go-critic · quasilyte · Aug 16, 2020 · Aug 16, 2020 · Aug 16, 2020 · Aug 16, 2020
diff --git a/checkers/testdata/typeDefFirst/negative_tests.go b/checkers/testdata/typeDefFirst/negative_tests.go
@@ -0,0 +1,11 @@
+package checker_test
+
+type rec struct{}
+
+func (r rec) Method()         {}
+func (r *rec) MethodWithRef() {}
+
+// Considering no type declaration in file it is also ok
+func (r recv) Method2() {}
+
+func JustFunction() {}
diff --git a/checkers/testdata/typeDefFirst/positive_tests.go b/checkers/testdata/typeDefFirst/positive_tests.go
@@ -0,0 +1,6 @@
+package checker_test
+
+func (r recv) MethodBefore() {}
+
+/*! definition of type 'recv' should appear before its methods */
+type recv struct{}
diff --git a/checkers/testdata/typeDefFirst/positive_tests2.go b/checkers/testdata/typeDefFirst/positive_tests2.go
@@ -0,0 +1,6 @@
+package checker_test
+
+func (r *reciv) Method() {}
+
+/*! definition of type 'reciv' should appear before its methods */
+type reciv struct{ x, y int }
diff --git a/checkers/typeDefFirst_checker.go b/checkers/typeDefFirst_checker.go
@@ -0,0 +1,80 @@
+package checkers
+
+import (
+	"go/ast"
+	"go/token"
+
+	"github.com/go-critic/go-critic/checkers/internal/astwalk"
+	"github.com/go-critic/go-critic/framework/linter"
+)
+
+func init() {
+	var info linter.CheckerInfo
+	info.Name = "typeDefFirst"
+	info.Tags = []string{"style", "experimental"}
+	info.Summary = "File-scoped checker, that requires type definition before its method definitions"
+	info.Before = `
+func (r rec) Method() {}
+type rec struct{}
+`
+	info.After = `
+type rec struct{}
+func (r rec) Method() {}
+`
+	collection.AddChecker(&info, func(ctx *linter.CheckerContext) linter.FileWalker {
+		return &typeDefFirstChecker{
+			ctx: ctx,
+		}
+	})
+}
+
+type typeDefFirstChecker struct {
+	astwalk.WalkHandler
+	ctx *linter.CheckerContext
+}
+
+func (c *typeDefFirstChecker) WalkFile(f *ast.File) {
+	if f.Decls == nil {
+		return
+	}
+
+	typeUsageMap := make(map[string]bool)
+	for _, declaration := range f.Decls {
+		switch decl := declaration.(type) {
+		case *ast.FuncDecl:
+			if decl.Recv != nil {
+				receiver := decl.Recv.List[0]
+				typeName := c.receiverType(receiver.Type)
+				typeUsageMap[typeName] = true
+			}
+
+		case *ast.GenDecl:
+			if decl.Tok != token.TYPE {
+				continue
+			}
+			for _, spec := range decl.Specs {
+				if spec, ok := spec.(*ast.TypeSpec); ok {
+					typeName := spec.Name.Name
+					if val, ok := typeUsageMap[typeName]; ok && val {
+						c.warn(decl, typeName)
+					}
+				}
+			}
+		}
+	}
+}
+
+func (c *typeDefFirstChecker) warn(cause ast.Node, typeName string) {
+	c.ctx.Warn(cause, "definition of type '%s' should appear before its methods", typeName)
+}
+
+func (c *typeDefFirstChecker) receiverType(e ast.Expr) string {
+	switch e := e.(type) {
+	case *ast.StarExpr:
+		return c.receiverType(e.X)
+	case *ast.Ident:
+		return e.Name
+	default:
+		panic("unreachable")
+	}
+}