[WIP] mgca: Add ConstArg representation for const items

compiler/rustc_ast/src/ast.rs

@@ -3617,7 +3617,7 @@ pub struct ConstItem {
     pub ident: Ident,
     pub generics: Generics,
     pub ty: P<Ty>,
-    pub expr: Option<P<Expr>>,
+    pub body: Option<P<AnonConst>>,
     pub define_opaque: Option<ThinVec<(NodeId, Path)>>,
 }
 

compiler/rustc_ast/src/mut_visit.rs

@@ -1307,12 +1307,12 @@ impl WalkItemKind for AssocItemKind {
 }
 
 fn walk_const_item<T: MutVisitor>(vis: &mut T, item: &mut ConstItem) {
-    let ConstItem { defaultness, ident, generics, ty, expr, define_opaque } = item;
+    let ConstItem { defaultness, ident, generics, ty, body, define_opaque } = item;
     visit_defaultness(vis, defaultness);
     vis.visit_ident(ident);
     vis.visit_generics(generics);
     vis.visit_ty(ty);
-    visit_opt(expr, |expr| vis.visit_expr(expr));
+    visit_opt(body, |body| vis.visit_anon_const(body));
     walk_define_opaques(vis, define_opaque);
 }
 

compiler/rustc_ast/src/visit.rs

@@ -448,13 +448,13 @@ impl WalkItemKind for ItemKind {
                 ident,
                 generics,
                 ty,
-                expr,
+                body,
                 define_opaque,
             }) => {
                 try_visit!(visitor.visit_ident(ident));
                 try_visit!(visitor.visit_generics(generics));
                 try_visit!(visitor.visit_ty(ty));
-                visit_opt!(visitor, visit_expr, expr);
+                visit_opt!(visitor, visit_anon_const, body);
                 try_visit!(walk_define_opaques(visitor, define_opaque));
             }
             ItemKind::Fn(func) => {
@@ -1044,13 +1044,13 @@ impl WalkItemKind for AssocItemKind {
                 ident,
                 generics,
                 ty,
-                expr,
+                body,
                 define_opaque,
             }) => {
                 try_visit!(visitor.visit_ident(ident));
                 try_visit!(visitor.visit_generics(generics));
                 try_visit!(visitor.visit_ty(ty));
-                visit_opt!(visitor, visit_expr, expr);
+                visit_opt!(visitor, visit_anon_const, body);
                 try_visit!(walk_define_opaques(visitor, define_opaque));
             }
             AssocItemKind::Fn(func) => {

compiler/rustc_ast_lowering/src/item.rs

@@ -170,37 +170,41 @@ 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(ident, ty, *m, body_id)
             }
             ItemKind::Const(box ast::ConstItem {
                 ident,
                 generics,
                 ty,
-                expr,
+                body,
                 define_opaque,
                 ..
             }) => {
                 let ident = self.lower_ident(*ident);
-                let (generics, (ty, body_id)) = self.lower_generics(
+                let (generics, (ty, body)) = 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 body = this.lower_const_item(body.as_deref().unwrap());
+                        (ty, body)
                     },
                 );
                 self.lower_define_opaque(hir_id, &define_opaque);
-                hir::ItemKind::Const(ident, ty, generics, body_id)
+                hir::ItemKind::Const(ident, ty, generics, body)
             }
             ItemKind::Fn(box Fn {
                 sig: FnSig { decl, header, span: fn_sig_span },
@@ -480,15 +484,14 @@ 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))
+    fn lower_const_item(&mut self, body: &AnonConst) -> &'hir hir::ConstArg<'hir> {
+        let mgca = self.tcx.features().min_generic_const_args();
+        if mgca && let Some(ct_arg) = self.try_lower_as_const_path(body) {
+            return ct_arg;
+        }
+        let anon = self.lower_anon_const_to_anon_const(body);
+        self.arena
+            .alloc(hir::ConstArg { hir_id: self.next_id(), kind: hir::ConstArgKind::Anon(anon) })
     }
 
     #[instrument(level = "debug", skip(self))]
@@ -786,7 +789,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 ident,
                 generics,
                 ty,
-                expr,
+                body,
                 define_opaque,
                 ..
             }) => {
@@ -797,14 +800,13 @@ 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)));
-
+                        let body = body.as_deref().map(|body| this.lower_const_item(body));
                         hir::TraitItemKind::Const(ty, body)
                     },
                 );
 
                 if define_opaque.is_some() {
-                    if expr.is_some() {
+                    if body.is_some() {
                         self.lower_define_opaque(hir_id, &define_opaque);
                     } else {
                         self.dcx().span_err(
@@ -814,7 +816,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     }
                 }
 
-                (*ident, generics, kind, expr.is_some())
+                (*ident, generics, kind, body.is_some())
             }
             AssocItemKind::Fn(box Fn {
                 sig, ident, generics, body: None, define_opaque, ..
@@ -978,7 +980,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 ident,
                 generics,
                 ty,
-                expr,
+                body,
                 define_opaque,
                 ..
             }) => (
@@ -990,8 +992,8 @@ 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);
+                        let body = this.lower_const_item(body.as_deref().unwrap());
                         hir::ImplItemKind::Const(ty, body)
                     },
                 ),

compiler/rustc_ast_lowering/src/lib.rs

@@ -2084,6 +2084,22 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
     }
 
+    /// Assumes mgca feature is enabled.
+    fn try_lower_as_const_path(&mut self, body: &AnonConst) -> Option<&'hir hir::ConstArg<'hir>> {
+        let ExprKind::Path(qself, path) = &body.value.kind else { return None };
+        let qpath = self.lower_qpath(
+            body.value.id,
+            qself,
+            path,
+            ParamMode::Optional,
+            AllowReturnTypeNotation::No,
+            ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+            None,
+        );
+        let ct_kind = hir::ConstArgKind::Path(qpath);
+        Some(self.arena.alloc(hir::ConstArg { hir_id: self.next_id(), kind: ct_kind }))
+    }
+
     /// Used when lowering a type argument that turned out to actually be a const argument.
     ///
     /// Only use for that purpose since otherwise it will create a duplicate def.

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1084,9 +1084,9 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     _ => visit::walk_item(self, item),
                 }
             }
-            ItemKind::Const(box ConstItem { defaultness, expr, .. }) => {
+            ItemKind::Const(box ConstItem { defaultness, body, .. }) => {
                 self.check_defaultness(item.span, *defaultness);
-                if expr.is_none() {
+                if body.is_none() {
                     self.dcx().emit_err(errors::ConstWithoutBody {
                         span: item.span,
                         replace_span: self.ending_semi_or_hi(item.span),
@@ -1436,7 +1436,7 @@ 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 { body: 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

@@ -214,15 +214,15 @@ impl<'a> State<'a> {
                 ident,
                 generics,
                 ty,
-                expr,
+                body,
                 define_opaque,
             }) => {
                 self.print_item_const(
                     *ident,
                     None,
                     generics,
                     ty,
-                    expr.as_deref(),
+                    body.as_deref().map(|ct| &*ct.value),
                     &item.vis,
                     ast::Safety::Default,
                     *defaultness,
@@ -563,15 +563,15 @@ impl<'a> State<'a> {
                 ident,
                 generics,
                 ty,
-                expr,
+                body,
                 define_opaque,
             }) => {
                 self.print_item_const(
                     *ident,
                     None,
                     generics,
                     ty,
-                    expr.as_deref(),
+                    body.as_deref().map(|ct| &*ct.value),
                     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 = ecx.anon_const_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(P(ecx.stmt_item(span, const_item)))

compiler/rustc_builtin_macros/src/global_allocator.rs

@@ -48,7 +48,7 @@ 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 = ecx.anon_const_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(P(ecx.stmt_item(span, const_item)))

compiler/rustc_builtin_macros/src/proc_macro_harness.rs

@@ -379,7 +379,7 @@ fn mk_decls(cx: &mut ExtCtxt<'_>, macros: &[ProcMacro]) -> P<ast::Item> {
             i
         });
 
-    let block = cx.expr_block(
+    let block = cx.anon_const_block(
         cx.block(span, thin_vec![cx.stmt_item(span, krate), cx.stmt_item(span, decls_static)]),
     );
 

compiler/rustc_builtin_macros/src/test.rs

@@ -294,8 +294,9 @@ 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(
-                        cx.expr_struct(
+                    body: Some(P(ast::AnonConst {
+                        id: ast::DUMMY_NODE_ID,
+                        value: cx.expr_struct(
                             sp,
                             test_path("TestDescAndFn"),
                             thin_vec![
@@ -376,7 +377,7 @@ pub(crate) fn expand_test_or_bench(
                         field("testfn", test_fn), // }
                     ],
                         ), // }
-                    ),
+                    })),
                 }
                 .into(),
             ),

compiler/rustc_const_eval/src/check_consts/qualifs.rs

@@ -7,6 +7,7 @@
 
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir::LangItem;
+use rustc_hir::def::DefKind;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, AdtDef, Ty};
@@ -368,8 +369,12 @@ where
         // check performed after the promotion. Verify that with an assertion.
         assert!(promoted.is_none() || Q::ALLOW_PROMOTED);
 
-        // Don't peek inside trait associated constants.
-        if promoted.is_none() && cx.tcx.trait_of_item(def).is_none() {
+        // Const items don't themselves have bodies -- they will have either a path or an anon const instead.
+        // FIXME(mgca): is this really the right behavior? should we return the qualifs of the anon const body instead?
+        //              (note also that original code ignored trait assoc items)
+        if promoted.is_none()
+            && !matches!(cx.tcx.def_kind(def), DefKind::Const | DefKind::AssocConst)
+        {
             let qualifs = cx.tcx.at(constant.span).mir_const_qualif(def);
 
             if !Q::in_qualifs(&qualifs) {

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -294,6 +294,29 @@ pub fn eval_to_const_value_raw_provider<'tcx>(
                 )
             },
         );
+    } else if let ty::InstanceKind::Item(def_id) = key.value.instance.def
+        && matches!(tcx.def_kind(def_id), DefKind::Const | DefKind::AssocConst)
+    {
+        let ct = tcx.const_of_item(def_id).instantiate(tcx, key.value.instance.args);
+        match ct.kind() {
+            ty::ConstKind::Unevaluated(_) => {
+                return Err(ErrorHandled::TooGeneric(DUMMY_SP));
+            }
+            ty::ConstKind::Value(cv) => return Ok(tcx.valtree_to_const_val(cv)),
+            ty::ConstKind::Error(guar) => {
+                return Err(ErrorHandled::Reported(
+                    ReportedErrorInfo::const_eval_error(guar),
+                    DUMMY_SP,
+                ));
+            }
+            ty::ConstKind::Expr(_) => return Err(ErrorHandled::TooGeneric(DUMMY_SP)),
+            ty::ConstKind::Param(_) | ty::ConstKind::Placeholder(_) => {
+                return Err(ErrorHandled::TooGeneric(DUMMY_SP));
+            }
+            ty::ConstKind::Infer(_) | ty::ConstKind::Bound(..) => {
+                bug!("unexpected constant {ct:?}")
+            }
+        }
     }
 
     tcx.eval_to_allocation_raw(key).map(|val| turn_into_const_value(tcx, val, key))

compiler/rustc_expand/src/build.rs

@@ -105,6 +105,10 @@ impl<'a> ExtCtxt<'a> {
         }
     }
 
+    pub fn anon_const_block(&self, b: P<ast::Block>) -> P<ast::AnonConst> {
+        P(self.anon_const(b.span, ast::ExprKind::Block(b, None)))
+    }
+
     pub fn const_ident(&self, span: Span, ident: Ident) -> ast::AnonConst {
         self.anon_const(span, ast::ExprKind::Path(None, self.path_ident(span, ident)))
     }
@@ -711,7 +715,7 @@ impl<'a> ExtCtxt<'a> {
         span: Span,
         ident: Ident,
         ty: P<ast::Ty>,
-        expr: P<ast::Expr>,
+        body: P<ast::AnonConst>,
     ) -> P<ast::Item> {
         let defaultness = ast::Defaultness::Final;
         self.item(
@@ -724,7 +728,7 @@ impl<'a> ExtCtxt<'a> {
                     // FIXME(generic_const_items): Pass the generics as a parameter.
                     generics: ast::Generics::default(),
                     ty,
-                    expr: Some(expr),
+                    body: Some(body),
                     define_opaque: None,
                 }
                 .into(),