Auto merge of #113522 - fmease:generic-consts, r=cjgillot

Implement generic const items

This implements generic parameters and where-clauses on free and associated const items under the experimental feature gate `generic_const_items`. See rust-lang/lang-team#214.

Tracking issue: #113521.
Fixes #104400.

This PR doesn't include rustfmt support as per [nightly style procedure](https://github.com/rust-lang/style-team/blob/master/nightly-style-procedure.md) and it doesn't add rustdoc JSON support (see [related Zulip topic](https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/Rustdoc.20JSON.3A.20Experimental.20rustc.20features)).

CC `@scottmcm` `@compiler-errors` `@WalterSmuts`
r? `@oli-obk`

<details><summary>Resolved Questions</summary>

* Q: Should `const ADD<const N: usize, const M: usize>: usize = N + M; ADD::<0, 1>` trigger the error *generic parameters may not be used in const operations* (which can be unlocked with `#![feature(generic_const_exprs)]`). Currently it doesn't. Or does this fall under [this paragraph](https://github.com/rust-lang/rust/blob/71f71a5397c42fec01f5c1045c638d961fa9f7ca/compiler/rustc_resolve/src/late.rs#L191)?
  * A: No, #113522 (comment)
* Q: Should `const UNUSED: () = () where String: Copy;` (with `#![feature(trivial_bounds)]` and with `UNUSED` unused) succeed compilation? Currently it doesn't: *evaluation of constant value failed // entering unreachable code*
  * A: Yes, but postponed until stabilization (latest), #113522 (comment)

</details>

compiler/rustc_ast/src/ast.rs

@@ -2947,6 +2947,7 @@ pub struct StaticItem {
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct ConstItem {
     pub defaultness: Defaultness,
+    pub generics: Generics,
     pub ty: P<Ty>,
     pub expr: Option<P<Expr>>,
 }
@@ -3058,6 +3059,7 @@ impl ItemKind {
         match self {
             Self::Fn(box Fn { generics, .. })
             | Self::TyAlias(box TyAlias { generics, .. })
+            | Self::Const(box ConstItem { generics, .. })
             | Self::Enum(_, generics)
             | Self::Struct(_, generics)
             | Self::Union(_, generics)

compiler/rustc_ast/src/mut_visit.rs

@@ -1149,10 +1149,11 @@ pub fn noop_flat_map_assoc_item<T: MutVisitor>(
 }
 
 fn visit_const_item<T: MutVisitor>(
-    ConstItem { defaultness, ty, expr }: &mut ConstItem,
+    ConstItem { defaultness, generics, ty, expr }: &mut ConstItem,
     visitor: &mut T,
 ) {
     visit_defaultness(defaultness, visitor);
+    visitor.visit_generics(generics);
     visitor.visit_ty(ty);
     visit_opt(expr, |expr| visitor.visit_expr(expr));
 }

compiler/rustc_ast/src/visit.rs

@@ -308,8 +308,12 @@ pub fn walk_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a Item) {
     match &item.kind {
         ItemKind::ExternCrate(_) => {}
         ItemKind::Use(use_tree) => visitor.visit_use_tree(use_tree, item.id, false),
-        ItemKind::Static(box StaticItem { ty, mutability: _, expr })
-        | ItemKind::Const(box ConstItem { ty, expr, .. }) => {
+        ItemKind::Static(box StaticItem { ty, mutability: _, expr }) => {
+            visitor.visit_ty(ty);
+            walk_list!(visitor, visit_expr, expr);
+        }
+        ItemKind::Const(box ConstItem { defaultness: _, generics, ty, expr }) => {
+            visitor.visit_generics(generics);
             visitor.visit_ty(ty);
             walk_list!(visitor, visit_expr, expr);
         }
@@ -677,7 +681,8 @@ pub fn walk_assoc_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a AssocItem,
     visitor.visit_ident(ident);
     walk_list!(visitor, visit_attribute, attrs);
     match kind {
-        AssocItemKind::Const(box ConstItem { ty, expr, .. }) => {
+        AssocItemKind::Const(box ConstItem { defaultness: _, generics, ty, expr }) => {
+            visitor.visit_generics(generics);
             visitor.visit_ty(ty);
             walk_list!(visitor, visit_expr, expr);
         }

compiler/rustc_ast_lowering/src/item.rs

@@ -231,9 +231,15 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
                 hir::ItemKind::Static(ty, *m, body_id)
             }
-            ItemKind::Const(box ast::ConstItem { ty, expr, .. }) => {
-                let (ty, body_id) = self.lower_const_item(ty, span, expr.as_deref());
-                hir::ItemKind::Const(ty, body_id)
+            ItemKind::Const(box ast::ConstItem { generics, ty, expr, .. }) => {
+                let (generics, (ty, body_id)) = self.lower_generics(
+                    generics,
+                    Const::No,
+                    id,
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    |this| this.lower_const_item(ty, span, expr.as_deref()),
+                );
+                hir::ItemKind::Const(ty, generics, body_id)
             }
             ItemKind::Fn(box Fn {
                 sig: FnSig { decl, header, span: fn_sig_span },
@@ -715,11 +721,23 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let trait_item_def_id = hir_id.expect_owner();
 
         let (generics, kind, has_default) = match &i.kind {
-            AssocItemKind::Const(box ConstItem { ty, expr, .. }) => {
-                let ty =
-                    self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::ConstTy));
-                let body = expr.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
-                (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body), body.is_some())
+            AssocItemKind::Const(box ConstItem { generics, ty, expr, .. }) => {
+                let (generics, kind) = self.lower_generics(
+                    &generics,
+                    Const::No,
+                    i.id,
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    |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)
+                    },
+                );
+                (generics, kind, expr.is_some())
             }
             AssocItemKind::Fn(box Fn { sig, generics, body: None, .. }) => {
                 let asyncness = sig.header.asyncness;
@@ -817,14 +835,19 @@ impl<'hir> LoweringContext<'_, 'hir> {
         self.lower_attrs(hir_id, &i.attrs);
 
         let (generics, kind) = match &i.kind {
-            AssocItemKind::Const(box ConstItem { ty, expr, .. }) => {
-                let ty =
-                    self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::ConstTy));
-                (
-                    hir::Generics::empty(),
-                    hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
-                )
-            }
+            AssocItemKind::Const(box ConstItem { generics, ty, expr, .. }) => self.lower_generics(
+                &generics,
+                Const::No,
+                i.id,
+                &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                |this| {
+                    let ty = this
+                        .lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::ConstTy));
+                    let body = this.lower_const_body(i.span, expr.as_deref());
+
+                    hir::ImplItemKind::Const(ty, body)
+                },
+            ),
             AssocItemKind::Fn(box Fn { sig, generics, body, .. }) => {
                 self.current_item = Some(i.span);
                 let asyncness = sig.header.asyncness;

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -569,6 +569,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(const_closures, "const closures are experimental");
     gate_all!(builtin_syntax, "`builtin #` syntax is unstable");
     gate_all!(explicit_tail_calls, "`become` expression is experimental");
+    gate_all!(generic_const_items, "generic const items are experimental");
 
     if !visitor.features.negative_bounds {
         for &span in spans.get(&sym::negative_bounds).iter().copied().flatten() {

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

@@ -30,10 +30,15 @@ impl<'a> State<'a> {
             ast::ForeignItemKind::Fn(box ast::Fn { defaultness, sig, generics, body }) => {
                 self.print_fn_full(sig, ident, generics, vis, *defaultness, body.as_deref(), attrs);
             }
-            ast::ForeignItemKind::Static(ty, mutbl, body) => {
-                let def = ast::Defaultness::Final;
-                self.print_item_const(ident, Some(*mutbl), ty, body.as_deref(), vis, def);
-            }
+            ast::ForeignItemKind::Static(ty, mutbl, body) => self.print_item_const(
+                ident,
+                Some(*mutbl),
+                &ast::Generics::default(),
+                ty,
+                body.as_deref(),
+                vis,
+                ast::Defaultness::Final,
+            ),
             ast::ForeignItemKind::TyAlias(box ast::TyAlias {
                 defaultness,
                 generics,
@@ -67,6 +72,7 @@ impl<'a> State<'a> {
         &mut self,
         ident: Ident,
         mutbl: Option<ast::Mutability>,
+        generics: &ast::Generics,
         ty: &ast::Ty,
         body: Option<&ast::Expr>,
         vis: &ast::Visibility,
@@ -82,6 +88,7 @@ impl<'a> State<'a> {
         };
         self.word_space(leading);
         self.print_ident(ident);
+        self.print_generic_params(&generics.params);
         self.word_space(":");
         self.print_type(ty);
         if body.is_some() {
@@ -92,6 +99,7 @@ impl<'a> State<'a> {
             self.word_space("=");
             self.print_expr(body);
         }
+        self.print_where_clause(&generics.where_clause);
         self.word(";");
         self.end(); // end the outer cbox
     }
@@ -158,20 +166,21 @@ impl<'a> State<'a> {
                 self.word(";");
             }
             ast::ItemKind::Static(box StaticItem { ty, mutability: mutbl, expr: body }) => {
-                let def = ast::Defaultness::Final;
                 self.print_item_const(
                     item.ident,
                     Some(*mutbl),
+                    &ast::Generics::default(),
                     ty,
                     body.as_deref(),
                     &item.vis,
-                    def,
+                    ast::Defaultness::Final,
                 );
             }
-            ast::ItemKind::Const(box ast::ConstItem { defaultness, ty, expr }) => {
+            ast::ItemKind::Const(box ast::ConstItem { defaultness, generics, ty, expr }) => {
                 self.print_item_const(
                     item.ident,
                     None,
+                    generics,
                     ty,
                     expr.as_deref(),
                     &item.vis,
@@ -515,8 +524,16 @@ impl<'a> State<'a> {
             ast::AssocItemKind::Fn(box ast::Fn { defaultness, sig, generics, body }) => {
                 self.print_fn_full(sig, ident, generics, vis, *defaultness, body.as_deref(), attrs);
             }
-            ast::AssocItemKind::Const(box ast::ConstItem { defaultness, ty, expr }) => {
-                self.print_item_const(ident, None, ty, expr.as_deref(), vis, *defaultness);
+            ast::AssocItemKind::Const(box ast::ConstItem { defaultness, generics, ty, expr }) => {
+                self.print_item_const(
+                    ident,
+                    None,
+                    generics,
+                    ty,
+                    expr.as_deref(),
+                    vis,
+                    *defaultness,
+                );
             }
             ast::AssocItemKind::Type(box ast::TyAlias {
                 defaultness,

compiler/rustc_builtin_macros/src/test.rs

@@ -255,6 +255,7 @@ pub fn expand_test_or_bench(
             ast::ItemKind::Const(
                 ast::ConstItem {
                     defaultness: ast::Defaultness::Final,
+                    generics: ast::Generics::default(),
                     ty: cx.ty(sp, ast::TyKind::Path(None, test_path("TestDescAndFn"))),
                     // test::TestDescAndFn {
                     expr: Some(

compiler/rustc_expand/src/build.rs

@@ -643,7 +643,16 @@ impl<'a> ExtCtxt<'a> {
             span,
             name,
             AttrVec::new(),
-            ast::ItemKind::Const(ast::ConstItem { defaultness, ty, expr: Some(expr) }.into()),
+            ast::ItemKind::Const(
+                ast::ConstItem {
+                    defaultness,
+                    // FIXME(generic_const_items): Pass the generics as a parameter.
+                    generics: ast::Generics::default(),
+                    ty,
+                    expr: Some(expr),
+                }
+                .into(),
+            ),
         )
     }
 

compiler/rustc_feature/src/active.rs

@@ -424,6 +424,8 @@ declare_features! (
     (incomplete, generic_associated_types_extended, "1.61.0", Some(95451), None),
     /// Allows non-trivial generic constants which have to have wfness manually propagated to callers
     (incomplete, generic_const_exprs, "1.56.0", Some(76560), None),
+    /// Allows generic parameters and where-clauses on free & associated const items.
+    (incomplete, generic_const_items, "CURRENT_RUSTC_VERSION", Some(113521), None),
     /// Allows using `..=X` as a patterns in slices.
     (active, half_open_range_patterns_in_slices, "1.66.0", Some(67264), None),
     /// Allows `if let` guard in match arms.

compiler/rustc_hir/src/hir.rs

@@ -3130,9 +3130,9 @@ impl<'hir> Item<'hir> {
     }
     /// Expect an [`ItemKind::Const`] or panic.
     #[track_caller]
-    pub fn expect_const(&self) -> (&'hir Ty<'hir>, BodyId) {
-        let ItemKind::Const(ty, body) = self.kind else { self.expect_failed("a constant") };
-        (ty, body)
+    pub fn expect_const(&self) -> (&'hir Ty<'hir>, &'hir Generics<'hir>, BodyId) {
+        let ItemKind::Const(ty, gen, body) = self.kind else { self.expect_failed("a constant") };
+        (ty, gen, body)
     }
     /// Expect an [`ItemKind::Fn`] or panic.
     #[track_caller]
@@ -3319,7 +3319,7 @@ pub enum ItemKind<'hir> {
     /// A `static` item.
     Static(&'hir Ty<'hir>, Mutability, BodyId),
     /// A `const` item.
-    Const(&'hir Ty<'hir>, BodyId),
+    Const(&'hir Ty<'hir>, &'hir Generics<'hir>, BodyId),
     /// A function declaration.
     Fn(FnSig<'hir>, &'hir Generics<'hir>, BodyId),
     /// A MBE macro definition (`macro_rules!` or `macro`).
@@ -3372,6 +3372,7 @@ impl ItemKind<'_> {
         Some(match *self {
             ItemKind::Fn(_, ref generics, _)
             | ItemKind::TyAlias(_, ref generics)
+            | ItemKind::Const(_, ref generics, _)
             | ItemKind::OpaqueTy(OpaqueTy { ref generics, .. })
             | ItemKind::Enum(_, ref generics)
             | ItemKind::Struct(_, ref generics)
@@ -3567,7 +3568,9 @@ impl<'hir> OwnerNode<'hir> {
         match self {
             OwnerNode::Item(Item {
                 kind:
-                    ItemKind::Static(_, _, body) | ItemKind::Const(_, body) | ItemKind::Fn(_, _, body),
+                    ItemKind::Static(_, _, body)
+                    | ItemKind::Const(_, _, body)
+                    | ItemKind::Fn(_, _, body),
                 ..
             })
             | OwnerNode::TraitItem(TraitItem {
@@ -3770,9 +3773,9 @@ impl<'hir> Node<'hir> {
     pub fn ty(self) -> Option<&'hir Ty<'hir>> {
         match self {
             Node::Item(it) => match it.kind {
-                ItemKind::TyAlias(ty, _) | ItemKind::Static(ty, _, _) | ItemKind::Const(ty, _) => {
-                    Some(ty)
-                }
+                ItemKind::TyAlias(ty, _)
+                | ItemKind::Static(ty, _, _)
+                | ItemKind::Const(ty, _, _) => Some(ty),
                 _ => None,
             },
             Node::TraitItem(it) => match it.kind {
@@ -3800,7 +3803,9 @@ impl<'hir> Node<'hir> {
         match self {
             Node::Item(Item {
                 kind:
-                    ItemKind::Static(_, _, body) | ItemKind::Const(_, body) | ItemKind::Fn(_, _, body),
+                    ItemKind::Static(_, _, body)
+                    | ItemKind::Const(_, _, body)
+                    | ItemKind::Fn(_, _, body),
                 ..
             })
             | Node::TraitItem(TraitItem {

compiler/rustc_hir/src/intravisit.rs

@@ -467,11 +467,17 @@ pub fn walk_item<'v, V: Visitor<'v>>(visitor: &mut V, item: &'v Item<'v>) {
         ItemKind::Use(ref path, _) => {
             visitor.visit_use(path, item.hir_id());
         }
-        ItemKind::Static(ref typ, _, body) | ItemKind::Const(ref typ, body) => {
+        ItemKind::Static(ref typ, _, body) => {
             visitor.visit_id(item.hir_id());
             visitor.visit_ty(typ);
             visitor.visit_nested_body(body);
         }
+        ItemKind::Const(ref typ, ref generics, body) => {
+            visitor.visit_id(item.hir_id());
+            visitor.visit_ty(typ);
+            visitor.visit_generics(generics);
+            visitor.visit_nested_body(body);
+        }
         ItemKind::Fn(ref sig, ref generics, body_id) => {
             visitor.visit_id(item.hir_id());
             visitor.visit_fn(