Disallow A { .. } if A has no fields

```
error: `A` has no fields, `..` needs at least one default field in the struct definition
  --> $DIR/empty-struct.rs:16:17
   |
LL |     let _ = A { .. };
   |             -   ^^
   |             |
   |             this type has no fields
```

Author: estebank

compiler/rustc_hir_typeck/src/expr.rs

@@ -1991,18 +1991,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         adt_ty: Ty<'tcx>,
         expected: Expectation<'tcx>,
         expr: &hir::Expr<'_>,
-        span: Span,
+        path_span: Span,
         variant: &'tcx ty::VariantDef,
         hir_fields: &'tcx [hir::ExprField<'tcx>],
         base_expr: &'tcx hir::StructTailExpr<'tcx>,
     ) {
         let tcx = self.tcx;
 
-        let adt_ty = self.try_structurally_resolve_type(span, adt_ty);
+        let adt_ty = self.try_structurally_resolve_type(path_span, adt_ty);
         let adt_ty_hint = expected.only_has_type(self).and_then(|expected| {
             self.fudge_inference_if_ok(|| {
                 let ocx = ObligationCtxt::new(self);
-                ocx.sup(&self.misc(span), self.param_env, expected, adt_ty)?;
+                ocx.sup(&self.misc(path_span), self.param_env, expected, adt_ty)?;
                 if !ocx.select_where_possible().is_empty() {
                     return Err(TypeError::Mismatch);
                 }
@@ -2012,11 +2012,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         });
         if let Some(adt_ty_hint) = adt_ty_hint {
             // re-link the variables that the fudging above can create.
-            self.demand_eqtype(span, adt_ty_hint, adt_ty);
+            self.demand_eqtype(path_span, adt_ty_hint, adt_ty);
         }
 
         let ty::Adt(adt, args) = adt_ty.kind() else {
-            span_bug!(span, "non-ADT passed to check_expr_struct_fields");
+            span_bug!(path_span, "non-ADT passed to check_expr_struct_fields");
         };
         let adt_kind = adt.adt_kind();
 
@@ -2107,7 +2107,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if adt_kind == AdtKind::Union && hir_fields.len() != 1 {
             struct_span_code_err!(
                 self.dcx(),
-                span,
+                path_span,
                 E0784,
                 "union expressions should have exactly one field",
             )
@@ -2167,6 +2167,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 });
                 return;
             }
+            if variant.fields.is_empty() {
+                let mut err = self.dcx().struct_span_err(
+                    span,
+                    format!(
+                        "`{adt_ty}` has no fields, `..` needs at least one default field in the \
+                         struct definition",
+                    ),
+                );
+                err.span_label(path_span, "this type has no fields");
+                err.emit();
+            }
             if !missing_mandatory_fields.is_empty() {
                 let s = pluralize!(missing_mandatory_fields.len());
                 let fields: Vec<_> =
@@ -2316,11 +2327,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 .collect();
 
             if !private_fields.is_empty() {
-                self.report_private_fields(adt_ty, span, expr.span, private_fields, hir_fields);
+                self.report_private_fields(
+                    adt_ty,
+                    path_span,
+                    expr.span,
+                    private_fields,
+                    hir_fields,
+                );
             } else {
                 self.report_missing_fields(
                     adt_ty,
-                    span,
+                    path_span,
                     remaining_fields,
                     variant,
                     hir_fields,

tests/ui/structs/default-field-values/empty-struct.rs

@@ -0,0 +1,21 @@
+#![feature(default_field_values)]
+
+// If an API wants users to always use `..` even if they specify all the fields, they should use a
+// sentinel field. As of now, that field can't be made private so it is only constructable with this
+// syntax, but this might change in the future.
+
+struct A {}
+struct B();
+struct C;
+struct D {
+    x: i32,
+}
+struct E(i32);
+
+fn main() {
+    let _ = A { .. }; //~ ERROR has no fields
+    let _ = B { .. }; //~ ERROR has no fields
+    let _ = C { .. }; //~ ERROR has no fields
+    let _ = D { x: 0, .. };
+    let _ = E { 0: 0, .. };
+}

tests/ui/structs/default-field-values/empty-struct.stderr

@@ -0,0 +1,26 @@
+error: `A` has no fields, `..` needs at least one default field in the struct definition
+  --> $DIR/empty-struct.rs:16:17
+   |
+LL |     let _ = A { .. };
+   |             -   ^^
+   |             |
+   |             this type has no fields
+
+error: `B` has no fields, `..` needs at least one default field in the struct definition
+  --> $DIR/empty-struct.rs:17:17
+   |
+LL |     let _ = B { .. };
+   |             -   ^^
+   |             |
+   |             this type has no fields
+
+error: `C` has no fields, `..` needs at least one default field in the struct definition
+  --> $DIR/empty-struct.rs:18:17
+   |
+LL |     let _ = C { .. };
+   |             -   ^^
+   |             |
+   |             this type has no fields
+
+error: aborting due to 3 previous errors
+