generic_const_args: allow paths to non type consts

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -464,6 +464,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     check_incompatible_features(sess, features);
     check_dependent_features(sess, features);
     check_new_solver_banned_features(sess, features);
+    check_features_requiring_new_solver(sess, features);
 
     let mut visitor = PostExpansionVisitor { sess, features };
 
@@ -738,3 +739,25 @@ fn check_new_solver_banned_features(sess: &Session, features: &Features) {
         });
     }
 }
+
+fn check_features_requiring_new_solver(sess: &Session, features: &Features) {
+    if sess.opts.unstable_opts.next_solver.globally {
+        return;
+    }
+
+    // Require the new solver with GCA, because the old solver can't implement GCA correctly as it
+    // does not support normalization obligations for free and inherent consts.
+    if let Some(gca_span) = features
+        .enabled_lang_features()
+        .iter()
+        .find(|feat| feat.gate_name == sym::generic_const_args)
+        .map(|feat| feat.attr_sp)
+    {
+        #[allow(rustc::symbol_intern_string_literal)]
+        sess.dcx().emit_err(errors::MissingDependentFeatures {
+            parent_span: gca_span,
+            parent: sym::generic_const_exprs,
+            missing: String::from("-Znext-solver=globally"),
+        });
+    }
+}

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -3041,7 +3041,14 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         span: Span,
     ) -> Result<(), ErrorGuaranteed> {
         let tcx = self.tcx();
-        if tcx.is_type_const(def_id) {
+        // FIXME(gca): Intentionally disallowing paths to inherent associated non-type constants
+        // until a refactoring for how generic args for IACs are represented has been landed.
+        let is_inherent_assoc_const = tcx.def_kind(def_id)
+            == DefKind::AssocConst { is_type_const: false }
+            && tcx.def_kind(tcx.parent(def_id)) == DefKind::Impl { of_trait: false };
+        if tcx.is_type_const(def_id)
+            || tcx.features().generic_const_args() && !is_inherent_assoc_const
+        {
             Ok(())
         } else {
             let mut err = self.dcx().struct_span_err(

compiler/rustc_middle/src/ty/context/impl_interner.rs

@@ -177,6 +177,9 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
     fn type_of_opaque_hir_typeck(self, def_id: LocalDefId) -> ty::EarlyBinder<'tcx, Ty<'tcx>> {
         self.type_of_opaque_hir_typeck(def_id)
     }
+    fn is_type_const(self, def_id: DefId) -> bool {
+        self.is_type_const(def_id)
+    }
     fn const_of_item(self, def_id: DefId) -> ty::EarlyBinder<'tcx, Const<'tcx>> {
         self.const_of_item(def_id)
     }

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs

@@ -1168,6 +1168,40 @@ where
         self.delegate.evaluate_const(param_env, uv)
     }
 
+    pub(super) fn evaluate_const_and_instantiate_normalizes_to_term(
+        &mut self,
+        goal: Goal<I, ty::NormalizesTo<I>>,
+        uv: ty::UnevaluatedConst<I>,
+    ) -> QueryResult<I> {
+        match self.evaluate_const(goal.param_env, uv) {
+            Some(evaluated) => {
+                self.instantiate_normalizes_to_term(goal, evaluated.into());
+                self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+            }
+            None => {
+                // HACK(khyperia): calling `resolve_vars_if_possible` here shouldn't be necessary,
+                // `try_evaluate_const` calls `resolve_vars_if_possible` already. However, we want
+                // to check `has_non_region_infer` against the type with vars resolved (i.e. check
+                // if there are vars we failed to resolve), so we need to call it again here.
+                // Perhaps we could split EvaluateConstErr::HasGenericsOrInfers into HasGenerics and
+                // HasInfers or something, make evaluate_const return that, and make this branch be
+                // based on that, rather than checking `has_non_region_infer`.
+                if self.resolve_vars_if_possible(uv).has_non_region_infer() {
+                    self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
+                } else {
+                    // We do not instantiate to the `uv` passed in, but rather
+                    // `goal.predicate.alias`. The `uv` passed in might correspond to the `impl`
+                    // form of a constant (with generic arguments corresponding to the impl block),
+                    // however, we want to structurally instantiate to the original, non-rebased,
+                    // trait `Self` form of the constant (with generic arguments being the trait
+                    // `Self` type).
+                    self.structurally_instantiate_normalizes_to_term(goal, goal.predicate.alias);
+                    self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+                }
+            }
+        }
+    }
+
     pub(super) fn is_transmutable(
         &mut self,
         src: I::Ty,

compiler/rustc_next_trait_solver/src/solve/normalizes_to/anon_const.rs

@@ -14,17 +14,23 @@ where
         &mut self,
         goal: Goal<I, ty::NormalizesTo<I>>,
     ) -> QueryResult<I> {
-        if let Some(normalized_const) = self.evaluate_const(
-            goal.param_env,
-            ty::UnevaluatedConst::new(
-                goal.predicate.alias.def_id().try_into().unwrap(),
-                goal.predicate.alias.args,
-            ),
-        ) {
-            self.instantiate_normalizes_to_term(goal, normalized_const.into());
-            self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+        let cx = self.cx();
+        let uv = goal.predicate.alias.expect_ct(self.cx());
+        // keep legacy behavior for array repeat expressions:
+        // when a constant is too generic to be evaluated, the legacy behavior is to return
+        // Ambiguous, whereas evaluate_const_and_instantiate_normalizes_to_term structurally
+        // instantiates to itself and returns Yes (if there are no inference variables)
+        let is_repeat_expr =
+            cx.anon_const_kind(uv.def.into()) == ty::AnonConstKind::RepeatExprCount;
+        if is_repeat_expr {
+            if let Some(normalized_const) = self.evaluate_const(goal.param_env, uv) {
+                self.instantiate_normalizes_to_term(goal, normalized_const.into());
+                self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+            } else {
+                self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
+            }
         } else {
-            self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
+            self.evaluate_const_and_instantiate_normalizes_to_term(goal, uv)
         }
     }
 }

compiler/rustc_next_trait_solver/src/solve/normalizes_to/free_alias.rs

@@ -30,13 +30,20 @@ where
                 .map(|pred| goal.with(cx, pred)),
         );
 
-        let actual = if free_alias.kind(cx).is_type() {
-            cx.type_of(free_alias.def_id()).instantiate(cx, free_alias.args).skip_norm_wip().into()
-        } else {
-            cx.const_of_item(free_alias.def_id())
-                .instantiate(cx, free_alias.args)
-                .skip_norm_wip()
-                .into()
+        let actual = match free_alias.kind(cx) {
+            ty::AliasTermKind::FreeTy { def_id } => {
+                cx.type_of(def_id).instantiate(cx, free_alias.args).skip_norm_wip().into()
+            }
+            ty::AliasTermKind::FreeConst { def_id } if cx.is_type_const(def_id) => {
+                cx.const_of_item(def_id).instantiate(cx, free_alias.args).skip_norm_wip().into()
+            }
+            ty::AliasTermKind::FreeConst { .. } => {
+                return self.evaluate_const_and_instantiate_normalizes_to_term(
+                    goal,
+                    free_alias.expect_ct(cx),
+                );
+            }
+            kind => panic!("expected free alias, found {kind:?}"),
         };
 
         self.instantiate_normalizes_to_term(goal, actual);

compiler/rustc_next_trait_solver/src/solve/normalizes_to/inherent.rs

@@ -52,13 +52,23 @@ where
                 .map(|pred| goal.with(cx, pred)),
         );
 
-        let normalized = if inherent.kind(cx).is_type() {
-            cx.type_of(inherent.def_id()).instantiate(cx, inherent_args).skip_norm_wip().into()
-        } else {
-            cx.const_of_item(inherent.def_id())
-                .instantiate(cx, inherent_args)
-                .skip_norm_wip()
-                .into()
+        let normalized = match inherent.kind(cx) {
+            ty::AliasTermKind::InherentTy { def_id } => {
+                cx.type_of(def_id).instantiate(cx, inherent_args).skip_norm_wip().into()
+            }
+            ty::AliasTermKind::InherentConst { def_id } if cx.is_type_const(def_id) => {
+                cx.const_of_item(def_id).instantiate(cx, inherent_args).skip_norm_wip().into()
+            }
+            ty::AliasTermKind::InherentConst { .. } => {
+                // FIXME(gca): This is dead code at the moment. It should eventually call
+                // self.evaluate_const like projected consts do in consider_impl_candidate in
+                // normalizes_to/mod.rs. However, how generic args are represented for IACs is up in
+                // the air right now.
+                // Will self.evaluate_const eventually take the inherent_args or the impl_args form
+                // of args? It might be either.
+                panic!("References to inherent associated consts should have been blocked");
+            }
+            kind => panic!("expected inherent alias, found {kind:?}"),
         };
         self.instantiate_normalizes_to_term(goal, normalized);
         self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)

compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs

@@ -384,19 +384,30 @@ where
 
             // Finally we construct the actual value of the associated type.
             let term = match goal.predicate.alias.kind(cx) {
-                ty::AliasTermKind::ProjectionTy { .. } => {
-                    cx.type_of(target_item_def_id).map_bound(|ty| ty.into())
+                ty::AliasTermKind::ProjectionTy { .. } => cx
+                    .type_of(target_item_def_id)
+                    .instantiate(cx, target_args)
+                    .skip_norm_wip()
+                    .into(),
+                ty::AliasTermKind::ProjectionConst { .. }
+                    if cx.is_type_const(target_item_def_id) =>
+                {
+                    cx.const_of_item(target_item_def_id)
+                        .instantiate(cx, target_args)
+                        .skip_norm_wip()
+                        .into()
                 }
                 ty::AliasTermKind::ProjectionConst { .. } => {
-                    cx.const_of_item(target_item_def_id).map_bound(|ct| ct.into())
+                    let uv = ty::UnevaluatedConst::new(
+                        target_item_def_id.try_into().unwrap(),
+                        target_args,
+                    );
+                    return ecx.evaluate_const_and_instantiate_normalizes_to_term(goal, uv);
                 }
                 kind => panic!("expected projection, found {kind:?}"),
             };
 
-            ecx.instantiate_normalizes_to_term(
-                goal,
-                term.instantiate(cx, target_args).skip_norm_wip(),
-            );
+            ecx.instantiate_normalizes_to_term(goal, term);
             ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
         })
     }

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -696,7 +696,10 @@ pub fn try_evaluate_const<'tcx>(
                     // logic does not go through type system normalization. If it did this would
                     // be a backwards compatibility problem as we do not enforce "syntactic" non-
                     // usage of generic parameters like we do here.
-                    if uv.args.has_non_region_param() || uv.args.has_non_region_infer() {
+                    if uv.args.has_non_region_param()
+                        || uv.args.has_non_region_infer()
+                        || uv.args.has_non_region_placeholders()
+                    {
                         return Err(EvaluateConstErr::HasGenericsOrInfers);
                     }
 

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -1060,7 +1060,7 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
             ty::ConstKind::Unevaluated(uv) => {
                 if !c.has_escaping_bound_vars() {
                     // Skip type consts as mGCA doesn't support evaluatable clauses
-                    if !tcx.is_type_const(uv.def) {
+                    if !tcx.is_type_const(uv.def) && !tcx.features().generic_const_args() {
                         let predicate = ty::Binder::dummy(ty::PredicateKind::Clause(
                             ty::ClauseKind::ConstEvaluatable(c),
                         ));

compiler/rustc_type_ir/src/interner.rs

@@ -205,6 +205,7 @@ pub trait Interner:
     fn type_of(self, def_id: Self::DefId) -> ty::EarlyBinder<Self, Self::Ty>;
     fn type_of_opaque_hir_typeck(self, def_id: Self::LocalDefId)
     -> ty::EarlyBinder<Self, Self::Ty>;
+    fn is_type_const(self, def_id: Self::DefId) -> bool;
     fn const_of_item(self, def_id: Self::DefId) -> ty::EarlyBinder<Self, Self::Const>;
     fn anon_const_kind(self, def_id: Self::DefId) -> ty::AnonConstKind;
 

compiler/rustc_type_ir/src/predicate.rs

@@ -732,16 +732,32 @@ impl<I: Interner> AliasTerm<I> {
             AliasTermKind::InherentTy { def_id } => AliasTyKind::Inherent { def_id },
             AliasTermKind::OpaqueTy { def_id } => AliasTyKind::Opaque { def_id },
             AliasTermKind::FreeTy { def_id } => AliasTyKind::Free { def_id },
-            AliasTermKind::InherentConst { .. }
+            kind @ (AliasTermKind::InherentConst { .. }
             | AliasTermKind::FreeConst { .. }
             | AliasTermKind::UnevaluatedConst { .. }
-            | AliasTermKind::ProjectionConst { .. } => {
-                panic!("Cannot turn `UnevaluatedConst` into `AliasTy`")
+            | AliasTermKind::ProjectionConst { .. }) => {
+                panic!("Cannot turn `{}` into `AliasTy`", kind.descr())
             }
         };
         ty::AliasTy { kind, args: self.args, _use_alias_ty_new_instead: () }
     }
 
+    pub fn expect_ct(self, interner: I) -> ty::UnevaluatedConst<I> {
+        let def_id = match self.kind(interner) {
+            AliasTermKind::InherentConst { def_id }
+            | AliasTermKind::FreeConst { def_id }
+            | AliasTermKind::UnevaluatedConst { def_id }
+            | AliasTermKind::ProjectionConst { def_id } => def_id,
+            kind @ (AliasTermKind::ProjectionTy { .. }
+            | AliasTermKind::InherentTy { .. }
+            | AliasTermKind::OpaqueTy { .. }
+            | AliasTermKind::FreeTy { .. }) => {
+                panic!("Cannot turn `{}` into `UnevaluatedConst`", kind.descr())
+            }
+        };
+        ty::UnevaluatedConst { def: def_id.try_into().unwrap(), args: self.args }
+    }
+
     // FIXME: remove this function (access the field instead)
     pub fn kind(self, _interner: I) -> AliasTermKind<I> {
         self.kind

src/doc/rustc-dev-guide/src/feature-gate-check.md

@@ -69,6 +69,8 @@ in `check_crate` and its AST visitor.
   (declared in `rustc_feature::INCOMPATIBLE_FEATURES`) are not used together.
 - `check_new_solver_banned_features`: Bans features incompatible with
   compiler mode for the next trait solver.
+- `check_features_requiring_new_solver`: Requires the new trait solver for
+  features incompatible with the old solver.
 - **Parser-gated spans**: Processes the `GatedSpans` recorded during parsing
   (see [Checking `GatedSpans`](#checking-gatedspans)).
 

tests/ui/const-generics/gca/ambiguous-on-failed-eval-with-vars-fail.next.stderr

@@ -0,0 +1,48 @@
+error[E0284]: type annotations needed for `([(); _], [(); 10])`
+  --> $DIR/ambiguous-on-failed-eval-with-vars-fail.rs:29:9
+   |
+LL |     let (mut arr, mut arr_with_weird_len) = free();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^   ------ type must be known at this point
+   |
+note: required by a const generic parameter in `free`
+  --> $DIR/ambiguous-on-failed-eval-with-vars-fail.rs:24:9
+   |
+LL | fn free<const N: usize>() -> ([(); N], [(); FREE::<N>]) {
+   |         ^^^^^^^^^^^^^^ required by this const generic parameter in `free`
+help: consider giving this pattern a type, where the value of const parameter `N` is specified
+   |
+LL |     let (mut arr, mut arr_with_weird_len): ([_; N], _) = free();
+   |                                          +++++++++++++
+
+error[E0271]: type mismatch resolving `2 == 10`
+  --> $DIR/ambiguous-on-failed-eval-with-vars-fail.rs:35:45
+   |
+LL |     let (mut arr, mut arr_with_weird_len) = free();
+   |                                             ^^^^^^ types differ
+
+error[E0284]: type annotations needed for `([(); _], [(); 10])`
+  --> $DIR/ambiguous-on-failed-eval-with-vars-fail.rs:46:9
+   |
+LL |     let (mut arr, mut arr_with_weird_len) = proj();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^   ------ type must be known at this point
+   |
+note: required by a const generic parameter in `proj`
+  --> $DIR/ambiguous-on-failed-eval-with-vars-fail.rs:41:9
+   |
+LL | fn proj<const N: usize>() -> ([(); N], [(); <S as Trait>::PROJ::<N>]) {
+   |         ^^^^^^^^^^^^^^ required by this const generic parameter in `proj`
+help: consider giving this pattern a type, where the value of const parameter `N` is specified
+   |
+LL |     let (mut arr, mut arr_with_weird_len): ([_; N], _) = proj();
+   |                                          +++++++++++++
+
+error[E0271]: type mismatch resolving `2 == 10`
+  --> $DIR/ambiguous-on-failed-eval-with-vars-fail.rs:52:45
+   |
+LL |     let (mut arr, mut arr_with_weird_len) = proj();
+   |                                             ^^^^^^ types differ
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0271, E0284.
+For more information about an error, try `rustc --explain E0271`.

tests/ui/const-generics/gca/ambiguous-on-failed-eval-with-vars-fail.old.stderr

@@ -0,0 +1,10 @@
+error: `generic_const_exprs` requires -Znext-solver=globally to be enabled
+  --> $DIR/ambiguous-on-failed-eval-with-vars-fail.rs:9:12
+   |
+LL | #![feature(generic_const_args)]
+   |            ^^^^^^^^^^^^^^^^^^
+   |
+   = help: enable all of these features
+
+error: aborting due to 1 previous error
+