Check for presence of field in typeck results before visiting it

To prevent ICEs where the struct type is tainted by errors.

Co-authored-by: Michael Goulet <michael@errs.io>

compiler/rustc_passes/src/dead.rs

@@ -464,7 +464,12 @@ impl<'tcx> Visitor<'tcx> for MarkSymbolVisitor<'tcx> {
  self.lookup_and_handle_method(expr.hir_id);
  }
  hir::ExprKind::Field(ref lhs, ..) => {
- self.handle_field_access(lhs, expr.hir_id);
+ if self.typeck_results().opt_field_index(expr.hir_id).is_some() {
+ self.handle_field_access(lhs, expr.hir_id);
+ } else {
+ // The struct type is probably tainted by errors.
+ self.tcx.dcx().span_delayed_bug(expr.span, "couldn't resolve index for field");
+ }
  }
  hir::ExprKind::Struct(qpath, fields, _) => {
  let res = self.typeck_results().qpath_res(qpath, expr.hir_id);

tests/ui/consts/const-eval/field-access-after-const-eval-fail-in-ty.rs

@@ -0,0 +1,5 @@
+// Regression test for issue #120615.
+
+fn main() {
+ [(); loop {}].field; //~ ERROR constant evaluation is taking a long time
+}

tests/ui/consts/const-eval/field-access-after-const-eval-fail-in-ty.stderr

@@ -0,0 +1,17 @@
+error: constant evaluation is taking a long time
+ --> $DIR/field-access-after-const-eval-fail-in-ty.rs:4:10
+ |
+LL | [(); loop {}].field;
+ | ^^^^^^^
+ |
+ = note: this lint makes sure the compiler doesn't get stuck due to infinite loops in const eval.
+ If your compilation actually takes a long time, you can safely allow the lint.
+help: the constant being evaluated
+ --> $DIR/field-access-after-const-eval-fail-in-ty.rs:4:10
+ |
+LL | [(); loop {}].field;
+ | ^^^^^^^
+ = note: `#[deny(long_running_const_eval)]` on by default
+
+error: aborting due to 1 previous error
+