Auto merge of #139558 - camelid:mgca-const-items, r=oli-obk,BoxyUwU

mgca: Add ConstArg representation for const items

tracking issue: #132980
fixes #131046
fixes #134641

As part of implementing `min_generic_const_args`, we need to distinguish const items that can be used in the type system, such as in associated const equality projections, from const items containing arbitrary const code, which must be kept out of the type system. Specifically, all "type consts" must be either concrete (no generics) or generic with a trivial expression like `N` or a path to another type const item.

To syntactically distinguish these cases, we require, for now at least, that users annotate all type consts with the `#[type_const]` attribute. Then, we validate that the const's right-hand side is indeed eligible to be a type const and represent it differently in the HIR.

We accomplish this representation using a new `ConstItemRhs` enum in the HIR, and a similar but simpler enum in the AST. When `#[type_const]` is **not** applied to a const (e.g. on stable), we represent const item right-hand sides (rhs's) as HIR bodies, like before. However, when the attribute is applied, we instead lower to a `hir::ConstArg`. This syntactically distinguishes between trivial const args (paths) and arbitrary expressions, which are represented using `AnonConst`s. Then in `generics_of`, we can take advantage of the existing machinery to bar the `AnonConst` rhs's from using parent generics.

Author: bors

compiler/rustc_ast/src/ast.rs

@@ -3759,10 +3759,29 @@ pub struct ConstItem {
     pub ident: Ident,
     pub generics: Generics,
     pub ty: Box<Ty>,
-    pub expr: Option<Box<Expr>>,
+    pub rhs: Option<ConstItemRhs>,
     pub define_opaque: Option<ThinVec<(NodeId, Path)>>,
 }
 
+#[derive(Clone, Encodable, Decodable, Debug, Walkable)]
+pub enum ConstItemRhs {
+    TypeConst(AnonConst),
+    Body(Box<Expr>),
+}
+
+impl ConstItemRhs {
+    pub fn span(&self) -> Span {
+        self.expr().span
+    }
+
+    pub fn expr(&self) -> &Expr {
+        match self {
+            ConstItemRhs::TypeConst(anon_const) => &anon_const.value,
+            ConstItemRhs::Body(expr) => expr,
+        }
+    }
+}
+
 // Adding a new variant? Please update `test_item` in `tests/ui/macros/stringify.rs`.
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum ItemKind {

compiler/rustc_ast/src/visit.rs

@@ -423,6 +423,7 @@ macro_rules! common_visitor_and_walkers {
             Closure,
             Const,
             ConstItem,
+            ConstItemRhs,
             Defaultness,
             Delegation,
             DelegationMac,

compiler/rustc_ast_lowering/src/item.rs

@@ -170,37 +170,36 @@ impl<'hir> LoweringContext<'_, 'hir> {
             }
             ItemKind::Static(box ast::StaticItem {
                 ident,
-                ty: t,
+                ty,
                 safety: _,
                 mutability: m,
                 expr: e,
                 define_opaque,
             }) => {
                 let ident = self.lower_ident(*ident);
-                let (ty, body_id) =
-                    self.lower_const_item(t, span, e.as_deref(), ImplTraitPosition::StaticTy);
+                let ty =
+                    self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::StaticTy));
+                let body_id = self.lower_const_body(span, e.as_deref());
                 self.lower_define_opaque(hir_id, define_opaque);
                 hir::ItemKind::Static(*m, ident, ty, body_id)
             }
             ItemKind::Const(box ast::ConstItem {
-                ident,
-                generics,
-                ty,
-                expr,
-                define_opaque,
-                ..
+                ident, generics, ty, rhs, define_opaque, ..
             }) => {
                 let ident = self.lower_ident(*ident);
-                let (generics, (ty, body_id)) = self.lower_generics(
+                let (generics, (ty, rhs)) = self.lower_generics(
                     generics,
                     id,
                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
-                        this.lower_const_item(ty, span, expr.as_deref(), ImplTraitPosition::ConstTy)
+                        let ty = this
+                            .lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::ConstTy));
+                        let rhs = this.lower_const_item_rhs(attrs, rhs.as_ref(), span);
+                        (ty, rhs)
                     },
                 );
                 self.lower_define_opaque(hir_id, &define_opaque);
-                hir::ItemKind::Const(ident, generics, ty, body_id)
+                hir::ItemKind::Const(ident, generics, ty, rhs)
             }
             ItemKind::Fn(box Fn {
                 sig: FnSig { decl, header, span: fn_sig_span },
@@ -462,17 +461,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }
     }
 
-    fn lower_const_item(
-        &mut self,
-        ty: &Ty,
-        span: Span,
-        body: Option<&Expr>,
-        impl_trait_position: ImplTraitPosition,
-    ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
-        let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(impl_trait_position));
-        (ty, self.lower_const_body(span, body))
-    }
-
     #[instrument(level = "debug", skip(self))]
     fn lower_use_tree(
         &mut self,
@@ -804,12 +792,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         let (ident, generics, kind, has_default) = match &i.kind {
             AssocItemKind::Const(box ConstItem {
-                ident,
-                generics,
-                ty,
-                expr,
-                define_opaque,
-                ..
+                ident, generics, ty, rhs, define_opaque, ..
             }) => {
                 let (generics, kind) = self.lower_generics(
                     generics,
@@ -818,14 +801,15 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     |this| {
                         let ty = this
                             .lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::ConstTy));
-                        let body = expr.as_ref().map(|x| this.lower_const_body(i.span, Some(x)));
-
-                        hir::TraitItemKind::Const(ty, body)
+                        let rhs = rhs
+                            .as_ref()
+                            .map(|rhs| this.lower_const_item_rhs(attrs, Some(rhs), i.span));
+                        hir::TraitItemKind::Const(ty, rhs)
                     },
                 );
 
                 if define_opaque.is_some() {
-                    if expr.is_some() {
+                    if rhs.is_some() {
                         self.lower_define_opaque(hir_id, &define_opaque);
                     } else {
                         self.dcx().span_err(
@@ -835,7 +819,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     }
                 }
 
-                (*ident, generics, kind, expr.is_some())
+                (*ident, generics, kind, rhs.is_some())
             }
             AssocItemKind::Fn(box Fn {
                 sig, ident, generics, body: None, define_opaque, ..
@@ -1022,12 +1006,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         let (ident, (generics, kind)) = match &i.kind {
             AssocItemKind::Const(box ConstItem {
-                ident,
-                generics,
-                ty,
-                expr,
-                define_opaque,
-                ..
+                ident, generics, ty, rhs, define_opaque, ..
             }) => (
                 *ident,
                 self.lower_generics(
@@ -1037,9 +1016,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     |this| {
                         let ty = this
                             .lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::ConstTy));
-                        let body = this.lower_const_body(i.span, expr.as_deref());
                         this.lower_define_opaque(hir_id, &define_opaque);
-                        hir::ImplItemKind::Const(ty, body)
+                        let rhs = this.lower_const_item_rhs(attrs, rhs.as_ref(), i.span);
+                        hir::ImplItemKind::Const(ty, rhs)
                     },
                 ),
             ),

compiler/rustc_ast_lowering/src/lib.rs

@@ -2327,6 +2327,33 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         self.arena.alloc(hir::ConstArg { hir_id: self.next_id(), kind: ct_kind })
     }
 
+    fn lower_const_item_rhs(
+        &mut self,
+        attrs: &[hir::Attribute],
+        rhs: Option<&ConstItemRhs>,
+        span: Span,
+    ) -> hir::ConstItemRhs<'hir> {
+        match rhs {
+            Some(ConstItemRhs::TypeConst(anon)) => {
+                hir::ConstItemRhs::TypeConst(self.lower_anon_const_to_const_arg(anon))
+            }
+            None if attr::contains_name(attrs, sym::type_const) => {
+                let const_arg = ConstArg {
+                    hir_id: self.next_id(),
+                    kind: hir::ConstArgKind::Error(
+                        DUMMY_SP,
+                        self.dcx().span_delayed_bug(DUMMY_SP, "no block"),
+                    ),
+                };
+                hir::ConstItemRhs::TypeConst(self.arena.alloc(const_arg))
+            }
+            Some(ConstItemRhs::Body(body)) => {
+                hir::ConstItemRhs::Body(self.lower_const_body(span, Some(body)))
+            }
+            None => hir::ConstItemRhs::Body(self.lower_const_body(span, None)),
+        }
+    }
+
     /// See [`hir::ConstArg`] for when to use this function vs
     /// [`Self::lower_anon_const_to_anon_const`].
     fn lower_anon_const_to_const_arg(&mut self, anon: &AnonConst) -> &'hir hir::ConstArg<'hir> {

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1239,9 +1239,9 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     }
                 });
             }
-            ItemKind::Const(box ConstItem { defaultness, expr, .. }) => {
+            ItemKind::Const(box ConstItem { defaultness, rhs, .. }) => {
                 self.check_defaultness(item.span, *defaultness);
-                if expr.is_none() {
+                if rhs.is_none() {
                     self.dcx().emit_err(errors::ConstWithoutBody {
                         span: item.span,
                         replace_span: self.ending_semi_or_hi(item.span),
@@ -1581,7 +1581,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
 
         if let AssocCtxt::Impl { .. } = ctxt {
             match &item.kind {
-                AssocItemKind::Const(box ConstItem { expr: None, .. }) => {
+                AssocItemKind::Const(box ConstItem { rhs: None, .. }) => {
                     self.dcx().emit_err(errors::AssocConstWithoutBody {
                         span: item.span,
                         replace_span: self.ending_semi_or_hi(item.span),

compiler/rustc_ast_pretty/src/pprust/state/item.rs

@@ -210,15 +210,15 @@ impl<'a> State<'a> {
                 ident,
                 generics,
                 ty,
-                expr,
+                rhs,
                 define_opaque,
             }) => {
                 self.print_item_const(
                     *ident,
                     None,
                     generics,
                     ty,
-                    expr.as_deref(),
+                    rhs.as_ref().map(|ct| ct.expr()),
                     &item.vis,
                     ast::Safety::Default,
                     *defaultness,
@@ -566,15 +566,15 @@ impl<'a> State<'a> {
                 ident,
                 generics,
                 ty,
-                expr,
+                rhs,
                 define_opaque,
             }) => {
                 self.print_item_const(
                     *ident,
                     None,
                     generics,
                     ty,
-                    expr.as_deref(),
+                    rhs.as_ref().map(|ct| ct.expr()),
                     vis,
                     ast::Safety::Default,
                     *defaultness,

compiler/rustc_builtin_macros/src/alloc_error_handler.rs

@@ -43,7 +43,7 @@ pub(crate) fn expand(
 
     // Generate anonymous constant serving as container for the allocator methods.
     let const_ty = ecx.ty(sig_span, TyKind::Tup(ThinVec::new()));
-    let const_body = ecx.expr_block(ecx.block(span, stmts));
+    let const_body = ast::ConstItemRhs::Body(ecx.expr_block(ecx.block(span, stmts)));
     let const_item = ecx.item_const(span, Ident::new(kw::Underscore, span), const_ty, const_body);
     let const_item = if is_stmt {
         Annotatable::Stmt(Box::new(ecx.stmt_item(span, const_item)))

compiler/rustc_builtin_macros/src/global_allocator.rs

@@ -47,7 +47,7 @@ pub(crate) fn expand(
 
     // Generate anonymous constant serving as container for the allocator methods.
     let const_ty = ecx.ty(ty_span, TyKind::Tup(ThinVec::new()));
-    let const_body = ecx.expr_block(ecx.block(span, stmts));
+    let const_body = ast::ConstItemRhs::Body(ecx.expr_block(ecx.block(span, stmts)));
     let const_item = ecx.item_const(span, Ident::new(kw::Underscore, span), const_ty, const_body);
     let const_item = if is_stmt {
         Annotatable::Stmt(Box::new(ecx.stmt_item(span, const_item)))

compiler/rustc_builtin_macros/src/proc_macro_harness.rs

@@ -385,9 +385,9 @@ fn mk_decls(cx: &mut ExtCtxt<'_>, macros: &[ProcMacro]) -> Box<ast::Item> {
         cx.attr_nested_word(sym::allow, sym::deprecated, span),
     ]);
 
-    let block = cx.expr_block(
+    let block = ast::ConstItemRhs::Body(cx.expr_block(
         cx.block(span, thin_vec![cx.stmt_item(span, krate), cx.stmt_item(span, decls_static)]),
-    );
+    ));
 
     let anon_constant = cx.item_const(
         span,

compiler/rustc_builtin_macros/src/test.rs

@@ -289,7 +289,7 @@ pub(crate) fn expand_test_or_bench(
                     ty: cx.ty(sp, ast::TyKind::Path(None, test_path("TestDescAndFn"))),
                     define_opaque: None,
                     // test::TestDescAndFn {
-                    expr: Some(
+                    rhs: Some(ast::ConstItemRhs::Body(
                         cx.expr_struct(
                             sp,
                             test_path("TestDescAndFn"),
@@ -371,7 +371,7 @@ pub(crate) fn expand_test_or_bench(
                         field("testfn", test_fn), // }
                     ],
                         ), // }
-                    ),
+                    )),
                 }
                 .into(),
             ),