diff --git a/src/cmd/compile/internal/noder/reader.go b/src/cmd/compile/internal/noder/reader.go
index 82add23abd..e97cf4e6b6 100644
--- a/src/cmd/compile/internal/noder/reader.go
+++ b/src/cmd/compile/internal/noder/reader.go
@@ -449,6 +449,8 @@ func (r *reader) interfaceType() *types.Type {
 	tpkg := types.LocalPkg // TODO(mdempsky): Remove after iexport is gone.
 
 	nmethods, nembeddeds := r.Len(), r.Len()
+	implicit := nmethods == 0 && nembeddeds == 1 && r.Bool()
+	assert(!implicit) // implicit interfaces only appear in constraints
 
 	fields := make([]*types.Field, nmethods+nembeddeds)
 	methods, embeddeds := fields[:nmethods], fields[nmethods:]
diff --git a/src/cmd/compile/internal/noder/reader2.go b/src/cmd/compile/internal/noder/reader2.go
index a8960fdbec..8d1f9087a5 100644
--- a/src/cmd/compile/internal/noder/reader2.go
+++ b/src/cmd/compile/internal/noder/reader2.go
@@ -295,6 +295,7 @@ func (r *reader2) unionType() *types2.Union {
 func (r *reader2) interfaceType() *types2.Interface {
 	methods := make([]*types2.Func, r.Len())
 	embeddeds := make([]types2.Type, r.Len())
+	implicit := len(methods) == 0 && len(embeddeds) == 1 && r.Bool()
 
 	for i := range methods {
 		pos := r.pos()
@@ -307,7 +308,11 @@ func (r *reader2) interfaceType() *types2.Interface {
 		embeddeds[i] = r.typ()
 	}
 
-	return types2.NewInterfaceType(methods, embeddeds)
+	iface := types2.NewInterfaceType(methods, embeddeds)
+	if implicit {
+		iface.MarkImplicit()
+	}
+	return iface
 }
 
 func (r *reader2) signature(recv *types2.Var, rtparams, tparams []*types2.TypeParam) *types2.Signature {
diff --git a/src/cmd/compile/internal/noder/writer.go b/src/cmd/compile/internal/noder/writer.go
index 432a2a7195..59e9409b97 100644
--- a/src/cmd/compile/internal/noder/writer.go
+++ b/src/cmd/compile/internal/noder/writer.go
@@ -396,6 +396,15 @@ func (w *writer) interfaceType(typ *types2.Interface) {
 	w.Len(typ.NumExplicitMethods())
 	w.Len(typ.NumEmbeddeds())
 
+	if typ.NumExplicitMethods() == 0 && typ.NumEmbeddeds() == 1 {
+		w.Bool(typ.IsImplicit())
+	} else {
+		// Implicit interfaces always have 0 explicit methods and 1
+		// embedded type, so we skip writing out the implicit flag
+		// otherwise as a space optimization.
+		assert(!typ.IsImplicit())
+	}
+
 	for i := 0; i < typ.NumExplicitMethods(); i++ {
 		m := typ.ExplicitMethod(i)
 		sig := m.Type().(*types2.Signature)