review comments

Author: estebank

compiler/rustc_middle/src/query/mod.rs

@@ -1863,11 +1863,10 @@ rustc_queries! {
         feedable
     }
 
-    /// Returns whether the impl or associated function has the `default` keyword.
+    /// Returns whether the field corresponding to the `DefId` has a default field value.
     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> {

compiler/rustc_resolve/src/diagnostics.rs

@@ -1970,54 +1970,7 @@ 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;
-            let mut iter = struct_expr.fields.iter().peekable();
-            let mut prev: Option<Span> = None;
-            while let Some(field) = iter.next() {
-                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![
-                                (
-                                    // Account for trailing commas and ensure we remove them.
-                                    match (prev, iter.peek()) {
-                                        (_, Some(next)) => field.span.with_hi(next.span.lo()),
-                                        (Some(prev), _) => field.span.with_lo(prev.hi()),
-                                        (None, None) => 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;
-                    }
-                }
-                prev = Some(field.span);
-            }
-        }
+        self.mention_default_field_values(source, ident, &mut err);
 
         let mut not_publicly_reexported = false;
         if let Some((this_res, outer_ident)) = outermost_res {
@@ -2200,6 +2153,85 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         err.emit();
     }
 
+    /// When a private field is being set that has a default field value, we suggest using `..` and
+    /// setting the value of that field implicitly with its default.
+    ///
+    /// If we encounter code like
+    /// ```text
+    /// struct Priv;
+    /// pub struct S {
+    ///     pub field: Priv = Priv,
+    /// }
+    /// ```
+    /// which is used from a place where `Priv` isn't accessible
+    /// ```text
+    /// let _ = S { field: m::Priv1 {} };
+    /// //                    ^^^^^ private struct
+    /// ```
+    /// we will suggest instead using the `default_field_values` syntax instead:
+    /// ```text
+    /// let _ = S { .. };
+    /// ```
+    fn mention_default_field_values(
+        &self,
+        source: &Option<ast::Expr>,
+        ident: Ident,
+        err: &mut Diag<'_>,
+    ) {
+        let Some(expr) = source else { return };
+        let ast::ExprKind::Struct(struct_expr) = &expr.kind else { return };
+        // We don't have to handle type-relative paths because they're forbidden in ADT
+        // expressions, but that would change with `#[feature(more_qualified_paths)]`.
+        let Some(Res::Def(_, def_id)) =
+            self.partial_res_map[&struct_expr.path.segments.iter().last().unwrap().id].full_res()
+        else {
+            return;
+        };
+        let Some(default_fields) = self.field_defaults(def_id) else { return };
+        if struct_expr.fields.is_empty() {
+            return;
+        }
+        let last_span = struct_expr.fields.iter().last().unwrap().span;
+        let mut iter = struct_expr.fields.iter().peekable();
+        let mut prev: Option<Span> = None;
+        while let Some(field) = iter.next() {
+            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![
+                            (
+                                // Account for trailing commas and ensure we remove them.
+                                match (prev, iter.peek()) {
+                                    (_, Some(next)) => field.span.with_hi(next.span.lo()),
+                                    (Some(prev), _) => field.span.with_lo(prev.hi()),
+                                    (None, None) => 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;
+                }
+            }
+            prev = Some(field.span);
+        }
+    }
+
     pub(crate) fn find_similarly_named_module_or_crate(
         &self,
         ident: Symbol,