feat: structure diamonds basic support

See TODO in the new comments.

src/Lean/Elab/Structure.lean

@@ -7,6 +7,7 @@ import Lean.Parser.Command
 import Lean.Meta.Closure
 import Lean.Meta.SizeOf
 import Lean.Meta.Injective
+import Lean.Meta.Structure
 import Lean.Elab.Command
 import Lean.Elab.DeclModifiers
 import Lean.Elab.DeclUtil
@@ -227,23 +228,30 @@ private def validStructType (type : Expr) : Bool :=
   | Expr.sort .. => true
   | _            => false
 
-private def checkParentIsStructure (parent : Expr) : TermElabM Name :=
-  match parent.getAppFn with
-  | Expr.const c _ _ => do
-    unless isStructure (← getEnv) c do
-      throwError "'{c}' is not a structure"
-    pure c
-  | _ => throwError "expected structure"
-
 private def findFieldInfo? (infos : Array StructFieldInfo) (fieldName : Name) : Option StructFieldInfo :=
   infos.find? fun info => info.name == fieldName
 
 private def containsFieldName (infos : Array StructFieldInfo) (fieldName : Name) : Bool :=
   (findFieldInfo? infos fieldName).isSome
 
+register_builtin_option structureDiamondWarning : Bool := {
+  defValue := true -- TODO: set as false after finishing support for diamonds
+  descr    := "enable/disable warning messages for structure diamonds"
+}
+
+/-- Return `some fieldName` if field `fieldName` of the parent structure `parentStructName` is already in `infos` -/
+private def findExistingField? (infos : Array StructFieldInfo) (parentStructName : Name) : CoreM (Option Name) := do
+  let fieldNames := getStructureFieldsFlattened (← getEnv) parentStructName
+  for fieldName in fieldNames do
+    if containsFieldName infos fieldName then
+      return some fieldName
+  return none
+
 private partial def processSubfields (structDeclName : Name) (parentFVar : Expr) (parentStructName : Name) (subfieldNames : Array Name)
     (infos : Array StructFieldInfo) (k : Array StructFieldInfo → TermElabM α) : TermElabM α :=
-  let rec loop (i : Nat) (infos : Array StructFieldInfo) := do
+  go 0 infos
+where
+  go (i : Nat) (infos : Array StructFieldInfo) := do
     if h : i < subfieldNames.size then
       let subfieldName := subfieldNames.get ⟨i, h⟩
       if containsFieldName infos subfieldName then
@@ -255,28 +263,65 @@ private partial def processSubfields (structDeclName : Name) (parentFVar : Expr)
            its default value is overwritten in the structure. -/
         let declName := structDeclName ++ subfieldName
         let infos := infos.push { name := subfieldName, declName, fvar := subfieldFVar, kind := StructFieldKind.fromParent }
-        loop (i+1) infos
+        go (i+1) infos
+    else
+      k infos
+
+private partial def copyNewFieldsFrom (view : StructView) (infos : Array StructFieldInfo) (parent : Expr) (parentStructName : Name) (k : Array StructFieldInfo → TermElabM α) : TermElabM α := do
+  let fieldNames := getStructureFieldsFlattened (← getEnv) parentStructName
+  let rec go (i : Nat) (infos : Array StructFieldInfo) : TermElabM α := do
+    if h : i < fieldNames.size then
+      let fieldName := fieldNames.get ⟨i, h⟩
+      let fieldType ← getFieldType parent fieldName
+      match (← findFieldInfo? infos fieldName) with
+      | some existingFieldInfo =>
+        let existingFieldType ← inferType existingFieldInfo.fvar
+        unless (← isDefEq fieldType existingFieldType) do
+          throwError "parent field type mismatch, field '{fieldName}' from parent '{parentStructName}' {← mkHasTypeButIsExpectedMsg fieldType existingFieldType}"
+        go (i+1) infos
+      | none =>
+        /- TODO: we are ignoring the following information from the `fieldName` declaraion at `parentStructName`.
+           - Binder annotation
+           - Visibility annotation (private/protected)
+           - `inferMod`
+           - Default value.
+         -/
+        withLocalDeclD fieldName fieldType fun fieldFVar => do
+          let fieldDeclName := view.declName ++ fieldName
+          let infos := infos.push { name := fieldName, declName := fieldDeclName, fvar := fieldFVar, value? := none,
+                                    kind := StructFieldKind.newField, inferMod := false }
+          go (i+1) infos
+    else
+      k infos
+  go 0 infos
+
+private partial def withParents (view : StructView) (k : Array StructFieldInfo → TermElabM α) : TermElabM α := do
+  go 0 #[]
+where
+  go (i : Nat) (infos : Array StructFieldInfo) : TermElabM α := do
+    if h : i < view.parents.size then
+      let parentStx := view.parents.get ⟨i, h⟩
+      withRef parentStx do
+      let parent ← Term.elabType parentStx
+      let parentStructName ← getStructureName parent
+      if let some existingFieldName ← findExistingField? infos parentStructName then
+        if structureDiamondWarning.get (← getOptions) then
+          logWarning s!"field '{existingFieldName}' from '{parentStructName}' has already been declared"
+        copyNewFieldsFrom view infos parent parentStructName fun infos => go (i+1) infos
+        -- TODO: if `class`, then we need to create a let-decl that stores the local instance for the `parentStructure`
+      else
+        let toParentName := Name.mkSimple $ "to" ++ parentStructName.eraseMacroScopes.getString! -- erase macro scopes?
+        if containsFieldName infos toParentName then
+          throwErrorAt parentStx "field '{toParentName}' has already been declared"
+        let env ← getEnv
+        let binfo := if view.isClass && isClass env parentStructName then BinderInfo.instImplicit else BinderInfo.default
+        withLocalDecl toParentName binfo parent fun parentFVar =>
+          let infos := infos.push { name := toParentName, declName := view.declName ++ toParentName, fvar := parentFVar, kind := StructFieldKind.subobject }
+          let subfieldNames := getStructureFieldsFlattened env parentStructName
+          processSubfields view.declName parentFVar parentStructName subfieldNames infos fun infos => go (i+1) infos
     else
       k infos
-  loop 0 infos
 
-private partial def withParents (view : StructView) (i : Nat) (infos : Array StructFieldInfo) (k : Array StructFieldInfo → TermElabM α) : TermElabM α := do
-  if h : i < view.parents.size then
-    let parentStx := view.parents.get ⟨i, h⟩
-    withRef parentStx do
-    let parent ← Term.elabType parentStx
-    let parentName ← checkParentIsStructure parent
-    let toParentName := Name.mkSimple $ "to" ++ parentName.eraseMacroScopes.getString! -- erase macro scopes?
-    if containsFieldName infos toParentName then
-      throwErrorAt parentStx "field '{toParentName}' has already been declared"
-    let env ← getEnv
-    let binfo := if view.isClass && isClass env parentName then BinderInfo.instImplicit else BinderInfo.default
-    withLocalDecl toParentName binfo parent fun parentFVar =>
-      let infos := infos.push { name := toParentName, declName := view.declName ++ toParentName, fvar := parentFVar, kind := StructFieldKind.subobject }
-      let subfieldNames := getStructureFieldsFlattened env parentName
-      processSubfields view.declName parentFVar parentName subfieldNames infos fun infos => withParents view (i+1) infos k
-  else
-    k infos
 
 private def elabFieldTypeValue (view : StructFieldView) : TermElabM (Option Expr × Option Expr) := do
   Term.withAutoBoundImplicit <| Term.elabBinders view.binders.getArgs fun params => do
@@ -516,7 +561,7 @@ private def elabStructureView (view : StructView) : TermElabM Unit := do
   let type ← Term.elabType view.type
   unless validStructType type do throwErrorAt view.type "expected Type"
   withRef view.ref do
-  withParents view 0 #[] fun fieldInfos =>
+  withParents view fun fieldInfos =>
   withFields view.fields 0 fieldInfos fun fieldInfos => do
     Term.synthesizeSyntheticMVarsNoPostponing
     let u ← getResultUniverse type
@@ -554,6 +599,7 @@ private def elabStructureView (view : StructView) : TermElabM Unit := do
         let projInstances := instParents.toList.map fun info => info.declName
         Term.applyAttributesAt view.declName view.modifiers.attrs AttributeApplicationTime.afterTypeChecking
         projInstances.forM fun declName => addInstance declName AttributeKind.global (eval_prio default)
+        -- TODO: we must create `to` functions for the parent structures that have been flattened, and mark them as instances (if class)
         let lctx ← getLCtx
         let fieldsWithDefault := fieldInfos.filter fun info => info.value?.isSome
         let defaultAuxDecls ← fieldsWithDefault.mapM fun info => do

tests/lean/diamond1.lean

@@ -0,0 +1,27 @@
+structure Bar (α : Type) where
+  a : α
+  b : Nat → α
+
+structure Baz (α : Type) where
+  a : α → α
+  c : Bool → α
+  d : Nat
+
+set_option structureDiamondWarning false in
+structure Foo (α : Type) extends Bar α, Baz α -- Error: parent field type mismatch
+
+set_option structureDiamondWarning false in
+structure Foo (α : Type) extends Bar (α → α), Baz α
+
+#print Foo
+
+def f (x : Nat) : Foo Nat :=
+  { a := fun y => x + y
+    b := (. + .)
+    c := fun _ => x
+    d := x }
+
+#print f
+
+set_option structureDiamondWarning true in
+structure Foo' (α : Type) extends Bar (α → α), Baz α -- Warning: parent field duplication

tests/lean/diamond1.lean.expected.out

@@ -0,0 +1,10 @@
+diamond1.lean:11:40-11:45: error: parent field type mismatch, field 'a' from parent 'Baz' has type
+  α → α : Type
+but is expected to have type
+  α : Type
+inductive Foo : Type → Type
+constructors:
+Foo.mk : {α : Type} → Bar (α → α) → (Bool → α) → Nat → Foo α
+def f : Nat → Foo Nat :=
+fun x => { toBar := { a := fun y => x + y, b := fun a a_1 => a + a_1 }, c := fun x_1 => x, d := x }
+diamond1.lean:27:47-27:52: warning: field 'a' from 'Baz' has already been declared

tests/lean/struct1.lean

@@ -11,8 +11,8 @@ structure S : Nat := -- error expected Type
 
 structure S extends Nat → Nat := -- error expected structure
 (x : Nat)
-
-structure S extends A Nat, A Bool := -- error field toA already declared
+set_option structureDiamondWarning true in
+structure S' extends A Nat, A Bool := -- error field toA already declared
 (x : Nat)
 
 structure S extends A Nat, B Bool := -- error field `x` from `B` has already been declared

tests/lean/struct1.lean.expected.out

@@ -1,7 +1,15 @@
 struct1.lean:9:14-9:17: error: expected Type
 struct1.lean:12:20-12:29: error: expected structure
-struct1.lean:15:27-15:33: error: field 'toA' has already been declared
-struct1.lean:18:27-18:33: error: field 'x' from 'B' has already been declared
+struct1.lean:15:28-15:34: warning: field 'x' from 'A' has already been declared
+struct1.lean:15:28-15:34: error: parent field type mismatch, field 'x' from parent 'A' has type
+  Bool : Type
+but is expected to have type
+  Nat : Type
+struct1.lean:18:27-18:33: warning: field 'x' from 'B' has already been declared
+struct1.lean:18:27-18:33: error: parent field type mismatch, field 'x' from parent 'B' has type
+  Bool : Type
+but is expected to have type
+  Nat : Type
 struct1.lean:29:1-29:2: error: field 'x' has already been declared
 struct1.lean:32:1-32:2: error: field 'x' has been declared in parent structure
 struct1.lean:35:6-35:10: error: type mismatch