Auto merge of #84571 - jedel1043:issue-49804-impl, r=petrochenkov

Parse unnamed fields of struct and union type

Added the `unnamed_fields` feature gate.

This is a prototype of [RFC 2102](#49804), so any suggestions are greatly appreciated.

r? `@petrochenkov`

compiler/rustc_ast/src/ast.rs

@@ -1861,6 +1861,10 @@ pub enum TyKind {
     Never,
     /// A tuple (`(A, B, C, D,...)`).
     Tup(Vec<P<Ty>>),
+    /// An anonymous struct type i.e. `struct { foo: Type }`
+    AnonymousStruct(Vec<FieldDef>, bool),
+    /// An anonymous union type i.e. `union { bar: Type }`
+    AnonymousUnion(Vec<FieldDef>, bool),
     /// A path (`module::module::...::Type`), optionally
     /// "qualified", e.g., `<Vec<T> as SomeTrait>::SomeType`.
     ///

compiler/rustc_ast/src/mut_visit.rs

@@ -484,6 +484,9 @@ pub fn noop_visit_ty<T: MutVisitor>(ty: &mut P<Ty>, vis: &mut T) {
             visit_vec(bounds, |bound| vis.visit_param_bound(bound));
         }
         TyKind::MacCall(mac) => vis.visit_mac_call(mac),
+        TyKind::AnonymousStruct(fields, ..) | TyKind::AnonymousUnion(fields, ..) => {
+            fields.flat_map_in_place(|field| vis.flat_map_field_def(field));
+        }
     }
     vis.visit_span(span);
     visit_lazy_tts(tokens, vis);

compiler/rustc_ast/src/visit.rs

@@ -404,6 +404,9 @@ pub fn walk_ty<'a, V: Visitor<'a>>(visitor: &mut V, typ: &'a Ty) {
         TyKind::Typeof(ref expression) => visitor.visit_anon_const(expression),
         TyKind::Infer | TyKind::ImplicitSelf | TyKind::Err => {}
         TyKind::MacCall(ref mac) => visitor.visit_mac_call(mac),
+        TyKind::AnonymousStruct(ref fields, ..) | TyKind::AnonymousUnion(ref fields, ..) => {
+            walk_list!(visitor, visit_field_def, fields)
+        }
         TyKind::Never | TyKind::CVarArgs => {}
     }
 }

compiler/rustc_ast_lowering/src/item.rs

@@ -789,7 +789,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }
     }
 
-    fn lower_field_def(&mut self, (index, f): (usize, &FieldDef)) -> hir::FieldDef<'hir> {
+    pub(super) fn lower_field_def(
+        &mut self,
+        (index, f): (usize, &FieldDef),
+    ) -> hir::FieldDef<'hir> {
         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
             let t = self.lower_path_ty(
                 &f.ty,

compiler/rustc_ast_lowering/src/lib.rs

@@ -1267,6 +1267,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let kind = match t.kind {
             TyKind::Infer => hir::TyKind::Infer,
             TyKind::Err => hir::TyKind::Err,
+            // FIXME(unnamed_fields): IMPLEMENTATION IN PROGRESS
+            TyKind::AnonymousStruct(ref _fields, _recovered) => {
+                self.sess.struct_span_err(t.span, "anonymous structs are unimplemented").emit();
+                hir::TyKind::Err
+            }
+            TyKind::AnonymousUnion(ref _fields, _recovered) => {
+                self.sess.struct_span_err(t.span, "anonymous unions are unimplemented").emit();
+                hir::TyKind::Err
+            }
             TyKind::Slice(ref ty) => hir::TyKind::Slice(self.lower_ty(ty, itctx)),
             TyKind::Ptr(ref mt) => hir::TyKind::Ptr(self.lower_mt(mt, itctx)),
             TyKind::Rptr(ref region, ref mt) => {

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -175,10 +175,30 @@ impl<'a> AstValidator<'a> {
                     }
                 }
             }
+            TyKind::AnonymousStruct(ref fields, ..) | TyKind::AnonymousUnion(ref fields, ..) => {
+                self.with_banned_assoc_ty_bound(|this| {
+                    walk_list!(this, visit_struct_field_def, fields)
+                });
+            }
             _ => visit::walk_ty(self, t),
         }
     }
 
+    fn visit_struct_field_def(&mut self, field: &'a FieldDef) {
+        if let Some(ident) = field.ident {
+            if ident.name == kw::Underscore {
+                self.check_anonymous_field(field);
+                self.visit_vis(&field.vis);
+                self.visit_ident(ident);
+                self.visit_ty_common(&field.ty);
+                self.walk_ty(&field.ty);
+                walk_list!(self, visit_attribute, &field.attrs);
+                return;
+            }
+        }
+        self.visit_field_def(field);
+    }
+
     fn err_handler(&self) -> &rustc_errors::Handler {
         &self.session.diagnostic()
     }
@@ -213,6 +233,66 @@ impl<'a> AstValidator<'a> {
         err.emit();
     }
 
+    fn check_anonymous_field(&self, field: &FieldDef) {
+        let FieldDef { ty, .. } = field;
+        match &ty.kind {
+            TyKind::AnonymousStruct(..) | TyKind::AnonymousUnion(..) => {
+                // We already checked for `kw::Underscore` before calling this function,
+                // so skip the check
+            }
+            TyKind::Path(..) => {
+                // 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
+            }
+            _ => {
+                let msg = "unnamed fields can only have struct or union types";
+                let label = "not a struct or union";
+                self.err_handler()
+                    .struct_span_err(field.span, msg)
+                    .span_label(ty.span, label)
+                    .emit();
+            }
+        }
+    }
+
+    fn deny_anonymous_struct(&self, ty: &Ty) {
+        match &ty.kind {
+            TyKind::AnonymousStruct(..) => {
+                self.err_handler()
+                    .struct_span_err(
+                        ty.span,
+                        "anonymous structs are not allowed outside of unnamed struct or union fields",
+                    )
+                    .span_label(ty.span, "anonymous struct declared here")
+                    .emit();
+            }
+            TyKind::AnonymousUnion(..) => {
+                self.err_handler()
+                    .struct_span_err(
+                        ty.span,
+                        "anonymous unions are not allowed outside of unnamed struct or union fields",
+                    )
+                    .span_label(ty.span, "anonymous union declared here")
+                    .emit();
+            }
+            _ => {}
+        }
+    }
+
+    fn deny_anonymous_field(&self, field: &FieldDef) {
+        if let Some(ident) = field.ident {
+            if ident.name == kw::Underscore {
+                self.err_handler()
+                    .struct_span_err(
+                        field.span,
+                        "anonymous fields are not allowed outside of structs or unions",
+                    )
+                    .span_label(ident.span, "anonymous field declared here")
+                    .emit()
+            }
+        }
+    }
+
     fn check_decl_no_pat(decl: &FnDecl, mut report_err: impl FnMut(Span, Option<Ident>, bool)) {
         for Param { pat, .. } in &decl.inputs {
             match pat.kind {
@@ -732,6 +812,71 @@ impl<'a> AstValidator<'a> {
             )
             .emit();
     }
+
+    fn visit_ty_common(&mut self, ty: &'a Ty) {
+        match ty.kind {
+            TyKind::BareFn(ref bfty) => {
+                self.check_fn_decl(&bfty.decl, SelfSemantic::No);
+                Self::check_decl_no_pat(&bfty.decl, |span, _, _| {
+                    struct_span_err!(
+                        self.session,
+                        span,
+                        E0561,
+                        "patterns aren't allowed in function pointer types"
+                    )
+                    .emit();
+                });
+                self.check_late_bound_lifetime_defs(&bfty.generic_params);
+            }
+            TyKind::TraitObject(ref bounds, ..) => {
+                let mut any_lifetime_bounds = false;
+                for bound in bounds {
+                    if let GenericBound::Outlives(ref lifetime) = *bound {
+                        if any_lifetime_bounds {
+                            struct_span_err!(
+                                self.session,
+                                lifetime.ident.span,
+                                E0226,
+                                "only a single explicit lifetime bound is permitted"
+                            )
+                            .emit();
+                            break;
+                        }
+                        any_lifetime_bounds = true;
+                    }
+                }
+                self.no_questions_in_bounds(bounds, "trait object types", false);
+            }
+            TyKind::ImplTrait(_, ref bounds) => {
+                if self.is_impl_trait_banned {
+                    struct_span_err!(
+                        self.session,
+                        ty.span,
+                        E0667,
+                        "`impl Trait` is not allowed in path parameters"
+                    )
+                    .emit();
+                }
+
+                if let Some(outer_impl_trait_sp) = self.outer_impl_trait {
+                    struct_span_err!(
+                        self.session,
+                        ty.span,
+                        E0666,
+                        "nested `impl Trait` is not allowed"
+                    )
+                    .span_label(outer_impl_trait_sp, "outer `impl Trait`")
+                    .span_label(ty.span, "nested `impl Trait` here")
+                    .emit();
+                }
+
+                if !bounds.iter().any(|b| matches!(b, GenericBound::Trait(..))) {
+                    self.err_handler().span_err(ty.span, "at least one trait must be specified");
+                }
+            }
+            _ => {}
+        }
+    }
 }
 
 /// Checks that generic parameters are in the correct order,
@@ -850,72 +995,8 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
     }
 
     fn visit_ty(&mut self, ty: &'a Ty) {
-        match ty.kind {
-            TyKind::BareFn(ref bfty) => {
-                self.check_fn_decl(&bfty.decl, SelfSemantic::No);
-                Self::check_decl_no_pat(&bfty.decl, |span, _, _| {
-                    struct_span_err!(
-                        self.session,
-                        span,
-                        E0561,
-                        "patterns aren't allowed in function pointer types"
-                    )
-                    .emit();
-                });
-                self.check_late_bound_lifetime_defs(&bfty.generic_params);
-            }
-            TyKind::TraitObject(ref bounds, ..) => {
-                let mut any_lifetime_bounds = false;
-                for bound in bounds {
-                    if let GenericBound::Outlives(ref lifetime) = *bound {
-                        if any_lifetime_bounds {
-                            struct_span_err!(
-                                self.session,
-                                lifetime.ident.span,
-                                E0226,
-                                "only a single explicit lifetime bound is permitted"
-                            )
-                            .emit();
-                            break;
-                        }
-                        any_lifetime_bounds = true;
-                    }
-                }
-                self.no_questions_in_bounds(bounds, "trait object types", false);
-            }
-            TyKind::ImplTrait(_, ref bounds) => {
-                if self.is_impl_trait_banned {
-                    struct_span_err!(
-                        self.session,
-                        ty.span,
-                        E0667,
-                        "`impl Trait` is not allowed in path parameters"
-                    )
-                    .emit();
-                }
-
-                if let Some(outer_impl_trait_sp) = self.outer_impl_trait {
-                    struct_span_err!(
-                        self.session,
-                        ty.span,
-                        E0666,
-                        "nested `impl Trait` is not allowed"
-                    )
-                    .span_label(outer_impl_trait_sp, "outer `impl Trait`")
-                    .span_label(ty.span, "nested `impl Trait` here")
-                    .emit();
-                }
-
-                if !bounds.iter().any(|b| matches!(b, GenericBound::Trait(..))) {
-                    self.err_handler().span_err(ty.span, "at least one trait must be specified");
-                }
-
-                self.walk_ty(ty);
-                return;
-            }
-            _ => {}
-        }
-
+        self.visit_ty_common(ty);
+        self.deny_anonymous_struct(ty);
         self.walk_ty(ty)
     }
 
@@ -929,6 +1010,11 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
         visit::walk_lifetime(self, lifetime);
     }
 
+    fn visit_field_def(&mut self, s: &'a FieldDef) {
+        self.deny_anonymous_field(s);
+        visit::walk_field_def(self, s)
+    }
+
     fn visit_item(&mut self, item: &'a Item) {
         if item.attrs.iter().any(|attr| self.session.is_proc_macro_attr(attr)) {
             self.has_proc_macro_decls = true;
@@ -1084,14 +1170,42 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     self.check_mod_file_item_asciionly(item.ident);
                 }
             }
-            ItemKind::Union(ref vdata, _) => {
-                if let VariantData::Tuple(..) | VariantData::Unit(..) = vdata {
-                    self.err_handler()
-                        .span_err(item.span, "tuple and unit unions are not permitted");
+            ItemKind::Struct(ref vdata, ref generics) => match vdata {
+                // Duplicating the `Visitor` logic allows catching all cases
+                // of `Anonymous(Struct, Union)` outside of a field struct or union.
+                //
+                // Inside `visit_ty` the validator catches every `Anonymous(Struct, Union)` it
+                // encounters, and only on `ItemKind::Struct` and `ItemKind::Union`
+                // it uses `visit_ty_common`, which doesn't contain that specific check.
+                VariantData::Struct(ref fields, ..) => {
+                    self.visit_vis(&item.vis);
+                    self.visit_ident(item.ident);
+                    self.visit_generics(generics);
+                    self.with_banned_assoc_ty_bound(|this| {
+                        walk_list!(this, visit_struct_field_def, fields);
+                    });
+                    walk_list!(self, visit_attribute, &item.attrs);
+                    return;
                 }
+                _ => {}
+            },
+            ItemKind::Union(ref vdata, ref generics) => {
                 if vdata.fields().is_empty() {
                     self.err_handler().span_err(item.span, "unions cannot have zero fields");
                 }
+                match vdata {
+                    VariantData::Struct(ref fields, ..) => {
+                        self.visit_vis(&item.vis);
+                        self.visit_ident(item.ident);
+                        self.visit_generics(generics);
+                        self.with_banned_assoc_ty_bound(|this| {
+                            walk_list!(this, visit_struct_field_def, fields);
+                        });
+                        walk_list!(self, visit_attribute, &item.attrs);
+                        return;
+                    }
+                    _ => {}
+                }
             }
             ItemKind::Const(def, .., None) => {
                 self.check_defaultness(item.span, def);

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -725,6 +725,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
         // involved, so we only emit errors where there are no other parsing errors.
         gate_all!(destructuring_assignment, "destructuring assignments are unstable");
     }
+    gate_all!(unnamed_fields, "unnamed fields are not yet fully implemented");
 
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).