Export all types referenced through other exported APIs, enforce

_tools/customlint/plugin.go

@@ -17,6 +17,7 @@ func (f *plugin) BuildAnalyzers() ([]*analysis.Analyzer, error) {
 	return []*analysis.Analyzer{
 		emptyCaseAnalyzer,
 		shadowAnalyzer,
+		unexportedAPIAnalyzer,
 	}, nil
 }
 

_tools/customlint/testdata/shadow/shadow.go.golden

@@ -62,13 +62,17 @@
 	type isType int
 
 	func F5() isType {
+	          ~
+!!! unexportedapi: exported API references unexported identifier isType
 		var isType isType // OK
 		return isType
 	}
 
 	type isAlias int
 
 	func F6() isAlias {
+	          ~
+!!! unexportedapi: exported API references unexported identifier isAlias
 		var isAlias isAlias // OK
 		return isAlias
 	}

_tools/customlint/testdata/unexportedapi/unexportedapi.go

@@ -0,0 +1,211 @@
+package unexportedapi
+
+type Foo struct {
+	Bar *oops
+}
+
+type oops struct {
+	v int
+}
+
+type Okay struct {
+	Sure  int
+	Value ***Okay2
+}
+
+type Okay2 struct {
+	VeryGood struct{}
+}
+
+func OkayFunc(v *Okay) *Okay2 {
+	if v == nil {
+		return nil
+	}
+	return **v.Value
+}
+
+// Test cases for various scenarios
+
+// Exported function with unexported parameter type
+func BadFunc(x unexported) {}
+
+// Exported function with unexported return type
+func AnotherBadFunc() *unexported {
+	return nil
+}
+
+// Exported function with unexported type in slice
+func SliceFunc(x []unexported) {}
+
+// Exported function with unexported type in map
+func MapFunc(x map[string]unexported) {}
+
+// Exported function with unexported type in map key
+func MapKeyFunc(x map[unexported]string) {}
+
+// Exported function with unexported type in channel
+func ChanFunc(x chan unexported) {}
+
+// Exported type alias to unexported type
+type BadAlias = unexported
+
+// Exported type with unexported embedded field (OK since unexported has no exported members)
+type OkayEmbed struct {
+	unexported
+}
+
+// Unexported type with exported field
+type unexportedWithExportedField struct {
+	ExportedField int
+}
+
+// Bad - exported type embedding unexported type with exported members
+type BadEmbed struct {
+	unexportedWithExportedField
+}
+
+// Unexported type - should not trigger
+type okayUnexported struct {
+	field unexported
+}
+
+// Exported interface with unexported type in method
+type BadInterface interface {
+	Method(x unexported)
+}
+
+// Exported interface with unexported return type
+type AnotherBadInterface interface {
+	Method() unexported
+}
+
+type unexported struct {
+	x int
+}
+
+// Exported function with multiple return values including unexported
+func MultiReturn() (int, unexported, error) {
+	return 0, unexported{}, nil
+}
+
+// Exported variable with unexported type
+var BadVar unexported
+
+// Exported const with unexported type (should not be possible, but let's be safe)
+// const BadConst unexported = unexported{} // This won't compile anyway
+
+// Array of unexported type
+type BadArray [10]unexported
+
+// Exported function with variadic unexported parameter
+func VariadicFunc(args ...unexported) {}
+
+// Exported type with method returning unexported type
+type ExportedWithMethod struct{}
+
+func (e ExportedWithMethod) Method() unexported {
+	return unexported{}
+}
+
+// Exported type with pointer receiver method returning unexported type
+func (e *ExportedWithMethod) PointerMethod() *unexported {
+	return nil
+}
+
+// Generic type with unexported type constraint (Go 1.18+)
+type GenericExported[T any] struct {
+	Value T
+}
+
+// Okay - unexported method on exported type (methods are not part of exported API unless on exported interface)
+func (e ExportedWithMethod) unexportedMethod() unexported {
+	return unexported{}
+}
+
+// Test variables initialized with function calls
+
+// Helper functions for testing
+func helperReturnsExported() *Okay2 {
+	return &Okay2{}
+}
+
+func helperReturnsUnexported() unexported {
+	return unexported{}
+}
+
+// Okay - exported variable initialized by calling unexported function that returns exported type
+var OkayVarFromUnexportedFunc = helperReturnsExported()
+
+// Bad - exported variable initialized by calling exported function that returns unexported type
+var BadVarFromFunc = helperReturnsUnexported()
+
+// Okay - exported variable with explicit type (implementation doesn't matter)
+var OkayVarExplicitType *Okay2 = helperReturnsExported()
+
+// Bad - exported variable with explicit unexported type
+var BadVarExplicitType unexported = helperReturnsUnexported()
+
+// Test type aliases
+type (
+	ExportedString   string
+	unexportedString string
+)
+
+// Okay - exported function using exported type alias
+func OkayTypeAlias(s ExportedString) {}
+
+// Bad - exported function using unexported type alias
+func BadTypeAlias(s unexportedString) {}
+
+// Test unexported types with exported methods (for interface satisfaction)
+type unexportedImpl struct {
+	value int
+}
+
+// Okay - exported method on unexported type (not part of public API, often used for interface satisfaction)
+func (u *unexportedImpl) ExportedMethod() int {
+	return u.value
+}
+
+// Okay - exported method on unexported type can return unexported types
+func (u *unexportedImpl) AnotherMethod() unexported {
+	return unexported{}
+}
+
+// Test for avoiding duplicate errors on embedded types with methods
+
+type BaseWithBadMethod struct{}
+
+// This method has an unexported return type - should be flagged once
+func (b *BaseWithBadMethod) GetUnexported() *unexported {
+	return nil
+}
+
+// This type embeds BaseWithBadMethod - should NOT re-report the GetUnexported method issue
+type DerivedEmbedding struct {
+	BaseWithBadMethod
+}
+
+// Test embedding unexported type with exported method that references unexported type
+type unexportedBase struct{}
+
+// This exported method on unexported type won't be checked (unexported type methods are skipped)
+func (u *unexportedBase) MethodWithBadReturn() *unexported {
+	return nil
+}
+
+// This embeds an unexported type - what happens?
+// OK because methods on unexported types aren't checked
+type EmbeddingUnexportedBase struct {
+	unexportedBase
+}
+
+// Test embedding unexported type with exported field that references unexported type
+type unexportedBaseWithField struct {
+	ExportedField *unexported
+}
+
+// Bad - embeds unexported type with exported field that references unexported type
+type EmbeddingUnexportedBaseWithField struct {
+	unexportedBaseWithField
+}