Introduce min_generic_const_args and directly represent paths

Co-authored-by: Boxy UwU <rust@boxyuwu.dev>
Co-authored-by: León Orell Valerian Liehr <me@fmease.dev>

compiler/rustc_ast_lowering/src/lib.rs

@@ -2052,6 +2052,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
     }
 
+    /// 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.
     #[instrument(level = "debug", skip(self))]
     fn lower_const_path_to_const_arg(
         &mut self,
@@ -2060,51 +2063,58 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         ty_id: NodeId,
         span: Span,
     ) -> &'hir hir::ConstArg<'hir> {
-        let ct_kind = match res {
-            Res::Def(DefKind::ConstParam, _) => {
-                let qpath = self.lower_qpath(
-                    ty_id,
-                    &None,
-                    path,
-                    ParamMode::Optional,
-                    AllowReturnTypeNotation::No,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Path),
-                    None,
-                );
-                hir::ConstArgKind::Path(qpath)
-            }
-            _ => {
-                // Construct an AnonConst where the expr is the "ty"'s path.
+        let tcx = self.tcx;
 
-                let parent_def_id = self.current_def_id_parent;
-                let node_id = self.next_node_id();
-                let span = self.lower_span(span);
-
-                // Add a definition for the in-band const def.
-                let def_id =
-                    self.create_def(parent_def_id, node_id, kw::Empty, DefKind::AnonConst, span);
-                let hir_id = self.lower_node_id(node_id);
+        // FIXME(min_generic_const_args): we only allow one-segment const paths for now
+        let ct_kind = if path.is_potential_trivial_const_arg()
+            && (tcx.features().min_generic_const_args()
+                || matches!(res, Res::Def(DefKind::ConstParam, _)))
+        {
+            let qpath = self.lower_qpath(
+                ty_id,
+                &None,
+                path,
+                ParamMode::Optional,
+                AllowReturnTypeNotation::No,
+                // FIXME(min_generic_const_args): update for `fn foo() -> Bar<FOO<impl Trait>>` support
+                ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                None,
+            );
+            hir::ConstArgKind::Path(qpath)
+        } else {
+            // Construct an AnonConst where the expr is the "ty"'s path.
+
+            let parent_def_id = self.current_def_id_parent;
+            let node_id = self.next_node_id();
+            let span = self.lower_span(span);
+
+            // Add a definition for the in-band const def.
+            // We're lowering a const argument that was originally thought to be a type argument,
+            // so the def collector didn't create the def ahead of time. That's why we have to do
+            // it here.
+            let def_id =
+                self.create_def(parent_def_id, node_id, kw::Empty, DefKind::AnonConst, span);
+            let hir_id = self.lower_node_id(node_id);
+
+            let path_expr = Expr {
+                id: ty_id,
+                kind: ExprKind::Path(None, path.clone()),
+                span,
+                attrs: AttrVec::new(),
+                tokens: None,
+            };
 
-                let path_expr = Expr {
-                    id: ty_id,
-                    kind: ExprKind::Path(None, path.clone()),
+            let ct = self.with_new_scopes(span, |this| {
+                self.arena.alloc(hir::AnonConst {
+                    def_id,
+                    hir_id,
+                    body: this.with_def_id_parent(def_id, |this| {
+                        this.lower_const_body(path_expr.span, Some(&path_expr))
+                    }),
                     span,
-                    attrs: AttrVec::new(),
-                    tokens: None,
-                };
-
-                let ct = self.with_new_scopes(span, |this| {
-                    self.arena.alloc(hir::AnonConst {
-                        def_id,
-                        hir_id,
-                        body: this.with_def_id_parent(def_id, |this| {
-                            this.lower_const_body(path_expr.span, Some(&path_expr))
-                        }),
-                        span,
-                    })
-                });
-                hir::ConstArgKind::Anon(ct)
-            }
+                })
+            });
+            hir::ConstArgKind::Anon(ct)
         };
 
         self.arena.alloc(hir::ConstArg {
@@ -2122,6 +2132,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
     #[instrument(level = "debug", skip(self))]
     fn lower_anon_const_to_const_arg_direct(&mut self, anon: &AnonConst) -> hir::ConstArg<'hir> {
+        let tcx = self.tcx;
         // Unwrap a block, so that e.g. `{ P }` is recognised as a parameter. Const arguments
         // currently have to be wrapped in curly brackets, so it's necessary to special-case.
         let expr = if let ExprKind::Block(block, _) = &anon.value.kind
@@ -2135,18 +2146,19 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         };
         let maybe_res =
             self.resolver.get_partial_res(expr.id).and_then(|partial_res| partial_res.full_res());
-        debug!("res={:?}", maybe_res);
-        // FIXME(min_generic_const_args): for now we only lower params to ConstArgKind::Path
-        if let Some(res) = maybe_res
-            && let Res::Def(DefKind::ConstParam, _) = res
-            && let ExprKind::Path(qself, path) = &expr.kind
+        // FIXME(min_generic_const_args): we only allow one-segment const paths for now
+        if let ExprKind::Path(None, path) = &expr.kind
+            && path.is_potential_trivial_const_arg()
+            && (tcx.features().min_generic_const_args()
+                || matches!(maybe_res, Some(Res::Def(DefKind::ConstParam, _))))
         {
             let qpath = self.lower_qpath(
                 expr.id,
-                qself,
+                &None,
                 path,
                 ParamMode::Optional,
                 AllowReturnTypeNotation::No,
+                // FIXME(min_generic_const_args): update for `fn foo() -> Bar<FOO<impl Trait>>` support
                 ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                 None,
             );

compiler/rustc_feature/src/unstable.rs

@@ -529,6 +529,8 @@ declare_features! (
     (unstable, macro_metavar_expr_concat, "1.81.0", Some(124225)),
     /// Allows `#[marker]` on certain traits allowing overlapping implementations.
     (unstable, marker_trait_attr, "1.30.0", Some(29864)),
+    /// Enables the generic const args MVP (only bare paths, not arbitrary computation).
+    (incomplete, min_generic_const_args, "CURRENT_RUSTC_VERSION", Some(132980)),
     /// A minimal, sound subset of specialization intended to be used by the
     /// standard library until the soundness issues with specialization
     /// are fixed.

compiler/rustc_hir_analysis/src/collect.rs

@@ -46,7 +46,7 @@ use tracing::{debug, instrument};
 
 use crate::check::intrinsic::intrinsic_operation_unsafety;
 use crate::errors;
-use crate::hir_ty_lowering::{HirTyLowerer, RegionInferReason};
+use crate::hir_ty_lowering::{FeedConstTy, HirTyLowerer, RegionInferReason};
 
 pub(crate) mod dump;
 mod generics_of;
@@ -88,6 +88,7 @@ pub fn provide(providers: &mut Providers) {
         coroutine_for_closure,
         opaque_ty_origin,
         rendered_precise_capturing_args,
+        const_param_default,
         ..*providers
     };
 }
@@ -1790,3 +1791,23 @@ fn rendered_precise_capturing_args<'tcx>(
         _ => None,
     })
 }
+
+fn const_param_default<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    def_id: LocalDefId,
+) -> ty::EarlyBinder<'tcx, Const<'tcx>> {
+    let default_ct = match tcx.hir_node_by_def_id(def_id) {
+        hir::Node::GenericParam(hir::GenericParam {
+            kind: hir::GenericParamKind::Const { default: Some(ct), .. },
+            ..
+        }) => ct,
+        _ => span_bug!(
+            tcx.def_span(def_id),
+            "`const_param_default` expected a generic parameter with a constant"
+        ),
+    };
+    let icx = ItemCtxt::new(tcx, def_id);
+    // FIXME(const_generics): investigate which places do and don't need const ty feeding
+    let ct = icx.lowerer().lower_const_arg(default_ct, FeedConstTy::No);
+    ty::EarlyBinder::bind(ct)
+}

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -223,11 +223,12 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
                 trace!(?predicates);
             }
             hir::GenericParamKind::Const { .. } => {
+                let param_def_id = param.def_id.to_def_id();
                 let ct_ty = tcx
-                    .type_of(param.def_id.to_def_id())
+                    .type_of(param_def_id)
                     .no_bound_vars()
                     .expect("const parameters cannot be generic");
-                let ct = icx.lowerer().lower_const_param(param.hir_id);
+                let ct = icx.lowerer().lower_const_param(param_def_id, param.hir_id);
                 predicates
                     .insert((ty::ClauseKind::ConstArgHasType(ct, ct_ty).upcast(tcx), param.span));
             }

compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs

@@ -19,7 +19,9 @@ use tracing::{debug, instrument};
 use super::errors::GenericsArgsErrExtend;
 use crate::bounds::Bounds;
 use crate::errors;
-use crate::hir_ty_lowering::{AssocItemQSelf, HirTyLowerer, PredicateFilter, RegionInferReason};
+use crate::hir_ty_lowering::{
+    AssocItemQSelf, FeedConstTy, HirTyLowerer, PredicateFilter, RegionInferReason,
+};
 
 impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     /// Add a `Sized` bound to the `bounds` if appropriate.
@@ -346,9 +348,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             hir::AssocItemConstraintKind::Equality { term } => {
                 let term = match term {
                     hir::Term::Ty(ty) => self.lower_ty(ty).into(),
-                    hir::Term::Const(ct) => {
-                        ty::Const::from_const_arg(tcx, ct, ty::FeedConstTy::No).into()
-                    }
+                    hir::Term::Const(ct) => self.lower_const_arg(ct, FeedConstTy::No).into(),
                 };
 
                 // Find any late-bound regions declared in `ty` that are not

compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs

@@ -115,17 +115,22 @@ fn generic_arg_mismatch_err(
             }
         }
         (GenericArg::Const(cnst), GenericParamDefKind::Type { .. }) => {
-            // FIXME(min_generic_const_args): once ConstArgKind::Path is used for non-params too,
-            // this should match against that instead of ::Anon
-            if let hir::ConstArgKind::Anon(anon) = cnst.kind
+            if let hir::ConstArgKind::Path(qpath) = cnst.kind
+                && let rustc_hir::QPath::Resolved(_, path) = qpath
+                && let Res::Def(DefKind::Fn { .. }, id) = path.res
+            {
+                err.help(format!("`{}` is a function item, not a type", tcx.item_name(id)));
+                err.help("function item types cannot be named directly");
+            } else if let hir::ConstArgKind::Anon(anon) = cnst.kind
                 && let body = tcx.hir().body(anon.body)
                 && let rustc_hir::ExprKind::Path(rustc_hir::QPath::Resolved(_, path)) =
                     body.value.kind
+                && let Res::Def(DefKind::Fn { .. }, id) = path.res
             {
-                if let Res::Def(DefKind::Fn { .. }, id) = path.res {
-                    err.help(format!("`{}` is a function item, not a type", tcx.item_name(id)));
-                    err.help("function item types cannot be named directly");
-                }
+                // FIXME(min_generic_const_args): this branch is dead once new const path lowering
+                // (for single-segment paths) is no longer gated
+                err.help(format!("`{}` is a function item, not a type", tcx.item_name(id)));
+                err.help("function item types cannot be named directly");
             }
         }
         _ => {}