Move Const::from_const_arg to HIR ty lowering

This makes more sense and will allow us to start lowering all paths to
`ConstArgKind::Path`, rather than just bare params.

Author: camelid

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;
@@ -86,6 +86,7 @@ pub fn provide(providers: &mut Providers) {
         coroutine_for_closure,
         is_type_alias_impl_trait,
         rendered_precise_capturing_args,
+        const_param_default,
         ..*providers
     };
 }
@@ -1873,3 +1874,22 @@ 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);
+    let ct = icx.lowerer().lower_const_arg(default_ct, FeedConstTy::No);
+    ty::EarlyBinder::bind(ct)
+}

compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs

@@ -20,7 +20,7 @@ use super::errors::GenericsArgsErrExtend;
 use crate::bounds::Bounds;
 use crate::errors;
 use crate::hir_ty_lowering::{
-    AssocItemQSelf, HirTyLowerer, OnlySelfBounds, PredicateFilter, RegionInferReason,
+    AssocItemQSelf, FeedConstTy, HirTyLowerer, OnlySelfBounds, PredicateFilter, RegionInferReason,
 };
 
 impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
@@ -392,9 +392,7 @@ 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/mod.rs

@@ -212,6 +212,23 @@ impl AssocItemQSelf {
     }
 }
 
+/// 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,
+}
+
 /// New-typed boolean indicating whether explicit late-bound lifetimes
 /// are present in a set of generic arguments.
 ///
@@ -495,8 +512,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         handle_ty_args(has_default, &inf.to_ty())
                     }
                     (GenericParamDefKind::Const { .. }, GenericArg::Const(ct)) => {
-                        ty::Const::from_const_arg(tcx, ct, ty::FeedConstTy::Param(param.def_id))
-                            .into()
+                        self.lowerer.lower_const_arg(ct, FeedConstTy::Param(param.def_id)).into()
                     }
                     (&GenericParamDefKind::Const { .. }, GenericArg::Infer(inf)) => {
                         self.lowerer.ct_infer(Some(param), inf.span).into()
@@ -930,8 +946,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                                 let term: ty::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()
+                                        self.lower_const_arg(ct, FeedConstTy::No).into()
                                     }
                                 };
                                 // FIXME(#97583): This isn't syntactically well-formed!
@@ -2007,6 +2022,60 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         }
     }
 
+    /// Convert a [`hir::ConstArg`] to a [`ty::Const`](Const).
+    #[instrument(skip(self), level = "debug")]
+    pub fn lower_const_arg(
+        &self,
+        const_arg: &hir::ConstArg<'tcx>,
+        feed: FeedConstTy,
+    ) -> Const<'tcx> {
+        let tcx = self.tcx();
+
+        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::Path(qpath) => {
+                // FIXME(min_generic_const_args): for now only params are lowered to ConstArgKind::Path
+                self.lower_const_arg_param(qpath, const_arg.hir_id)
+            }
+            hir::ConstArgKind::Anon(anon) => Const::from_anon_const(tcx, anon.def_id),
+        }
+    }
+
+    /// Lower a use of a const param to a [`Const`].
+    ///
+    /// IMPORTANT: `qpath` must be a const param, otherwise this will panic
+    fn lower_const_arg_param(&self, qpath: hir::QPath<'tcx>, hir_id: HirId) -> Const<'tcx> {
+        let tcx = self.tcx();
+
+        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),
+        }
+    }
+
     fn lower_delegation_ty(&self, idx: hir::InferDelegationKind) -> Ty<'tcx> {
         let delegation_sig = self.tcx().inherit_sig_for_delegation_item(self.item_def_id());
         match idx {
@@ -2146,7 +2215,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 let length = match length {
                     hir::ArrayLen::Infer(inf) => self.ct_infer(None, inf.span),
                     hir::ArrayLen::Body(constant) => {
-                        ty::Const::from_const_arg(tcx, constant, ty::FeedConstTy::No)
+                        self.lower_const_arg(constant, FeedConstTy::No)
                     }
                 };
 

compiler/rustc_hir_analysis/src/lib.rs

@@ -96,13 +96,15 @@ use rustc_hir::def::DefKind;
 use rustc_middle::middle;
 use rustc_middle::mir::interpret::GlobalId;
 use rustc_middle::query::Providers;
-use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_middle::ty::{self, Const, Ty, TyCtxt};
 use rustc_session::parse::feature_err;
 use rustc_span::Span;
 use rustc_span::symbol::sym;
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits;
 
+use self::hir_ty_lowering::{FeedConstTy, HirTyLowerer};
+
 rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 
 fn require_c_abi_if_c_variadic(tcx: TyCtxt<'_>, decl: &hir::FnDecl<'_>, abi: Abi, span: Span) {
@@ -227,3 +229,13 @@ pub fn lower_ty<'tcx>(tcx: TyCtxt<'tcx>, hir_ty: &hir::Ty<'tcx>) -> Ty<'tcx> {
     let env_def_id = tcx.hir().get_parent_item(hir_ty.hir_id);
     collect::ItemCtxt::new(tcx, env_def_id.def_id).lower_ty(hir_ty)
 }
+
+/// This is for rustdoc and clippy.
+pub fn lower_const_arg<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    hir_ct: &hir::ConstArg<'tcx>,
+    feed: FeedConstTy,
+) -> Const<'tcx> {
+    let env_def_id = tcx.hir().get_parent_item(hir_ct.hir_id);
+    collect::ItemCtxt::new(tcx, env_def_id.def_id).lowerer().lower_const_arg(hir_ct, feed)
+}

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -13,8 +13,8 @@ use rustc_hir_analysis::hir_ty_lowering::generics::{
     check_generic_arg_count_for_call, lower_generic_args,
 };
 use rustc_hir_analysis::hir_ty_lowering::{
-    ExplicitLateBound, GenericArgCountMismatch, GenericArgCountResult, GenericArgsLowerer,
-    GenericPathSegment, HirTyLowerer, IsMethodCall, RegionInferReason,
+    ExplicitLateBound, FeedConstTy, GenericArgCountMismatch, GenericArgCountResult,
+    GenericArgsLowerer, GenericPathSegment, HirTyLowerer, IsMethodCall, RegionInferReason,
 };
 use rustc_infer::infer::canonical::{Canonical, OriginalQueryValues, QueryResponse};
 use rustc_infer::infer::{DefineOpaqueTypes, InferResult};
@@ -477,7 +477,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             hir::ArrayLen::Infer(inf) => self.ct_infer(None, inf.span),
             hir::ArrayLen::Body(const_arg) => {
                 let span = const_arg.span();
-                let c = ty::Const::from_const_arg(self.tcx, const_arg, ty::FeedConstTy::No);
+                let c = self.lowerer().lower_const_arg(const_arg, FeedConstTy::No);
                 self.register_wf_obligation(c.into(), span, ObligationCauseCode::WellFormed(None));
                 self.normalize(span, c)
             }
@@ -489,8 +489,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         const_arg: &'tcx hir::ConstArg<'tcx>,
         param_def_id: DefId,
     ) -> ty::Const<'tcx> {
-        let ct =
-            ty::Const::from_const_arg(self.tcx, const_arg, ty::FeedConstTy::Param(param_def_id));
+        let ct = self.lowerer().lower_const_arg(const_arg, FeedConstTy::Param(param_def_id));
         self.register_wf_obligation(
             ct.into(),
             self.tcx.hir().span(const_arg.hir_id),

compiler/rustc_middle/src/ty/consts.rs

@@ -2,13 +2,12 @@ use either::Either;
 use rustc_data_structures::intern::Interned;
 use rustc_error_messages::MultiSpan;
 use rustc_hir::def::{DefKind, Res};
-use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_hir::{self as hir, HirId};
+use rustc_hir::def_id::LocalDefId;
+use rustc_hir::{self as hir};
 use rustc_macros::HashStable;
 use rustc_type_ir::{self as ir, TypeFlags, WithCachedTypeInfo};
 use tracing::{debug, instrument};
 
-use crate::middle::resolve_bound_vars as rbv;
 use crate::mir::interpret::{ErrorHandled, LitToConstInput, Scalar};
 use crate::ty::{self, GenericArgs, ParamEnv, ParamEnvAnd, Ty, TyCtxt, TypeVisitableExt};
 
@@ -183,46 +182,7 @@ 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")]
-    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::Path(qpath) => {
-                // FIXME(min_generic_const_args): for now only params are lowered to ConstArgKind::Path
-                Self::from_param(tcx, qpath, const_arg.hir_id)
-            }
-            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")]
@@ -249,34 +209,6 @@ 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(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
@@ -527,20 +459,3 @@ impl<'tcx> Const<'tcx> {
         matches!(self.kind(), ty::ConstKind::Infer(_))
     }
 }
-
-pub 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"
-        ),
-    };
-    ty::EarlyBinder::bind(Const::from_const_arg(tcx, default_ct, FeedConstTy::No))
-}

compiler/rustc_middle/src/ty/mod.rs

@@ -70,7 +70,7 @@ pub use self::closure::{
     place_to_string_for_capture,
 };
 pub use self::consts::{
-    Const, ConstInt, ConstKind, Expr, ExprKind, FeedConstTy, ScalarInt, UnevaluatedConst, ValTree,
+    Const, ConstInt, ConstKind, Expr, ExprKind, ScalarInt, UnevaluatedConst, ValTree,
 };
 pub use self::context::{
     CtxtInterners, CurrentGcx, DeducedParamAttrs, Feed, FreeRegionInfo, GlobalCtxt, Lift, TyCtxt,
@@ -2119,7 +2119,6 @@ pub fn provide(providers: &mut Providers) {
         incoherent_impls: trait_def::incoherent_impls_provider,
         trait_impls_in_crate: trait_def::trait_impls_in_crate_provider,
         traits: trait_def::traits_provider,
-        const_param_default: consts::const_param_default,
         vtable_allocation: vtable::vtable_allocation_provider,
         ..*providers
     };