Auto merge of #28816 - petrochenkov:unistruct, r=nrc

This patch uses the same data structures for structs and enum variants in AST and HIR. These changes in data structures lead to noticeable simplification in most of code dealing with them.
I didn't touch the top level, i.e. `ItemStruct` is still `ItemStruct` and not `ItemEnum` with one variant, like in the type checker.
As part of this patch, structures and variants get the `kind` field making distinction between "normal" structs, tuple structs and unit structs explicit instead of relying on the number of fields and presence of constructor `NodeId`. In particular, we can now distinguish empty tuple structs from unit structs, which was impossible before! Comprehensive tests for empty structs are added and some improvements to empty struct feature gates are made. Some tests don't pass due to issue #28692 , they are still there for completeness, but are commented out.
This patch fixes issue mentioned in #16819 (comment), now emptiness of tuple structs is checked after expansion.
It also touches #28750 by providing span for visit_struct_def
cc #28336

r? @nrc

Author: bors

src/doc/reference.md

@@ -2357,7 +2357,7 @@ The currently implemented features of the reference compiler are:
                               terms of encapsulation).
 * - `default_type_parameter_fallback` - Allows type parameter defaults to
                                         influence type inference.
-* - `braced_empty_structs` - Allows use of empty structs with braces.
+* - `braced_empty_structs` - Allows use of empty structs and enum variants with braces.
 
 If a feature is promoted to a language feature, then all existing programs will
 start to receive compilation warnings about `#![feature]` directives which enabled

src/librustc/front/map/collector.rs

@@ -134,40 +134,29 @@ impl<'ast> Visitor<'ast> for NodeCollector<'ast> {
             ItemEnum(ref enum_definition, _) => {
                 for v in &enum_definition.variants {
                     let variant_def_index =
-                        self.insert_def(v.node.id,
+                        self.insert_def(v.node.data.id(),
                                         NodeVariant(&**v),
                                         DefPathData::EnumVariant(v.node.name));
 
-                    match v.node.kind {
-                        TupleVariantKind(ref args) => {
-                            for arg in args {
-                                self.create_def_with_parent(Some(variant_def_index),
-                                                            arg.id,
-                                                            DefPathData::PositionalField);
-                            }
-                        }
-                        StructVariantKind(ref def) => {
-                            for field in &def.fields {
-                                self.create_def_with_parent(
-                                    Some(variant_def_index),
-                                    field.node.id,
-                                    DefPathData::Field(field.node.kind));
-                            }
-                        }
+                    for field in v.node.data.fields() {
+                        self.create_def_with_parent(
+                            Some(variant_def_index),
+                            field.node.id,
+                            DefPathData::Field(field.node.kind));
                     }
                 }
             }
             ItemForeignMod(..) => {
             }
             ItemStruct(ref struct_def, _) => {
                 // If this is a tuple-like struct, register the constructor.
-                if let Some(ctor_id) = struct_def.ctor_id {
-                    self.insert_def(ctor_id,
+                if !struct_def.is_struct() {
+                    self.insert_def(struct_def.id(),
                                     NodeStructCtor(&**struct_def),
                                     DefPathData::StructCtor);
                 }
 
-                for field in &struct_def.fields {
+                for field in struct_def.fields() {
                     self.create_def(field.node.id, DefPathData::Field(field.node.kind));
                 }
             }