Use correct hir_id for array const arg infers

Author: BoxyUwU

Diff for: compiler/rustc_ast_lowering/src/lib.rs

@@ -2013,7 +2013,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             ExprKind::Underscore => {
                 if self.tcx.features().generic_arg_infer() {
                     let ct_kind = hir::ConstArgKind::Infer(self.lower_span(c.value.span));
-                    self.arena.alloc(hir::ConstArg { hir_id: self.next_id(), kind: ct_kind })
+                    self.arena
+                        .alloc(hir::ConstArg { hir_id: self.lower_node_id(c.id), kind: ct_kind })
                 } else {
                     feature_err(
                         &self.tcx.sess,

Diff for: compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1389,10 +1389,14 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             // `ConstArgKind::Path`. We never actually access this `DefId`
             // anywhere so we don't need to encode it for other crates.
             if def_kind == DefKind::AnonConst
-                && matches!(
-                    tcx.hir_node_by_def_id(local_id),
-                    hir::Node::ConstArg(hir::ConstArg { kind: hir::ConstArgKind::Path(..), .. })
-                )
+                && match tcx.hir_node_by_def_id(local_id) {
+                    hir::Node::ConstArg(hir::ConstArg { kind, .. }) => match kind {
+                        // Skip encoding defs for these as they should not have had a `DefId` created
+                        hir::ConstArgKind::Path(..) | hir::ConstArgKind::Infer(..) => true,
+                        hir::ConstArgKind::Anon(..) => false,
+                    },
+                    _ => false,
+                }
             {
                 continue;
             }

Diff for: tests/ui/const-generics/generic_arg_infer/array-repeat-expr-lib.rs

@@ -0,0 +1,12 @@
+//@ check-pass
+
+#![feature(generic_arg_infer)]
+#![crate_type = "lib"]
+
+// Test that encoding the hallucinated `DefId` for the `_` const argument doesn't
+// ICE (see #133468). This requires this to be a library crate.
+
+pub fn foo() {
+    let s: [u8; 10];
+    s = [0; _];
+}