Fix an ICE that I just made worse

compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs

@@ -1,7 +1,6 @@
 use rustc_errors::DelayDm;
 use rustc_hir as hir;
 use rustc_index::vec::Idx;
-use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
 use rustc_middle::mir::{self, Field};
 use rustc_middle::thir::{FieldPat, Pat, PatKind};
@@ -227,13 +226,6 @@ fn type_may_have_partial_eq_impl(&self, ty: Ty<'tcx>) -> bool {
         // using `PartialEq::eq` in this scenario in the past.)
         let partial_eq_trait_id =
             self.tcx().require_lang_item(hir::LangItem::PartialEq, Some(self.span));
-        let any_ty = self
-            .infcx
-            .next_ty_var(TypeVariableOrigin {
-                kind: TypeVariableOriginKind::MiscVariable,
-                span: self.span,
-            })
-            .into();
         let obligation: PredicateObligation<'_> = predicate_for_trait_def(
             self.tcx(),
             self.param_env,
@@ -241,7 +233,7 @@ fn type_may_have_partial_eq_impl(&self, ty: Ty<'tcx>) -> bool {
             partial_eq_trait_id,
             0,
             ty,
-            [any_ty],
+            [ty.into()],
         );
         // FIXME: should this call a `predicate_must_hold` variant instead?
 

src/test/ui/pattern/usefulness/const-partial_eq-fallback-ice.rs

@@ -0,0 +1,20 @@
+#![allow(warnings)]
+
+struct MyType;
+
+impl PartialEq<usize> for MyType {
+    fn eq(&self, y: &usize) -> bool {
+        true
+    }
+}
+
+const CONSTANT: &&MyType = &&MyType;
+
+fn main() {
+    if let CONSTANT = &&MyType {
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        println!("semantic!");
+    } else {
+        println!("structural!");
+    }
+}

src/test/ui/pattern/usefulness/const-partial_eq-fallback-ice.stderr

@@ -0,0 +1,8 @@
+error: to use a constant of type `MyType` in a pattern, `MyType` must be annotated with `#[derive(PartialEq, Eq)]`
+  --> $DIR/const-partial_eq-fallback-ice.rs:14:12
+   |
+LL |     if let CONSTANT = &&MyType {
+   |            ^^^^^^^^
+
+error: aborting due to previous error
+