Auto merge of #69366 - Centril:unified-items, r=petrochenkov

parse: unify item parsing & filter illegal item kinds

This PR fully unifies item parsing into a single `fn parse_item_common` method which produces `Option<Item>`. The `Item` is then mapped into `ForeignItem` and `AssocItem` as necessary by transforming the `*Kind` and converting contextually bad variants into `None`, thereby filtering them away.

The PR does not yet unmerge the definition of `ForeignItemKind` from `AssocItemKind`. I've left that as future work as it didn't feel like this parser-focused PR would be the best one to deal with it. Changes to the AST data structures are instead kept to a reasonable minimum.

Based on https://github.com/rust-lang/rust/pull/69361.

Fixes https://github.com/rust-lang/rust/issues/48137.
RELNOTES: Now, `item` macro fragments can be interpolated into `impl`, `trait`, and `extern` contexts. See `src/test/ui/parser/issue-48137-macros-cannot-interpolate-impl-items.rs` for the relevant test.

r? @petrochenkov
cc @estebank

4 files changed, 56 insertions, 49 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 849950e939a..19c705fa997 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -2106,7 +2106,7 @@ pub enum Const {
 /// For details see the [RFC #2532](https://github.com/rust-lang/rfcs/pull/2532).
 #[derive(Copy, Clone, PartialEq, RustcEncodable, RustcDecodable, Debug, HashStable_Generic)]
 pub enum Defaultness {
-    Default,
+    Default(Span),
     Final,
 }
 
@@ -2411,15 +2411,15 @@ impl VariantData {
     }
 }
 
-/// An item.
-///
-/// The name might be a dummy name in case of anonymous items.
+/// An item definition.
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
 pub struct Item<K = ItemKind> {
     pub attrs: Vec<Attribute>,
     pub id: NodeId,
     pub span: Span,
     pub vis: Visibility,
+    /// The name of the item.
+    /// It might be a dummy name in case of anonymous items.
     pub ident: Ident,
 
     pub kind: K,
@@ -2506,11 +2506,11 @@ pub enum ItemKind {
     /// A constant item (`const`).
     ///
     /// E.g., `const FOO: i32 = 42;`.
-    Const(P<Ty>, Option<P<Expr>>),
+    Const(Defaultness, P<Ty>, Option<P<Expr>>),
     /// A function declaration (`fn`).
     ///
     /// E.g., `fn foo(bar: usize) -> usize { .. }`.
-    Fn(FnSig, Generics, Option<P<Block>>),
+    Fn(Defaultness, FnSig, Generics, Option<P<Block>>),
     /// A module declaration (`mod`).
     ///
     /// E.g., `mod foo;` or `mod foo { .. }`.
@@ -2524,7 +2524,7 @@ pub enum ItemKind {
     /// A type alias (`type`).
     ///
     /// E.g., `type Foo = Bar<u8>;`.
-    TyAlias(Generics, GenericBounds, Option<P<Ty>>),
+    TyAlias(Defaultness, Generics, GenericBounds, Option<P<Ty>>),
     /// An enum definition (`enum`).
     ///
     /// E.g., `enum Foo<A, B> { C<A>, D<B> }`.
@@ -2571,30 +2571,41 @@ pub enum ItemKind {
 }
 
 impl ItemKind {
-    pub fn descriptive_variant(&self) -> &str {
-        match *self {
+    pub fn article(&self) -> &str {
+        use ItemKind::*;
+        match self {
+            Use(..) | Static(..) | Const(..) | Fn(..) | Mod(..) | GlobalAsm(..) | TyAlias(..)
+            | Struct(..) | Union(..) | Trait(..) | TraitAlias(..) | MacroDef(..) => "a",
+            ExternCrate(..) | ForeignMod(..) | Mac(..) | Enum(..) | Impl { .. } => "an",
+        }
+    }
+
+    pub fn descr(&self) -> &str {
+        match self {
             ItemKind::ExternCrate(..) => "extern crate",
-            ItemKind::Use(..) => "use",
+            ItemKind::Use(..) => "`use` import",
             ItemKind::Static(..) => "static item",
             ItemKind::Const(..) => "constant item",
             ItemKind::Fn(..) => "function",
             ItemKind::Mod(..) => "module",
-            ItemKind::ForeignMod(..) => "foreign module",
-            ItemKind::GlobalAsm(..) => "global asm",
+            ItemKind::ForeignMod(..) => "extern block",
+            ItemKind::GlobalAsm(..) => "global asm item",
             ItemKind::TyAlias(..) => "type alias",
             ItemKind::Enum(..) => "enum",
             ItemKind::Struct(..) => "struct",
             ItemKind::Union(..) => "union",
             ItemKind::Trait(..) => "trait",
             ItemKind::TraitAlias(..) => "trait alias",
-            ItemKind::Mac(..) | ItemKind::MacroDef(..) | ItemKind::Impl { .. } => "item",
+            ItemKind::Mac(..) => "item macro invocation",
+            ItemKind::MacroDef(..) => "macro definition",
+            ItemKind::Impl { .. } => "implementation",
         }
     }
 
     pub fn generics(&self) -> Option<&Generics> {
         match self {
-            Self::Fn(_, generics, _)
-            | Self::TyAlias(generics, ..)
+            Self::Fn(_, _, generics, _)
+            | Self::TyAlias(_, generics, ..)
             | Self::Enum(_, generics)
             | Self::Struct(_, generics)
             | Self::Union(_, generics)
@@ -2613,19 +2624,7 @@ pub type ForeignItemKind = AssocItemKind;
 
 /// Represents associated items.
 /// These include items in `impl` and `trait` definitions.
-#[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
-pub struct AssocItem {
-    pub attrs: Vec<Attribute>,
-    pub id: NodeId,
-    pub span: Span,
-    pub vis: Visibility,
-    pub ident: Ident,
-
-    pub defaultness: Defaultness,
-    pub kind: AssocItemKind,
-    /// See `Item::tokens` for what this is.
-    pub tokens: Option<TokenStream>,
-}
+pub type AssocItem = Item<AssocItemKind>;
 
 /// Represents non-free item kinds.
 ///
@@ -2638,13 +2637,22 @@ pub struct AssocItem {
 pub enum AssocItemKind {
     /// A constant, `const $ident: $ty $def?;` where `def ::= "=" $expr? ;`.
     /// If `def` is parsed, then the constant is provided, and otherwise required.
-    Const(P<Ty>, Option<P<Expr>>),
+    Const(Defaultness, P<Ty>, Option<P<Expr>>),
     /// A static item (`static FOO: u8`).
     Static(P<Ty>, Mutability, Option<P<Expr>>),
     /// A function.
-    Fn(FnSig, Generics, Option<P<Block>>),
+    Fn(Defaultness, FnSig, Generics, Option<P<Block>>),
     /// A type.
-    TyAlias(Generics, GenericBounds, Option<P<Ty>>),
+    TyAlias(Defaultness, Generics, GenericBounds, Option<P<Ty>>),
     /// A macro expanding to items.
     Macro(Mac),
 }
+
+impl AssocItemKind {
+    pub fn defaultness(&self) -> Defaultness {
+        match *self {
+            Self::Const(def, ..) | Self::Fn(def, ..) | Self::TyAlias(def, ..) => def,
+            Self::Macro(..) | Self::Static(..) => Defaultness::Final,
+        }
+    }
+}
diff --git a/src/libsyntax/attr/mod.rs b/src/libsyntax/attr/mod.rs
index 313f5269235..cd485e71378 100644
--- a/src/libsyntax/attr/mod.rs
+++ b/src/libsyntax/attr/mod.rs
@@ -722,6 +722,6 @@ macro_rules! derive_has_attrs {
 }
 
 derive_has_attrs! {
-    Item, Expr, Local, ast::ForeignItem, ast::StructField, ast::AssocItem, ast::Arm,
+    Item, Expr, Local, ast::ForeignItem, ast::StructField, ast::Arm,
     ast::Field, ast::FieldPat, ast::Variant, ast::Param, GenericParam
 }
diff --git a/src/libsyntax/mut_visit.rs b/src/libsyntax/mut_visit.rs
index 02f790dfbb4..05bb07cd4b9 100644
--- a/src/libsyntax/mut_visit.rs
+++ b/src/libsyntax/mut_visit.rs
@@ -890,11 +890,11 @@ pub fn noop_visit_item_kind<T: MutVisitor>(kind: &mut ItemKind, vis: &mut T) {
     match kind {
         ItemKind::ExternCrate(_orig_name) => {}
         ItemKind::Use(use_tree) => vis.visit_use_tree(use_tree),
-        ItemKind::Static(ty, _, expr) | ItemKind::Const(ty, expr) => {
+        ItemKind::Static(ty, _, expr) | ItemKind::Const(_, ty, expr) => {
             vis.visit_ty(ty);
             visit_opt(expr, |expr| vis.visit_expr(expr));
         }
-        ItemKind::Fn(sig, generics, body) => {
+        ItemKind::Fn(_, sig, generics, body) => {
             visit_fn_sig(sig, vis);
             vis.visit_generics(generics);
             visit_opt(body, |body| vis.visit_block(body));
@@ -902,7 +902,7 @@ pub fn noop_visit_item_kind<T: MutVisitor>(kind: &mut ItemKind, vis: &mut T) {
         ItemKind::Mod(m) => vis.visit_mod(m),
         ItemKind::ForeignMod(nm) => vis.visit_foreign_mod(nm),
         ItemKind::GlobalAsm(_ga) => {}
-        ItemKind::TyAlias(generics, bounds, ty) => {
+        ItemKind::TyAlias(_, generics, bounds, ty) => {
             vis.visit_generics(generics);
             visit_bounds(bounds, vis);
             visit_opt(ty, |ty| vis.visit_ty(ty));
@@ -948,8 +948,7 @@ pub fn noop_flat_map_assoc_item<T: MutVisitor>(
     mut item: P<AssocItem>,
     visitor: &mut T,
 ) -> SmallVec<[P<AssocItem>; 1]> {
-    let AssocItem { id, ident, vis, defaultness: _, attrs, kind, span, tokens: _ } =
-        item.deref_mut();
+    let Item { id, ident, vis, attrs, kind, span, tokens: _ } = item.deref_mut();
     walk_nested_item(visitor, id, span, ident, vis, attrs, kind);
     smallvec![item]
 }
@@ -968,16 +967,16 @@ pub fn walk_nested_item(
     visitor.visit_vis(vis);
     visit_attrs(attrs, visitor);
     match kind {
-        AssocItemKind::Const(ty, expr) | AssocItemKind::Static(ty, _, expr) => {
+        AssocItemKind::Const(_, ty, expr) | AssocItemKind::Static(ty, _, expr) => {
             visitor.visit_ty(ty);
             visit_opt(expr, |expr| visitor.visit_expr(expr));
         }
-        AssocItemKind::Fn(sig, generics, body) => {
+        AssocItemKind::Fn(_, sig, generics, body) => {
             visitor.visit_generics(generics);
             visit_fn_sig(sig, visitor);
             visit_opt(body, |body| visitor.visit_block(body));
         }
-        AssocItemKind::TyAlias(generics, bounds, ty) => {
+        AssocItemKind::TyAlias(_, generics, bounds, ty) => {
             visitor.visit_generics(generics);
             visit_bounds(bounds, visitor);
             visit_opt(ty, |ty| visitor.visit_ty(ty));
@@ -1049,7 +1048,7 @@ pub fn noop_flat_map_foreign_item<T: MutVisitor>(
     mut item: P<ForeignItem>,
     visitor: &mut T,
 ) -> SmallVec<[P<ForeignItem>; 1]> {
-    let ForeignItem { ident, attrs, id, kind, vis, span, tokens: _ } = item.deref_mut();
+    let Item { ident, attrs, id, kind, vis, span, tokens: _ } = item.deref_mut();
     walk_nested_item(visitor, id, span, ident, vis, attrs, kind);
     smallvec![item]
 }
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index bd35918dba7..96149ad7947 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -298,11 +298,11 @@ pub fn walk_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a Item) {
             }
         }
         ItemKind::Use(ref use_tree) => visitor.visit_use_tree(use_tree, item.id, false),
-        ItemKind::Static(ref typ, _, ref expr) | ItemKind::Const(ref typ, ref expr) => {
+        ItemKind::Static(ref typ, _, ref expr) | ItemKind::Const(_, ref typ, ref expr) => {
             visitor.visit_ty(typ);
             walk_list!(visitor, visit_expr, expr);
         }
-        ItemKind::Fn(ref sig, ref generics, ref body) => {
+        ItemKind::Fn(_, ref sig, ref generics, ref body) => {
             visitor.visit_generics(generics);
             let kind = FnKind::Fn(FnCtxt::Free, item.ident, sig, &item.vis, body.as_deref());
             visitor.visit_fn(kind, item.span, item.id)
@@ -312,7 +312,7 @@ pub fn walk_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a Item) {
             walk_list!(visitor, visit_foreign_item, &foreign_module.items);
         }
         ItemKind::GlobalAsm(ref ga) => visitor.visit_global_asm(ga),
-        ItemKind::TyAlias(ref generics, ref bounds, ref ty) => {
+        ItemKind::TyAlias(_, ref generics, ref bounds, ref ty) => {
             visitor.visit_generics(generics);
             walk_list!(visitor, visit_param_bound, bounds);
             walk_list!(visitor, visit_ty, ty);
@@ -526,7 +526,7 @@ pub fn walk_pat<'a, V: Visitor<'a>>(visitor: &mut V, pattern: &'a Pat) {
 }
 
 pub fn walk_foreign_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a ForeignItem) {
-    let ForeignItem { id, span, ident, vis, attrs, kind, tokens: _ } = item;
+    let Item { id, span, ident, vis, attrs, kind, tokens: _ } = item;
     walk_nested_item(visitor, *id, *span, *ident, vis, attrs, kind, FnCtxt::Foreign);
 }
 
@@ -610,7 +610,7 @@ pub fn walk_fn<'a, V: Visitor<'a>>(visitor: &mut V, kind: FnKind<'a>, _span: Spa
 }
 
 pub fn walk_assoc_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a AssocItem, ctxt: AssocCtxt) {
-    let AssocItem { id, span, ident, vis, attrs, kind, tokens: _, defaultness: _ } = item;
+    let Item { id, span, ident, vis, attrs, kind, tokens: _ } = item;
     walk_nested_item(visitor, *id, *span, *ident, vis, attrs, kind, FnCtxt::Assoc(ctxt));
 }
 
@@ -628,16 +628,16 @@ fn walk_nested_item<'a, V: Visitor<'a>>(
     visitor.visit_ident(ident);
     walk_list!(visitor, visit_attribute, attrs);
     match kind {
-        AssocItemKind::Const(ty, expr) | AssocItemKind::Static(ty, _, expr) => {
+        AssocItemKind::Const(_, ty, expr) | AssocItemKind::Static(ty, _, expr) => {
             visitor.visit_ty(ty);
             walk_list!(visitor, visit_expr, expr);
         }
-        AssocItemKind::Fn(sig, generics, body) => {
+        AssocItemKind::Fn(_, sig, generics, body) => {
             visitor.visit_generics(generics);
             let kind = FnKind::Fn(ctxt, ident, sig, vis, body.as_deref());
             visitor.visit_fn(kind, span, id);
         }
-        AssocItemKind::TyAlias(generics, bounds, ty) => {
+        AssocItemKind::TyAlias(_, generics, bounds, ty) => {
             visitor.visit_generics(generics);
             walk_list!(visitor, visit_param_bound, bounds);
             walk_list!(visitor, visit_ty, ty);