Introduce feature(generic_const_parameter_types)

rust-lang · matthiaskrgr · Feb 21, 2025 · Feb 26, 2025 · Feb 27, 2025 · Feb 28, 2025
commit df5b279ca962a84c83a6766dbb02cf23b866082a
diff --git a/compiler/rustc_feature/src/unstable.rs b/compiler/rustc_feature/src/unstable.rs
@@ -508,6 +508,8 @@ declare_features! (
     (incomplete, generic_const_exprs, "1.56.0", Some(76560)),
     /// Allows generic parameters and where-clauses on free & associated const items.
     (incomplete, generic_const_items, "1.73.0", Some(113521)),
+    /// Allows the type of const generics to depend on generic parameters
+    (incomplete, generic_const_parameter_types, "CURRENT_RUSTC_VERSION", Some(137626)),
     /// Allows any generic constants being used as pattern type range ends
     (incomplete, generic_pattern_types, "1.86.0", Some(136574)),
     /// Allows registering static items globally, possibly across crates, to iterate over at runtime.

diff --git a/compiler/rustc_hir_analysis/src/check/wfcheck.rs b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
@@ -966,7 +966,7 @@ fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) -> Result<(),
             let ty = tcx.type_of(param.def_id).instantiate_identity();
 
             if tcx.features().unsized_const_params() {
-                enter_wf_checking_ctxt(tcx, hir_ty.span, param.def_id, |wfcx| {
+                enter_wf_checking_ctxt(tcx, hir_ty.span, tcx.local_parent(param.def_id), |wfcx| {
                     wfcx.register_bound(
                         ObligationCause::new(
                             hir_ty.span,
@@ -980,7 +980,7 @@ fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) -> Result<(),
                     Ok(())
                 })
             } else if tcx.features().adt_const_params() {
-                enter_wf_checking_ctxt(tcx, hir_ty.span, param.def_id, |wfcx| {
+                enter_wf_checking_ctxt(tcx, hir_ty.span, tcx.local_parent(param.def_id), |wfcx| {
                     wfcx.register_bound(
                         ObligationCause::new(
                             hir_ty.span,

diff --git a/compiler/rustc_hir_analysis/src/collect.rs b/compiler/rustc_hir_analysis/src/collect.rs
@@ -1822,6 +1822,9 @@ fn const_param_default<'tcx>(
         ),
     };
     let icx = ItemCtxt::new(tcx, def_id);
-    let ct = icx.lowerer().lower_const_arg(default_ct, FeedConstTy::Param(def_id.to_def_id()));
+    let identity_args = ty::GenericArgs::identity_for_item(tcx, def_id);
+    let ct = icx
+        .lowerer()
+        .lower_const_arg(default_ct, FeedConstTy::Param(def_id.to_def_id(), identity_args));
     ty::EarlyBinder::bind(ct)
 }
diff --git a/compiler/rustc_hir_analysis/src/collect/predicates_of.rs b/compiler/rustc_hir_analysis/src/collect/predicates_of.rs
@@ -223,10 +223,7 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
             }
             hir::GenericParamKind::Const { .. } => {
                 let param_def_id = param.def_id.to_def_id();
-                let ct_ty = tcx
-                    .type_of(param_def_id)
-                    .no_bound_vars()
-                    .expect("const parameters cannot be generic");
+                let ct_ty = tcx.type_of(param_def_id).instantiate_identity();
                 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));

diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs
@@ -273,7 +273,7 @@ pub fn lower_generic_args<'tcx: 'a, 'a>(
 
                             // We lower to an infer even when the feature gate is not enabled
                             // as it is useful for diagnostics to be able to see a `ConstKind::Infer`
-                            args.push(ctx.provided_kind(param, arg));
+                            args.push(ctx.provided_kind(&args, param, arg));
                             args_iter.next();
                             params.next();
                         }

diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
@@ -232,12 +232,15 @@ impl AssocItemQSelf {
 /// Use this enum with `<dyn HirTyLowerer>::lower_const_arg` to instruct it with the
 /// desired behavior.
 #[derive(Debug, Clone, Copy)]
-pub enum FeedConstTy {
+pub enum FeedConstTy<'a, 'tcx> {
     /// Feed the type.
     ///
     /// The `DefId` belongs to the const param that we are supplying
     /// this (anon) const arg to.
-    Param(DefId),
+    ///
+    /// The list of generic args is used to instantiate the parameters
+    /// used by the type of the const param specified by `DefId`.
+    Param(DefId, &'a [ty::GenericArg<'tcx>]),
     /// Don't feed the type.
     No,
 }
@@ -298,6 +301,7 @@ pub trait GenericArgsLowerer<'a, 'tcx> {
 
     fn provided_kind(
         &mut self,
+        preceding_args: &[ty::GenericArg<'tcx>],
         param: &ty::GenericParamDef,
         arg: &GenericArg<'tcx>,
     ) -> ty::GenericArg<'tcx>;
@@ -481,6 +485,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
 
             fn provided_kind(
                 &mut self,
+                preceding_args: &[ty::GenericArg<'tcx>],
                 param: &ty::GenericParamDef,
                 arg: &GenericArg<'tcx>,
             ) -> ty::GenericArg<'tcx> {
@@ -526,7 +531,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     (GenericParamDefKind::Const { .. }, GenericArg::Const(ct)) => self
                         .lowerer
                         // Ambig portions of `ConstArg` are handled in the match arm below
-                        .lower_const_arg(ct.as_unambig_ct(), FeedConstTy::Param(param.def_id))
+                        .lower_const_arg(
+                            ct.as_unambig_ct(),
+                            FeedConstTy::Param(param.def_id, preceding_args),
+                        )
                         .into(),
                     (&GenericParamDefKind::Const { .. }, GenericArg::Infer(inf)) => {
                         self.lowerer.ct_infer(Some(param), inf.span).into()
@@ -582,8 +590,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         let ty = tcx
                             .at(self.span)
                             .type_of(param.def_id)
-                            .no_bound_vars()
-                            .expect("const parameter types cannot be generic");
+                            .instantiate(tcx, preceding_args);
                         if let Err(guar) = ty.error_reported() {
                             return ty::Const::new_error(tcx, guar).into();
                         }
@@ -2107,14 +2114,50 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     pub fn lower_const_arg(
         &self,
         const_arg: &hir::ConstArg<'tcx>,
-        feed: FeedConstTy,
+        feed: FeedConstTy<'_, 'tcx>,
     ) -> Const<'tcx> {
         let tcx = self.tcx();
 
-        if let FeedConstTy::Param(param_def_id) = feed
+        if let FeedConstTy::Param(param_def_id, args) = feed
             && let hir::ConstArgKind::Anon(anon) = &const_arg.kind
         {
-            tcx.feed_anon_const_type(anon.def_id, tcx.type_of(param_def_id));
+            let anon_const_type = tcx.type_of(param_def_id).instantiate(tcx, args);
+
+            // We must error if the instantiated type has any inference variables as we will
+            // use this type to feed the `type_of` and query results must not contain inference
+            // variables otherwise we will ICE.
+            //
+            // We also error if the type contains any regions as effectively any region will wind
+            // up as a region variable in mir borrowck. It would also be somewhat concerning if
+            // hir typeck was using equality but mir borrowck wound up using subtyping as that could
+            // result in a non-infer in hir typeck but a region variable in borrowck.
+            //
+            // FIXME(generic_const_parameter_types): Ideally we remove these errors one day when
+            // we have the ability to intermix typeck of anon const const args with the parent
+            // bodies typeck.
+            if tcx.features().generic_const_parameter_types()
+                && (anon_const_type.has_free_regions() || anon_const_type.has_erased_regions())
+            {
+                let e = tcx.dcx().span_err(
+                    const_arg.span(),
+                    "anonymous constants with lifetimes in their type are not yet supported",
+                );
+                tcx.feed_anon_const_type(anon.def_id, ty::EarlyBinder::bind(Ty::new_error(tcx, e)));
+                return ty::Const::new_error(tcx, e);
+            }
+            if anon_const_type.has_non_region_infer() {
+                let e = tcx.dcx().span_err(
+                    const_arg.span(),
+                    "anonymous constants with inferred types are not yet supported",
+                );
+                tcx.feed_anon_const_type(anon.def_id, ty::EarlyBinder::bind(Ty::new_error(tcx, e)));
+                return ty::Const::new_error(tcx, e);
+            }
+
+            tcx.feed_anon_const_type(
+                anon.def_id,
+                ty::EarlyBinder::bind(tcx.type_of(param_def_id).instantiate(tcx, args)),
+            );
         }
 
         let hir_id = const_arg.hir_id;
@@ -2230,10 +2273,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let expr = &tcx.hir_body(anon.body).value;
         debug!(?expr);
 
-        let ty = tcx
-            .type_of(anon.def_id)
-            .no_bound_vars()
-            .expect("const parameter types cannot be generic");
+        // FIXME(generic_const_parameter_types): We should use the proper generic args
+        // here. It's only used as a hint for literals so doesn't matter too much to use the right
+        // generic arguments, just weaker type inference.
+        let ty = tcx.type_of(anon.def_id).instantiate_identity();
 
         match self.try_lower_anon_const_lit(ty, expr) {
             Some(v) => v,

diff --git a/compiler/rustc_hir_analysis/src/lib.rs b/compiler/rustc_hir_analysis/src/lib.rs
@@ -260,7 +260,7 @@ pub fn lower_ty<'tcx>(tcx: TyCtxt<'tcx>, hir_ty: &hir::Ty<'tcx>) -> Ty<'tcx> {
 pub fn lower_const_arg_for_rustdoc<'tcx>(
     tcx: TyCtxt<'tcx>,
     hir_ct: &hir::ConstArg<'tcx>,
-    feed: FeedConstTy,
+    feed: FeedConstTy<'_, 'tcx>,
 ) -> 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)

diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
@@ -519,7 +519,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub(crate) fn lower_const_arg(
         &self,
         const_arg: &'tcx hir::ConstArg<'tcx>,
-        feed: FeedConstTy,
+        feed: FeedConstTy<'_, 'tcx>,
     ) -> ty::Const<'tcx> {
         let ct = self.lowerer().lower_const_arg(const_arg, feed);
         self.register_wf_obligation(
@@ -1135,7 +1135,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             .last()
             .is_some_and(|GenericPathSegment(def_id, _)| tcx.generics_of(*def_id).has_self);
 
-        let (res, self_ctor_args) = if let Res::SelfCtor(impl_def_id) = res {
+        let (res, implicit_args) = if let Res::Def(DefKind::ConstParam, def) = res {
+            // types of const parameters are somewhat special as they are part of
+            // the same environment as the const parameter itself. this means that
+            // unlike most paths `type-of(N)` can return a type naming parameters
+            // introduced by the containing item, rather than provided through `N`.
+            //
+            // for example given `<T, const M: usize, const N: [T; M]>` and some
+            // `let a = N;` expression. The path to `N` would wind up with no args
+            // (as it has no args), but instantiating the early binder on `typeof(N)`
+            // requires providing generic arguments for `[T, M, N]`.
+            (res, Some(ty::GenericArgs::identity_for_item(tcx, tcx.parent(def))))
+        } else if let Res::SelfCtor(impl_def_id) = res {
             let ty = LoweredTy::from_raw(
                 self,
                 span,
@@ -1261,6 +1272,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
             fn provided_kind(
                 &mut self,
+                preceding_args: &[ty::GenericArg<'tcx>],
                 param: &ty::GenericParamDef,
                 arg: &GenericArg<'tcx>,
             ) -> ty::GenericArg<'tcx> {
@@ -1280,7 +1292,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     (GenericParamDefKind::Const { .. }, GenericArg::Const(ct)) => self
                         .fcx
                         // Ambiguous parts of `ConstArg` are handled in the match arms below
-                        .lower_const_arg(ct.as_unambig_ct(), FeedConstTy::Param(param.def_id))
+                        .lower_const_arg(
+                            ct.as_unambig_ct(),
+                            FeedConstTy::Param(param.def_id, preceding_args),
+                        )
                         .into(),
                     (&GenericParamDefKind::Const { .. }, GenericArg::Infer(inf)) => {
                         self.fcx.ct_infer(Some(param), inf.span).into()
@@ -1320,7 +1335,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         }
 
-        let args_raw = self_ctor_args.unwrap_or_else(|| {
+        let args_raw = implicit_args.unwrap_or_else(|| {
             lower_generic_args(
                 self,
                 def_id,

diff --git a/compiler/rustc_hir_typeck/src/method/confirm.rs b/compiler/rustc_hir_typeck/src/method/confirm.rs
@@ -426,6 +426,7 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
 
             fn provided_kind(
                 &mut self,
+                preceding_args: &[ty::GenericArg<'tcx>],
                 param: &ty::GenericParamDef,
                 arg: &GenericArg<'tcx>,
             ) -> ty::GenericArg<'tcx> {
@@ -446,7 +447,10 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
                     (GenericParamDefKind::Const { .. }, GenericArg::Const(ct)) => self
                         .cfcx
                         // We handle the ambig portions of `ConstArg` in the match arms below
-                        .lower_const_arg(ct.as_unambig_ct(), FeedConstTy::Param(param.def_id))
+                        .lower_const_arg(
+                            ct.as_unambig_ct(),
+                            FeedConstTy::Param(param.def_id, preceding_args),
+                        )
                         .into(),
                     (GenericParamDefKind::Const { .. }, GenericArg::Infer(inf)) => {
                         self.cfcx.ct_infer(Some(param), inf.span).into()

diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
@@ -342,6 +342,7 @@ impl ParamConst {
         ParamConst::new(def.index, def.name)
     }
 
+    #[instrument(level = "debug")]
     pub fn find_ty_from_env<'tcx>(self, env: ParamEnv<'tcx>) -> Ty<'tcx> {
         let mut candidates = env.caller_bounds().iter().filter_map(|clause| {
             // `ConstArgHasType` are never desugared to be higher ranked.

diff --git a/compiler/rustc_resolve/messages.ftl b/compiler/rustc_resolve/messages.ftl
@@ -125,9 +125,6 @@ resolve_const_param_in_enum_discriminant =
 resolve_const_param_in_non_trivial_anon_const =
     const parameters may only be used as standalone arguments, i.e. `{$name}`
 
-resolve_const_param_in_ty_of_const_param =
-    const parameters may not be used in the type of const parameters
-
 resolve_constructor_private_if_any_field_private =
     a constructor is private if any of the fields is private
 
@@ -250,9 +247,6 @@ resolve_lifetime_param_in_enum_discriminant =
 resolve_lifetime_param_in_non_trivial_anon_const =
     lifetime parameters may not be used in const expressions
 
-resolve_lifetime_param_in_ty_of_const_param =
-    lifetime parameters may not be used in the type of const parameters
-
 resolve_lowercase_self =
     attempt to use a non-constant value in a constant
     .suggestion = try using `Self`
@@ -437,9 +431,6 @@ resolve_type_param_in_enum_discriminant =
 resolve_type_param_in_non_trivial_anon_const =
     type parameters may not be used in const expressions
 
-resolve_type_param_in_ty_of_const_param =
-    type parameters may not be used in the type of const parameters
-
 resolve_undeclared_label =
     use of undeclared label `{$name}`
     .label = undeclared label `{$name}`

diff --git a/compiler/rustc_resolve/src/diagnostics.rs b/compiler/rustc_resolve/src/diagnostics.rs
@@ -890,9 +890,9 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             ResolutionError::ForwardDeclaredGenericParam => {
                 self.dcx().create_err(errs::ForwardDeclaredGenericParam { span })
             }
-            ResolutionError::ParamInTyOfConstParam { name, param_kind: is_type } => self
-                .dcx()
-                .create_err(errs::ParamInTyOfConstParam { span, name, param_kind: is_type }),
+            ResolutionError::ParamInTyOfConstParam { name } => {
+                self.dcx().create_err(errs::ParamInTyOfConstParam { span, name })
+            }
             ResolutionError::ParamInNonTrivialAnonConst { name, param_kind: is_type } => {
                 self.dcx().create_err(errs::ParamInNonTrivialAnonConst {
                     span,

diff --git a/compiler/rustc_resolve/src/errors.rs b/compiler/rustc_resolve/src/errors.rs
@@ -347,19 +347,6 @@ pub(crate) struct ParamInTyOfConstParam {
     #[label]
     pub(crate) span: Span,
     pub(crate) name: Symbol,
-    #[subdiagnostic]
-    pub(crate) param_kind: Option<ParamKindInTyOfConstParam>,
-}
-
-#[derive(Debug)]
-#[derive(Subdiagnostic)]
-pub(crate) enum ParamKindInTyOfConstParam {
-    #[note(resolve_type_param_in_ty_of_const_param)]
-    Type,
-    #[note(resolve_const_param_in_ty_of_const_param)]
-    Const,
-    #[note(resolve_lifetime_param_in_ty_of_const_param)]
-    Lifetime,
 }
 
 #[derive(Diagnostic)]