Detect struct construction with private field in field with default

When trying to construct a struct that has a public field of a private type, suggest using `..` if that field has a default value.

```
error[E0603]: struct `Priv1` is private
  --> $DIR/non-exhaustive-ctor.rs:25:39
   |
LL |     let _ = S { field: (), field1: m::Priv1 {} };
   |                            ------     ^^^^^ private struct
   |                            |
   |                            while setting this field
   |
note: the struct `Priv1` is defined here
  --> $DIR/non-exhaustive-ctor.rs:14:4
   |
LL |    struct Priv1 {}
   |    ^^^^^^^^^^^^
help: the field `field1` you're trying to set has a default value, you can use `..` to use it
   |
LL |     let _ = S { field: (), .. };
   |                            ~~
```

Author: estebank

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -424,6 +424,7 @@ provide! { tcx, def_id, other, cdata,
 
     crate_extern_paths => { cdata.source().paths().cloned().collect() }
     expn_that_defined => { cdata.get_expn_that_defined(def_id.index, tcx.sess) }
+    default_field => { cdata.get_default_field(def_id.index) }
     is_doc_hidden => { cdata.get_attr_flags(def_id.index).contains(AttrFlags::IS_DOC_HIDDEN) }
     doc_link_resolutions => { tcx.arena.alloc(cdata.get_doc_link_resolutions(def_id.index)) }
     doc_link_traits_in_scope => {

compiler/rustc_middle/src/query/mod.rs

@@ -1711,6 +1711,13 @@ rustc_queries! {
         feedable
     }
 
+    /// Returns the default `const` value for a struct field.
+    query default_field(def_id: DefId) -> Option<DefId> {
+        desc { |tcx| "looking up the `const` corresponding to the default for `{}`", tcx.def_path_str(def_id) }
+        separate_provide_extern
+        feedable
+    }
+
     query check_well_formed(key: LocalDefId) -> Result<(), ErrorGuaranteed> {
         desc { |tcx| "checking that `{}` is well-formed", tcx.def_path_str(key) }
         ensure_forwards_result_if_red

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -396,14 +396,18 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
             // The fields are not expanded yet.
             return;
         }
-        let fields = fields
+        let field_name = |i, field: &ast::FieldDef| {
+            field.ident.unwrap_or_else(|| Ident::from_str_and_span(&format!("{i}"), field.span))
+        };
+        let field_names: Vec<_> =
+            fields.iter().enumerate().map(|(i, field)| field_name(i, field)).collect();
+        let defaults = fields
             .iter()
             .enumerate()
-            .map(|(i, field)| {
-                field.ident.unwrap_or_else(|| Ident::from_str_and_span(&format!("{i}"), field.span))
-            })
+            .filter_map(|(i, field)| field.default.as_ref().map(|_| field_name(i, field).name))
             .collect();
-        self.r.field_names.insert(def_id, fields);
+        self.r.field_names.insert(def_id, field_names);
+        self.r.field_defaults.insert(def_id, defaults);
     }
 
     fn insert_field_visibilities_local(&mut self, def_id: DefId, fields: &[ast::FieldDef]) {

compiler/rustc_resolve/src/diagnostics.rs

@@ -1754,8 +1754,15 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
     }
 
     fn report_privacy_error(&mut self, privacy_error: &PrivacyError<'ra>) {
-        let PrivacyError { ident, binding, outermost_res, parent_scope, single_nested, dedup_span } =
-            *privacy_error;
+        let PrivacyError {
+            ident,
+            binding,
+            outermost_res,
+            parent_scope,
+            single_nested,
+            dedup_span,
+            ref source,
+        } = *privacy_error;
 
         let res = binding.res();
         let ctor_fields_span = self.ctor_fields_span(binding);
@@ -1771,6 +1778,44 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         let mut err =
             self.dcx().create_err(errors::IsPrivate { span: ident.span, ident_descr, ident });
 
+        if let Some(expr) = source
+            && let ast::ExprKind::Struct(struct_expr) = &expr.kind
+            && let Some(Res::Def(_, def_id)) = self.partial_res_map
+                [&struct_expr.path.segments.iter().last().unwrap().id]
+                .full_res()
+            && let Some(default_fields) = self.field_defaults(def_id)
+            && !struct_expr.fields.is_empty()
+        {
+            let last_span = struct_expr.fields.iter().last().unwrap().span;
+            for field in &struct_expr.fields[..] {
+                if field.expr.span.overlaps(ident.span) {
+                    err.span_label(field.ident.span, "while setting this field");
+                    if default_fields.contains(&field.ident.name) {
+                        let sugg = if last_span == field.span {
+                            vec![(field.span, "..".to_string())]
+                        } else {
+                            vec![
+                                (field.span, String::new()),
+                                (last_span.shrink_to_hi(), ", ..".to_string()),
+                            ]
+                        };
+                        err.multipart_suggestion_verbose(
+                            format!(
+                                "the type `{ident}` of field `{}` is private, but you can \
+                                 construct the default value defined for it in `{}` using `..` in \
+                                 the struct initializer expression",
+                                field.ident,
+                                self.tcx.item_name(def_id),
+                            ),
+                            sugg,
+                            Applicability::MachineApplicable,
+                        );
+                        break;
+                    }
+                }
+            }
+        }
+
         let mut not_publicly_reexported = false;
         if let Some((this_res, outer_ident)) = outermost_res {
             let import_suggestions = self.lookup_import_candidates(

compiler/rustc_resolve/src/ident.rs

@@ -893,6 +893,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                         binding,
                         dedup_span: path_span,
                         outermost_res: None,
+                        source: None,
                         parent_scope: *parent_scope,
                         single_nested: path_span != root_span,
                     });
@@ -1389,7 +1390,16 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         parent_scope: &ParentScope<'ra>,
         ignore_import: Option<Import<'ra>>,
     ) -> PathResult<'ra> {
-        self.resolve_path_with_ribs(path, opt_ns, parent_scope, None, None, None, ignore_import)
+        self.resolve_path_with_ribs(
+            path,
+            opt_ns,
+            parent_scope,
+            None,
+            None,
+            None,
+            None,
+            ignore_import,
+        )
     }
 
     #[instrument(level = "debug", skip(self))]
@@ -1406,6 +1416,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             path,
             opt_ns,
             parent_scope,
+            None,
             finalize,
             None,
             ignore_binding,
@@ -1418,6 +1429,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         path: &[Segment],
         opt_ns: Option<Namespace>, // `None` indicates a module path in import
         parent_scope: &ParentScope<'ra>,
+        source: Option<PathSource<'_>>,
         finalize: Option<Finalize>,
         ribs: Option<&PerNS<Vec<Rib<'ra>>>>,
         ignore_binding: Option<NameBinding<'ra>>,
@@ -1592,6 +1604,11 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                     // the user it is not accessible.
                     for error in &mut self.privacy_errors[privacy_errors_len..] {
                         error.outermost_res = Some((res, ident));
+                        error.source = match source {
+                            Some(PathSource::Struct(Some(expr)))
+                            | Some(PathSource::Expr(Some(expr))) => Some(expr.clone()),
+                            _ => None,
+                        };
                     }
 
                     let maybe_assoc = opt_ns != Some(MacroNS) && PathSource::Type.is_expected(res);

compiler/rustc_resolve/src/late.rs

@@ -403,7 +403,7 @@ pub(crate) enum PathSource<'a> {
     // Paths in path patterns `Path`.
     Pat,
     // Paths in struct expressions and patterns `Path { .. }`.
-    Struct,
+    Struct(Option<&'a Expr>),
     // Paths in tuple struct patterns `Path(..)`.
     TupleStruct(Span, &'a [Span]),
     // `m::A::B` in `<T as m::A>::B::C`.
@@ -419,7 +419,7 @@ pub(crate) enum PathSource<'a> {
 impl<'a> PathSource<'a> {
     fn namespace(self) -> Namespace {
         match self {
-            PathSource::Type | PathSource::Trait(_) | PathSource::Struct => TypeNS,
+            PathSource::Type | PathSource::Trait(_) | PathSource::Struct(_) => TypeNS,
             PathSource::Expr(..)
             | PathSource::Pat
             | PathSource::TupleStruct(..)
@@ -435,7 +435,7 @@ impl<'a> PathSource<'a> {
             PathSource::Type
             | PathSource::Expr(..)
             | PathSource::Pat
-            | PathSource::Struct
+            | PathSource::Struct(_)
             | PathSource::TupleStruct(..)
             | PathSource::ReturnTypeNotation => true,
             PathSource::Trait(_)
@@ -450,7 +450,7 @@ impl<'a> PathSource<'a> {
             PathSource::Type => "type",
             PathSource::Trait(_) => "trait",
             PathSource::Pat => "unit struct, unit variant or constant",
-            PathSource::Struct => "struct, variant or union type",
+            PathSource::Struct(_) => "struct, variant or union type",
             PathSource::TupleStruct(..) => "tuple struct or tuple variant",
             PathSource::TraitItem(ns) => match ns {
                 TypeNS => "associated type",
@@ -531,7 +531,7 @@ impl<'a> PathSource<'a> {
                     || matches!(res, Res::Def(DefKind::Const | DefKind::AssocConst, _))
             }
             PathSource::TupleStruct(..) => res.expected_in_tuple_struct_pat(),
-            PathSource::Struct => matches!(
+            PathSource::Struct(_) => matches!(
                 res,
                 Res::Def(
                     DefKind::Struct
@@ -571,8 +571,8 @@ impl<'a> PathSource<'a> {
             (PathSource::Trait(_), false) => E0405,
             (PathSource::Type, true) => E0573,
             (PathSource::Type, false) => E0412,
-            (PathSource::Struct, true) => E0574,
-            (PathSource::Struct, false) => E0422,
+            (PathSource::Struct(_), true) => E0574,
+            (PathSource::Struct(_), false) => E0422,
             (PathSource::Expr(..), true) | (PathSource::Delegation, true) => E0423,
             (PathSource::Expr(..), false) | (PathSource::Delegation, false) => E0425,
             (PathSource::Pat | PathSource::TupleStruct(..), true) => E0532,
@@ -1454,11 +1454,13 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
         path: &[Segment],
         opt_ns: Option<Namespace>, // `None` indicates a module path in import
         finalize: Option<Finalize>,
+        source: PathSource<'ast>,
     ) -> PathResult<'ra> {
         self.r.resolve_path_with_ribs(
             path,
             opt_ns,
             &self.parent_scope,
+            Some(source),
             finalize,
             Some(&self.ribs),
             None,
@@ -1961,7 +1963,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                 | PathSource::ReturnTypeNotation => false,
                 PathSource::Expr(..)
                 | PathSource::Pat
-                | PathSource::Struct
+                | PathSource::Struct(_)
                 | PathSource::TupleStruct(..)
                 | PathSource::Delegation => true,
             };
@@ -3794,7 +3796,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                     self.smart_resolve_path(pat.id, qself, path, PathSource::Pat);
                 }
                 PatKind::Struct(ref qself, ref path, ref _fields, ref rest) => {
-                    self.smart_resolve_path(pat.id, qself, path, PathSource::Struct);
+                    self.smart_resolve_path(pat.id, qself, path, PathSource::Struct(None));
                     self.record_patterns_with_skipped_bindings(pat, rest);
                 }
                 PatKind::Or(ref ps) => {
@@ -4200,6 +4202,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
             qself,
             path,
             ns,
+            source,
             path_span,
             source.defer_to_typeck(),
             finalize,
@@ -4245,7 +4248,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                     std_path.push(Segment::from_ident(Ident::with_dummy_span(sym::std)));
                     std_path.extend(path);
                     if let PathResult::Module(_) | PathResult::NonModule(_) =
-                        self.resolve_path(&std_path, Some(ns), None)
+                        self.resolve_path(&std_path, Some(ns), None, source)
                     {
                         // Check if we wrote `str::from_utf8` instead of `std::str::from_utf8`
                         let item_span =
@@ -4316,6 +4319,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
         qself: &Option<P<QSelf>>,
         path: &[Segment],
         primary_ns: Namespace,
+        source: PathSource<'ast>,
         span: Span,
         defer_to_typeck: bool,
         finalize: Finalize,
@@ -4324,7 +4328,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
 
         for (i, &ns) in [primary_ns, TypeNS, ValueNS].iter().enumerate() {
             if i == 0 || ns != primary_ns {
-                match self.resolve_qpath(qself, path, ns, finalize)? {
+                match self.resolve_qpath(qself, path, ns, source, finalize)? {
                     Some(partial_res)
                         if partial_res.unresolved_segments() == 0 || defer_to_typeck =>
                     {
@@ -4360,6 +4364,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
         qself: &Option<P<QSelf>>,
         path: &[Segment],
         ns: Namespace,
+        source: PathSource<'ast>,
         finalize: Finalize,
     ) -> Result<Option<PartialRes>, Spanned<ResolutionError<'ra>>> {
         debug!(
@@ -4421,7 +4426,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
             )));
         }
 
-        let result = match self.resolve_path(path, Some(ns), Some(finalize)) {
+        let result = match self.resolve_path(path, Some(ns), Some(finalize), source) {
             PathResult::NonModule(path_res) => path_res,
             PathResult::Module(ModuleOrUniformRoot::Module(module)) if !module.is_normal() => {
                 PartialRes::new(module.res().unwrap())
@@ -4642,7 +4647,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
             }
 
             ExprKind::Struct(ref se) => {
-                self.smart_resolve_path(expr.id, &se.qself, &se.path, PathSource::Struct);
+                self.smart_resolve_path(expr.id, &se.qself, &se.path, PathSource::Struct(parent));
                 // This is the same as `visit::walk_expr(self, expr);`, but we want to pass the
                 // parent in for accurate suggestions when encountering `Foo { bar }` that should
                 // have been `Foo { bar: self.bar }`.