Allow non-return ZST places to appear without proceding defs in BB

src/analyze/local_def.rs

@@ -720,8 +720,8 @@ impl<'tcx, 'ctx> Analyzer<'tcx, 'ctx> {
         let mut subst = HashMap::new();
         for (param_idx, param_ty) in bb_ty.as_ref().params.iter_enumerated() {
             if let Some(param_local) = bb_ty.local_of_param(param_idx) {
-                // unit return may use _0 without preceeding def
-                if param_local == mir::RETURN_PLACE {
+                // BBs may use locals without preceding def when they're ZST
+                if param_local == mir::RETURN_PLACE || param_local > self.body.arg_count.into() {
                     subst.extend(
                         bb_ty
                             .as_ref()
@@ -730,7 +730,7 @@ impl<'tcx, 'ctx> Analyzer<'tcx, 'ctx> {
                             .skip_while(|idx| idx.index() <= param_idx.index())
                             .map(|idx| (idx, idx + 1)),
                     );
-                    if bb_ty.as_ref().params.len() == 1 {
+                    if bb_ty.as_ref().params.len() - 1 == param_idx.index() {
                         params.push(rty::RefinedType::new(
                             rty::Type::unit(),
                             param_ty.refinement.clone(),

tests/ui/fail/closure_field_1.rs

@@ -0,0 +1,14 @@
+//@error-in-other-file: Unsat
+//@compile-flags: -C debug-assertions=off
+
+struct S<F> {
+    f: F,
+}
+
+fn main() {
+    let s = S {
+        f: |x: i32| x + 1,
+    };
+    let x = (s.f)(1);
+    assert!(x == 1);
+}

tests/ui/pass/closure_field_1.rs

@@ -0,0 +1,14 @@
+//@check-pass
+//@compile-flags: -C debug-assertions=off
+
+struct S<F> {
+    f: F,
+}
+
+fn main() {
+    let s = S {
+        f: |x: i32| x + 1,
+    };
+    let x = (s.f)(1);
+    assert!(x == 2);
+}