feat: add builtin attribute to support elaboration of mutual inductives/structures (#6166)

This PR is a prerequisite for #6125.

src/Lean/Elab.lean

@@ -23,6 +23,7 @@ import Lean.Elab.Quotation
 import Lean.Elab.Syntax
 import Lean.Elab.Do
 import Lean.Elab.StructInst
+import Lean.Elab.MutualInductive
 import Lean.Elab.Inductive
 import Lean.Elab.Structure
 import Lean.Elab.Print

src/Lean/Elab/MutualInductive.lean

@@ -0,0 +1,47 @@
+/-
+Copyright (c) 2024 Lean FRO, LLC. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Leonardo de Moura, Kyle Miller
+-/
+prelude
+import Lean.Util.ForEachExprWhere
+import Lean.Util.ReplaceLevel
+import Lean.Util.ReplaceExpr
+import Lean.Util.CollectLevelParams
+import Lean.Meta.Constructions
+import Lean.Meta.CollectFVars
+import Lean.Meta.SizeOf
+import Lean.Meta.Injective
+import Lean.Meta.IndPredBelow
+import Lean.Elab.Command
+import Lean.Elab.ComputedFields
+import Lean.Elab.DefView
+import Lean.Elab.DeclUtil
+import Lean.Elab.Deriving.Basic
+import Lean.Elab.DeclarationRange
+
+/-!
+# Elaborator framework for (mutual) inductives
+
+Note(kmill): This file currently is a stub for adding a builtin attribute for https://github.com/leanprover/lean4/pull/6125
+
+-/
+
+namespace Lean.Elab.Command
+open Meta
+
+structure InductiveElabDescr where
+  deriving Inhabited
+
+/--
+Environment extension to register inductive type elaborator commands.
+-/
+builtin_initialize inductiveElabAttr : KeyedDeclsAttribute InductiveElabDescr ←
+  unsafe KeyedDeclsAttribute.init {
+    builtinName := `builtin_inductive_elab,
+    name := `inductive_elab,
+    descr    := "Register an elaborator for inductive types",
+    valueTypeName := ``InductiveElabDescr
+  }
+
+end Lean.Elab.Command