Structurally resolve before builtin_index in EUV

Author: compiler-errors

compiler/rustc_hir_typeck/src/expr_use_visitor.rs

@@ -1741,7 +1741,11 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
             }
 
             PatKind::Slice(before, ref slice, after) => {
-                let Some(element_ty) = place_with_id.place.ty().builtin_index() else {
+                let Some(element_ty) = self
+                    .cx
+                    .try_structurally_resolve_type(pat.span, place_with_id.place.ty())
+                    .builtin_index()
+                else {
                     debug!("explicit index of non-indexable type {:?}", place_with_id);
                     return Err(self
                         .cx

tests/ui/traits/next-solver/typeck/index-of-projection.rs

@@ -0,0 +1,13 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+// Fixes a regression in `rustc_attr` where we weren't normalizing the
+// output type of a index operator performing a `Ty::builtin_index` call,
+// leading to an ICE.
+
+fn main() {
+    let mut vec = [1, 2, 3];
+    let x = || {
+        let [..] = &vec[..];
+    };
+}