bugs in the past though so if possible I would like to keep it.

r? `@compiler-errors`

2 files changed, 7 insertions, 20 deletions

diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 71c9058a696..f02e3a9aa29 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -2173,4 +2173,11 @@ rustc_queries! {
         desc { "traits in scope for documentation links for a module" }
         separate_provide_extern
     }
+
+    /// Used in `super_combine_consts` to ICE if the type of the two consts are definitely not going to end up being
+    /// equal to eachother. This might return `Ok` even if the types are unequal, but will never return `Err` if
+    /// the types might be equal.
+    query check_tys_might_be_eq(arg: Canonical<'tcx, (ty::ParamEnv<'tcx>, Ty<'tcx>, Ty<'tcx>)>) -> Result<(), NoSolution> {
+        desc { "check whether two const param are definitely not equal to eachother"}
+    }
 }
diff --git a/compiler/rustc_middle/src/ty/relate.rs b/compiler/rustc_middle/src/ty/relate.rs
index 8b08ea9a58c..33b509ec490 100644
--- a/compiler/rustc_middle/src/ty/relate.rs
+++ b/compiler/rustc_middle/src/ty/relate.rs
@@ -9,7 +9,6 @@ use crate::ty::{self, Expr, ImplSubject, Term, TermKind, Ty, TyCtxt, TypeFoldabl
 use crate::ty::{GenericArg, GenericArgKind, SubstsRef};
 use rustc_hir as ast;
 use rustc_hir::def_id::DefId;
-use rustc_span::DUMMY_SP;
 use rustc_target::spec::abi;
 use std::iter;
 
@@ -594,25 +593,6 @@ pub fn super_relate_consts<'tcx, R: TypeRelation<'tcx>>(
     debug!("{}.super_relate_consts(a = {:?}, b = {:?})", relation.tag(), a, b);
     let tcx = relation.tcx();
 
-    let a_ty;
-    let b_ty;
-    if relation.tcx().features().adt_const_params {
-        a_ty = tcx.normalize_erasing_regions(relation.param_env(), a.ty());
-        b_ty = tcx.normalize_erasing_regions(relation.param_env(), b.ty());
-    } else {
-        a_ty = tcx.erase_regions(a.ty());
-        b_ty = tcx.erase_regions(b.ty());
-    }
-    if a_ty != b_ty {
-        relation.tcx().sess.delay_span_bug(
-            DUMMY_SP,
-            &format!(
-                "cannot relate constants ({:?}, {:?}) of different types: {} != {}",
-                a, b, a_ty, b_ty
-            ),
-        );
-    }
-
     // HACK(const_generics): We still need to eagerly evaluate consts when
     // relating them because during `normalize_param_env_or_error`,
     // we may relate an evaluated constant in a obligation against