Setup ty::Const functions for ConstArg

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -470,13 +470,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     (&GenericParamDefKind::Type { has_default, .. }, GenericArg::Infer(inf)) => {
                         handle_ty_args(has_default, &inf.to_ty())
                     }
-                    (GenericParamDefKind::Const { .. }, GenericArg::Const(ct)) => match &ct.kind {
+                    (GenericParamDefKind::Const { .. }, GenericArg::Const(ct)) => {
-                        hir::ConstArgKind::Anon(anon) => {
+                        ty::Const::from_const_arg(tcx, ct, ty::FeedConstTy::Param(param.def_id))
-                            let did = anon.def_id;
+                            .into()
-                            tcx.feed_anon_const_type(did, tcx.type_of(param.def_id));
-                            ty::Const::from_anon_const(tcx, did).into()
                     }
-                    },
                     (&GenericParamDefKind::Const { .. }, hir::GenericArg::Infer(inf)) => {
                         self.lowerer.ct_infer(Some(param), inf.span).into()
                     }

compiler/rustc_middle/src/ty/consts.rs

@@ -3,9 +3,9 @@ use crate::mir::interpret::{ErrorHandled, LitToConstInput, Scalar};
 use crate::ty::{self, GenericArgs, ParamEnv, ParamEnvAnd, Ty, TyCtxt, TypeVisitableExt};
 use rustc_data_structures::intern::Interned;
 use rustc_error_messages::MultiSpan;
-use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
-use rustc_hir::def_id::LocalDefId;
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::{self as hir, HirId};
 use rustc_macros::HashStable;
 use rustc_type_ir::{self as ir, TypeFlags, WithCachedTypeInfo};
 use tracing::{debug, instrument};
@@ -182,16 +182,52 @@ impl<'tcx> rustc_type_ir::inherent::Const<TyCtxt<'tcx>> for Const<'tcx> {
     }
 }
 
+/// In some cases, [`hir::ConstArg`]s that are being used in the type system
+/// through const generics need to have their type "fed" to them
+/// using the query system.
+///
+/// Use this enum with [`Const::from_const_arg`] to instruct it with the
+/// desired behavior.
+#[derive(Debug, Clone, Copy)]
+pub enum FeedConstTy {
+    /// Feed the type.
+    ///
+    /// The `DefId` belongs to the const param that we are supplying
+    /// this (anon) const arg to.
+    Param(DefId),
+    /// Don't feed the type.
+    No,
+}
+
 impl<'tcx> Const<'tcx> {
+    /// Convert a [`hir::ConstArg`] to a [`ty::Const`](Self).
+    #[instrument(skip(tcx), level = "debug")]
+    #[allow(irrefutable_let_patterns)]
+    pub fn from_const_arg(
+        tcx: TyCtxt<'tcx>,
+        const_arg: &'tcx hir::ConstArg<'tcx>,
+        feed: FeedConstTy,
+    ) -> Self {
+        if let FeedConstTy::Param(param_def_id) = feed
+            && let hir::ConstArgKind::Anon(anon) = &const_arg.kind
+        {
+            tcx.feed_anon_const_type(anon.def_id, tcx.type_of(param_def_id));
+        }
+
+        match const_arg.kind {
+            hir::ConstArgKind::Anon(anon) => Self::from_anon_const(tcx, anon.def_id),
+        }
+    }
+
     /// Literals and const generic parameters are eagerly converted to a constant, everything else
     /// becomes `Unevaluated`.
     #[instrument(skip(tcx), level = "debug")]
     pub fn from_anon_const(tcx: TyCtxt<'tcx>, def: LocalDefId) -> Self {
         let body_id = match tcx.hir_node_by_def_id(def) {
             hir::Node::AnonConst(ac) => ac.body,
-            _ => span_bug!(
+            node => span_bug!(
                 tcx.def_span(def.to_def_id()),
-                "from_anon_const can only process anonymous constants"
+                "from_anon_const can only process anonymous constants, not {node:?}"
             ),
         };
 
@@ -200,7 +236,7 @@ impl<'tcx> Const<'tcx> {
 
         let ty = tcx.type_of(def).no_bound_vars().expect("const parameter types cannot be generic");
 
-        match Self::try_from_lit_or_param(tcx, ty, expr) {
+        match Self::try_from_lit(tcx, ty, expr) {
             Some(v) => v,
             None => ty::Const::new_unevaluated(
                 tcx,
@@ -212,12 +248,36 @@ impl<'tcx> Const<'tcx> {
         }
     }
 
+    /// Lower a const param to a [`Const`].
+    ///
+    /// IMPORTANT: `qpath` must be a const param, otherwise this will panic
+    fn from_param(tcx: TyCtxt<'tcx>, qpath: hir::QPath<'tcx>, hir_id: HirId) -> Self {
+        let hir::QPath::Resolved(_, &hir::Path { res: Res::Def(DefKind::ConstParam, def_id), .. }) =
+            qpath
+        else {
+            span_bug!(qpath.span(), "non-param {qpath:?} passed to Const::from_param")
+        };
+
+        match tcx.named_bound_var(hir_id) {
+            Some(rbv::ResolvedArg::EarlyBound(_)) => {
+                // Find the name and index of the const parameter by indexing the generics of
+                // the parent item and construct a `ParamConst`.
+                let item_def_id = tcx.parent(def_id);
+                let generics = tcx.generics_of(item_def_id);
+                let index = generics.param_def_id_to_index[&def_id];
+                let name = tcx.item_name(def_id);
+                ty::Const::new_param(tcx, ty::ParamConst::new(index, name))
+            }
+            Some(rbv::ResolvedArg::LateBound(debruijn, index, _)) => {
+                ty::Const::new_bound(tcx, debruijn, ty::BoundVar::from_u32(index))
+            }
+            Some(rbv::ResolvedArg::Error(guar)) => ty::Const::new_error(tcx, guar),
+            arg => bug!("unexpected bound var resolution for {:?}: {arg:?}", hir_id),
+        }
+    }
+
     #[instrument(skip(tcx), level = "debug")]
-    fn try_from_lit_or_param(
+    fn try_from_lit(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, expr: &'tcx hir::Expr<'tcx>) -> Option<Self> {
-        tcx: TyCtxt<'tcx>,
-        ty: Ty<'tcx>,
-        expr: &'tcx hir::Expr<'tcx>,
-    ) -> Option<Self> {
         // 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 = match &expr.kind {
@@ -250,34 +310,19 @@ impl<'tcx> Const<'tcx> {
             }
         }
 
-        // FIXME(const_generics): We currently have to special case parameters because `min_const_generics`
+        if let hir::ExprKind::Path(
-        // does not provide the parents generics to anonymous constants. We still allow generic const
+            qpath @ hir::QPath::Resolved(
-        // parameters by themselves however, e.g. `N`. These constants would cause an ICE if we were to
-        // ever try to instantiate the generic parameters in their bodies.
-        match expr.kind {
-            hir::ExprKind::Path(hir::QPath::Resolved(
                 _,
-                &hir::Path { res: Res::Def(DefKind::ConstParam, def_id), .. },
+                &hir::Path { res: Res::Def(DefKind::ConstParam, _), .. },
-            )) => {
+            ),
-                match tcx.named_bound_var(expr.hir_id) {
+        ) = expr.kind
-                    Some(rbv::ResolvedArg::EarlyBound(_)) => {
+        {
-                        // Find the name and index of the const parameter by indexing the generics of
+            // FIXME: once ConstArgKind::Path added, uncomment span_bug and delete return
-                        // the parent item and construct a `ParamConst`.
+            // span_bug!(expr.span, "try_from_lit: received const param which shouldn't be possible")
-                        let item_def_id = tcx.parent(def_id);
+            return Some(Self::from_param(tcx, qpath, expr.hir_id));
-                        let generics = tcx.generics_of(item_def_id);
-                        let index = generics.param_def_id_to_index[&def_id];
-                        let name = tcx.item_name(def_id);
-                        Some(ty::Const::new_param(tcx, ty::ParamConst::new(index, name)))
-                    }
-                    Some(rbv::ResolvedArg::LateBound(debruijn, index, _)) => {
-                        Some(ty::Const::new_bound(tcx, debruijn, ty::BoundVar::from_u32(index)))
-                    }
-                    Some(rbv::ResolvedArg::Error(guar)) => Some(ty::Const::new_error(tcx, guar)),
-                    arg => bug!("unexpected bound var resolution for {:?}: {arg:?}", expr.hir_id),
-                }
-            }
-            _ => None,
         }
+
+        None
     }
 
     #[inline]

compiler/rustc_middle/src/ty/mod.rs

@@ -86,7 +86,7 @@ pub use self::closure::{
     CAPTURE_STRUCT_LOCAL,
 };
 pub use self::consts::{
-    Const, ConstInt, ConstKind, Expr, ExprKind, ScalarInt, UnevaluatedConst, ValTree,
+    Const, ConstInt, ConstKind, Expr, ExprKind, FeedConstTy, ScalarInt, UnevaluatedConst, ValTree,
 };
 pub use self::context::{
     tls, CtxtInterners, CurrentGcx, DeducedParamAttrs, Feed, FreeRegionInfo, GlobalCtxt, Lift,