Auto merge of #131045 - compiler-errors:remove-unnamed_fields, r=wesleywiser

Retire the `unnamed_fields` feature for now

`#![feature(unnamed_fields)]` was implemented in part in #115131 and #115367, however work on that feature has (afaict) stalled and in the mean time there have been some concerns raised (e.g.[^1][^2]) about whether `unnamed_fields` is worthwhile to have in the language, especially in its current desugaring. Because it represents a compiler implementation burden including a new kind of anonymous ADT and additional complication to field selection, and is quite prone to bugs today, I'm choosing to remove the feature.

However, since I'm not one to really write a bunch of words, I'm specifically *not* going to de-RFC this feature. This PR essentially *rolls back* the state of this feature to "RFC accepted but not yet implemented"; however if anyone wants to formally unapprove the RFC from the t-lang side, then please be my guest. I'm just not totally willing to summarize the various language-facing reasons for why this feature is or is not worthwhile, since I'm coming from the compiler side mostly.

Fixes #117942
Fixes #121161
Fixes #121263
Fixes #121299
Fixes #121722
Fixes #121799
Fixes #126969
Fixes #131041

Tracking:
* #49804

[^1]: https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Unnamed.20struct.2Funion.20fields
[^2]: #49804 (comment)

Author: bors

compiler/rustc_ast/src/ast.rs

@@ -2167,10 +2167,6 @@ pub enum TyKind {
     Never,
     /// A tuple (`(A, B, C, D,...)`).
     Tup(ThinVec<P<Ty>>),
-    /// An anonymous struct type i.e. `struct { foo: Type }`.
-    AnonStruct(NodeId, ThinVec<FieldDef>),
-    /// An anonymous union type i.e. `union { bar: Type }`.
-    AnonUnion(NodeId, ThinVec<FieldDef>),
     /// A path (`module::module::...::Type`), optionally
     /// "qualified", e.g., `<Vec<T> as SomeTrait>::SomeType`.
     ///
@@ -2227,10 +2223,6 @@ impl TyKind {
             None
         }
     }
-
-    pub fn is_anon_adt(&self) -> bool {
-        matches!(self, TyKind::AnonStruct(..) | TyKind::AnonUnion(..))
-    }
 }
 
 /// Syntax used to declare a trait object.

compiler/rustc_ast/src/mut_visit.rs

@@ -519,10 +519,6 @@ pub fn walk_ty<T: MutVisitor>(vis: &mut T, ty: &mut P<Ty>) {
             visit_vec(bounds, |bound| vis.visit_param_bound(bound, BoundKind::Impl));
         }
         TyKind::MacCall(mac) => vis.visit_mac_call(mac),
-        TyKind::AnonStruct(id, fields) | TyKind::AnonUnion(id, fields) => {
-            vis.visit_id(id);
-            fields.flat_map_in_place(|field| vis.flat_map_field_def(field));
-        }
     }
     visit_lazy_tts(vis, tokens);
     vis.visit_span(span);

compiler/rustc_ast/src/util/classify.rs

@@ -287,12 +287,6 @@ fn type_trailing_braced_mac_call(mut ty: &ast::Ty) -> Option<&ast::MacCall> {
             | ast::TyKind::Pat(..)
             | ast::TyKind::Dummy
             | ast::TyKind::Err(..) => break None,
-
-            // These end in brace, but cannot occur in a let-else statement.
-            // They are only parsed as fields of a data structure. For the
-            // purpose of denying trailing braces in the expression of a
-            // let-else, we can disregard these.
-            ast::TyKind::AnonStruct(..) | ast::TyKind::AnonUnion(..) => break None,
         }
     }
 }

compiler/rustc_ast/src/visit.rs

@@ -535,9 +535,6 @@ pub fn walk_ty<'a, V: Visitor<'a>>(visitor: &mut V, typ: &'a Ty) -> V::Result {
         TyKind::Err(_guar) => {}
         TyKind::MacCall(mac) => try_visit!(visitor.visit_mac_call(mac)),
         TyKind::Never | TyKind::CVarArgs => {}
-        TyKind::AnonStruct(_id, ref fields) | TyKind::AnonUnion(_id, ref fields) => {
-            walk_list!(visitor, visit_field_def, fields);
-        }
     }
     V::Result::output()
 }

compiler/rustc_ast_lowering/src/lib.rs

@@ -1259,46 +1259,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let kind = match &t.kind {
             TyKind::Infer => hir::TyKind::Infer,
             TyKind::Err(guar) => hir::TyKind::Err(*guar),
-            // Lower the anonymous structs or unions in a nested lowering context.
-            //
-            // ```
-            // struct Foo {
-            //     _: union {
-            // //     ^__________________  <-- within the nested lowering context,
-            //         /* fields */ //   |     we lower all fields defined into an
-            //     } //                  |     owner node of struct or union item
-            // //  ^_____________________|
-            // }
-            // ```
-            TyKind::AnonStruct(node_id, fields) | TyKind::AnonUnion(node_id, fields) => {
-                // Here its `def_id` is created in `build_reduced_graph`.
-                let def_id = self.local_def_id(*node_id);
-                debug!(?def_id);
-                let owner_id = hir::OwnerId { def_id };
-                self.with_hir_id_owner(*node_id, |this| {
-                    let fields = this.arena.alloc_from_iter(
-                        fields.iter().enumerate().map(|f| this.lower_field_def(f)),
-                    );
-                    let span = t.span;
-                    let variant_data =
-                        hir::VariantData::Struct { fields, recovered: ast::Recovered::No };
-                    // FIXME: capture the generics from the outer adt.
-                    let generics = hir::Generics::empty();
-                    let kind = match t.kind {
-                        TyKind::AnonStruct(..) => hir::ItemKind::Struct(variant_data, generics),
-                        TyKind::AnonUnion(..) => hir::ItemKind::Union(variant_data, generics),
-                        _ => unreachable!(),
-                    };
-                    hir::OwnerNode::Item(this.arena.alloc(hir::Item {
-                        ident: Ident::new(kw::Empty, span),
-                        owner_id,
-                        kind,
-                        span: this.lower_span(span),
-                        vis_span: this.lower_span(span.shrink_to_lo()),
-                    }))
-                });
-                hir::TyKind::AnonAdt(hir::ItemId { owner_id })
-            }
             TyKind::Slice(ty) => hir::TyKind::Slice(self.lower_ty(ty, itctx)),
             TyKind::Ptr(mt) => hir::TyKind::Ptr(self.lower_mt(mt, itctx)),
             TyKind::Ref(region, mt) => {

compiler/rustc_ast_passes/messages.ftl

@@ -1,7 +1,3 @@
-ast_passes_anon_struct_or_union_not_allowed =
-    anonymous {$struct_or_union}s are not allowed outside of unnamed struct or union fields
-    .label = anonymous {$struct_or_union} declared here
-
 ast_passes_assoc_const_without_body =
     associated constant in `impl` without body
     .suggestion = provide a definition for the constant
@@ -160,14 +156,6 @@ ast_passes_inherent_cannot_be = inherent impls cannot be {$annotation}
     .type = inherent impl for this type
     .only_trait = only trait implementations may be annotated with {$annotation}
 
-ast_passes_invalid_unnamed_field =
-    unnamed fields are not allowed outside of structs or unions
-    .label = unnamed field declared here
-
-ast_passes_invalid_unnamed_field_ty =
-    unnamed fields can only have struct or union types
-    .label = not a struct or union
-
 ast_passes_item_invalid_safety = items outside of `unsafe extern {"{ }"}` cannot be declared with `safe` safety qualifier
     .suggestion = remove safe from this item
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -244,9 +244,6 @@ impl<'a> AstValidator<'a> {
                     }
                 }
             }
-            TyKind::AnonStruct(_, ref fields) | TyKind::AnonUnion(_, ref fields) => {
-                walk_list!(self, visit_struct_field_def, fields)
-            }
             _ => visit::walk_ty(self, t),
         }
     }
@@ -255,7 +252,6 @@ impl<'a> AstValidator<'a> {
         if let Some(ident) = field.ident
             && ident.name == kw::Underscore
         {
-            self.check_unnamed_field_ty(&field.ty, ident.span);
             self.visit_vis(&field.vis);
             self.visit_ident(ident);
             self.visit_ty_common(&field.ty);
@@ -294,39 +290,6 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    fn check_unnamed_field_ty(&self, ty: &Ty, span: Span) {
-        if matches!(
-            &ty.kind,
-            // We already checked for `kw::Underscore` before calling this function,
-            // so skip the check
-            TyKind::AnonStruct(..) | TyKind::AnonUnion(..)
-            // If the anonymous field contains a Path as type, we can't determine
-            // if the path is a valid struct or union, so skip the check
-            | TyKind::Path(..)
-        ) {
-            return;
-        }
-        self.dcx().emit_err(errors::InvalidUnnamedFieldTy { span, ty_span: ty.span });
-    }
-
-    fn deny_anon_struct_or_union(&self, ty: &Ty) {
-        let struct_or_union = match &ty.kind {
-            TyKind::AnonStruct(..) => "struct",
-            TyKind::AnonUnion(..) => "union",
-            _ => return,
-        };
-        self.dcx().emit_err(errors::AnonStructOrUnionNotAllowed { struct_or_union, span: ty.span });
-    }
-
-    fn deny_unnamed_field(&self, field: &FieldDef) {
-        if let Some(ident) = field.ident
-            && ident.name == kw::Underscore
-        {
-            self.dcx()
-                .emit_err(errors::InvalidUnnamedField { span: field.span, ident_span: ident.span });
-        }
-    }
-
     fn check_trait_fn_not_const(&self, constness: Const, parent: &TraitOrTraitImpl) {
         let Const::Yes(span) = constness else {
             return;
@@ -890,15 +853,9 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
 
     fn visit_ty(&mut self, ty: &'a Ty) {
         self.visit_ty_common(ty);
-        self.deny_anon_struct_or_union(ty);
         self.walk_ty(ty)
     }
 
-    fn visit_field_def(&mut self, field: &'a FieldDef) {
-        self.deny_unnamed_field(field);
-        visit::walk_field_def(self, field)
-    }
-
     fn visit_item(&mut self, item: &'a Item) {
         if item.attrs.iter().any(|attr| attr.is_proc_macro_attr()) {
             self.has_proc_macro_decls = true;

compiler/rustc_ast_passes/src/errors.rs

@@ -814,33 +814,6 @@ pub(crate) struct NegativeBoundWithParentheticalNotation {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(ast_passes_invalid_unnamed_field_ty)]
-pub(crate) struct InvalidUnnamedFieldTy {
-    #[primary_span]
-    pub span: Span,
-    #[label]
-    pub ty_span: Span,
-}
-
-#[derive(Diagnostic)]
-#[diag(ast_passes_invalid_unnamed_field)]
-pub(crate) struct InvalidUnnamedField {
-    #[primary_span]
-    pub span: Span,
-    #[label]
-    pub ident_span: Span,
-}
-
-#[derive(Diagnostic)]
-#[diag(ast_passes_anon_struct_or_union_not_allowed)]
-pub(crate) struct AnonStructOrUnionNotAllowed {
-    #[primary_span]
-    #[label]
-    pub span: Span,
-    pub struct_or_union: &'static str,
-}
-
 #[derive(Diagnostic)]
 #[diag(ast_passes_match_arm_with_no_body)]
 pub(crate) struct MatchArmWithNoBody {

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -541,7 +541,6 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(builtin_syntax, "`builtin #` syntax is unstable");
     gate_all!(explicit_tail_calls, "`become` expression is experimental");
     gate_all!(generic_const_items, "generic const items are experimental");
-    gate_all!(unnamed_fields, "unnamed fields are not yet fully implemented");
     gate_all!(fn_delegation, "functions delegation is not yet fully implemented");
     gate_all!(postfix_match, "postfix match is experimental");
     gate_all!(mut_ref, "mutable by-reference bindings are experimental");

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1174,14 +1174,6 @@ impl<'a> State<'a> {
                 }
                 self.pclose();
             }
-            ast::TyKind::AnonStruct(_, fields) => {
-                self.head("struct");
-                self.print_record_struct_body(fields, ty.span);
-            }
-            ast::TyKind::AnonUnion(_, fields) => {
-                self.head("union");
-                self.print_record_struct_body(fields, ty.span);
-            }
             ast::TyKind::Paren(typ) => {
                 self.popen();
                 self.print_type(typ);