Add facet type values and an instruction that produces them (#4460)

Still to do:
* Represent facet type values in a canonical form
* Produce & consume facet type values instead of interface values
* `type` should be associated with a canonical facet type value
* Support `&` on facet type values
* Type check and enforce requirements in facet types

---------

Co-authored-by: Josh L <josh11b@users.noreply.github.com>

toolchain/check/context.cpp

@@ -1060,8 +1060,8 @@ class TypeCompleter {
   template <typename InstT>
     requires(
         InstT::Kind
-            .template IsAnyOf<SemIR::AssociatedEntityType, SemIR::FunctionType,
-                              SemIR::GenericClassType,
+            .template IsAnyOf<SemIR::AssociatedEntityType, SemIR::FacetType,
+                              SemIR::FunctionType, SemIR::GenericClassType,
                               SemIR::GenericInterfaceType, SemIR::InterfaceType,
                               SemIR::UnboundElementType, SemIR::WhereExpr>())
   auto BuildValueReprForInst(SemIR::TypeId /*type_id*/, InstT /*inst*/) const

toolchain/check/eval.cpp

@@ -126,6 +126,9 @@ class EvalContext {
   auto interfaces() -> const ValueStore<SemIR::InterfaceId>& {
     return sem_ir().interfaces();
   }
+  auto facet_types() -> CanonicalValueStore<SemIR::FacetTypeId>& {
+    return sem_ir().facet_types();
+  }
   auto specifics() -> const SemIR::SpecificStore& {
     return sem_ir().specifics();
   }
@@ -1106,6 +1109,20 @@ static auto MakeConstantForCall(EvalContext& eval_context, SemIRLoc loc,
   return SemIR::ConstantId::NotConstant;
 }
 
+// Creates a FacetType constant.
+static auto MakeFacetTypeResult(Context& context,
+                                SemIR::TypeId base_facet_type_id,
+                                SemIR::InstBlockId requirement_block_id,
+                                Phase phase) -> SemIR::ConstantId {
+  SemIR::FacetTypeId facet_type_id = context.sem_ir().facet_types().Add(
+      SemIR::FacetTypeInfo{.base_facet_type_id = base_facet_type_id,
+                           .requirement_block_id = requirement_block_id});
+  return MakeConstantResult(context,
+                            SemIR::FacetType{.type_id = SemIR::TypeId::TypeType,
+                                             .facet_type_id = facet_type_id},
+                            phase);
+}
+
 // Implementation for `TryEvalInst`, wrapping `Context` with `EvalContext`.
 static auto TryEvalInstInContext(EvalContext& eval_context,
                                  SemIR::InstId inst_id, SemIR::Inst inst)
@@ -1287,6 +1304,23 @@ static auto TryEvalInstInContext(EvalContext& eval_context,
           Phase::Template);
     }
 
+    case CARBON_KIND(SemIR::FacetType facet_type): {
+      SemIR::FacetTypeInfo info =
+          eval_context.facet_types().Get(facet_type.facet_type_id);
+      Phase phase = Phase::Template;
+      SemIR::TypeId base_facet_type_id =
+          GetConstantValue(eval_context, info.base_facet_type_id, &phase);
+      // TODO: process & canonicalize requirements
+      SemIR::InstBlockId requirement_block_id = info.requirement_block_id;
+      // If nothing changed, can reuse this instruction.
+      if (base_facet_type_id == info.base_facet_type_id &&
+          requirement_block_id == info.requirement_block_id) {
+        return MakeConstantResult(eval_context.context(), inst, phase);
+      }
+      return MakeFacetTypeResult(eval_context.context(), base_facet_type_id,
+                                 requirement_block_id, phase);
+    }
+
     case CARBON_KIND(SemIR::InterfaceDecl interface_decl): {
       // If the interface has generic parameters, we don't produce an interface
       // type, but a callable whose return value is an interface type.
@@ -1445,10 +1479,15 @@ static auto TryEvalInstInContext(EvalContext& eval_context,
       return eval_context.GetConstantValue(typed_inst.facet_id);
     }
     case CARBON_KIND(SemIR::WhereExpr typed_inst): {
-      // TODO: This currently ignores the requirements and just produces the
-      // left-hand type argument to the `where`.
-      return eval_context.GetConstantValue(
-          eval_context.insts().Get(typed_inst.period_self_id).type_id());
+      SemIR::TypeId base_facet_type_id =
+          eval_context.insts().Get(typed_inst.period_self_id).type_id();
+      Phase phase = Phase::Template;
+      base_facet_type_id =
+          GetConstantValue(eval_context, base_facet_type_id, &phase);
+      SemIR::InstBlockId requirement_block_id = typed_inst.requirements_id;
+      // TODO: process & canonicalize requirements
+      return MakeFacetTypeResult(eval_context.context(), base_facet_type_id,
+                                 requirement_block_id, phase);
     }
 
     // `not true` -> `false`, `not false` -> `true`.

toolchain/check/handle_where.cpp

@@ -79,6 +79,7 @@ auto HandleParseNode(Context& context, Parse::RequirementEqualEqualId node_id)
   auto rhs = context.node_stack().PopExpr();
   auto lhs = context.node_stack().PopExpr();
   // TODO: type check lhs and rhs are comparable
+  // TODO: require that at least one side uses a designator
 
   // Build up the list of arguments for the `WhereExpr` inst.
   context.args_type_info_stack().AddInstId(
@@ -103,6 +104,7 @@ auto HandleParseNode(Context& context, Parse::RequirementImplsId node_id)
     context.emitter().Emit(rhs_node, ImplsOnNonFacetType);
     rhs_as_type.inst_id = SemIR::InstId::BuiltinError;
   }
+  // TODO: require that at least one side uses a designator
 
   // Build up the list of arguments for the `WhereExpr` inst.
   context.args_type_info_stack().AddInstId(

toolchain/check/testdata/impl/fail_todo_impl_assoc_const.carbon

@@ -10,7 +10,7 @@
 
 interface I { let T:! type; }
 
-// CHECK:STDERR: fail_todo_impl_assoc_const.carbon:[[@LINE+10]]:1: error: semantics TODO: `impl of interface with associated constant` [SemanticsTodo]
+// CHECK:STDERR: fail_todo_impl_assoc_const.carbon:[[@LINE+10]]:1: error: semantics TODO: `impl as non-interface` [SemanticsTodo]
 // CHECK:STDERR: impl bool as I where .T = bool {}
 // CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 // CHECK:STDERR:
@@ -50,6 +50,7 @@ impl bool as I where .T = bool {}
 // CHECK:STDOUT:   %.6: type = assoc_entity_type %ImplicitAs.type.3, %Convert.type.2 [template]
 // CHECK:STDOUT:   %.7: %.6 = assoc_entity element0, imports.%import_ref.6 [template]
 // CHECK:STDOUT:   %.8: %.4 = assoc_entity element0, imports.%import_ref.7 [symbolic]
+// CHECK:STDOUT:   %.9: type = facet_type <facet-type %I.type+requirements> [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {
@@ -90,7 +91,7 @@ impl bool as I where .T = bool {}
 // CHECK:STDOUT:     %.loc23_27.2: ref type = temporary_storage
 // CHECK:STDOUT:     %.loc23_27.3: ref type = temporary %.loc23_27.2, %bool.make_type.loc23_27
 // CHECK:STDOUT:     %.loc23_27.4: %.1 = converted %bool.make_type.loc23_27, <error> [template = <error>]
-// CHECK:STDOUT:     %.loc23_16: type = where_expr %.Self [template = constants.%I.type] {
+// CHECK:STDOUT:     %.loc23_16: type = where_expr %.Self [template = constants.%.9] {
 // CHECK:STDOUT:       requirement_rewrite %T.ref, <error>
 // CHECK:STDOUT:     }
 // CHECK:STDOUT:   }