Add Unnecessary Type Conversion Lint Rule

formatter/fmt.go

@@ -10,8 +10,9 @@ import (
 
 // rule set
 const (
-	UnnecessaryElse   = "unnecessary-else"
-	SimplifySliceExpr = "simplify-slice-range"
+	UnnecessaryElse     = "unnecessary-else"
+	UnnecessaryTypeConv = "unnecessary-type-conversion"
+	SimplifySliceExpr   = "simplify-slice-range"
 )
 
 // IssueFormatter is the interface that wraps the Format method.
@@ -41,6 +42,8 @@ func getFormatter(rule string) IssueFormatter {
 		return &UnnecessaryElseFormatter{}
 	case SimplifySliceExpr:
 		return &SimplifySliceExpressionFormatter{}
+	case UnnecessaryTypeConv:
+		return &UnnecessaryTypeConversionFormatter{}
 	default:
 		return &GeneralIssueFormatter{}
 	}

formatter/fmt_test.go

@@ -296,3 +296,43 @@ func main() {
 		})
 	}
 }
+
+func TestUnnecessaryTypeConversionFormatter(t *testing.T) {
+	formatter := &UnnecessaryTypeConversionFormatter{}
+
+	issue := internal.Issue{
+		Rule:       "unnecessary-type-conversion",
+		Filename:   "test.go",
+		Start:      token.Position{Line: 5, Column: 10},
+		End:        token.Position{Line: 5, Column: 20},
+		Message:    "unnecessary type conversion",
+		Suggestion: "Remove the type conversion. Change `int(myInt)` to just `myInt`.",
+		Note:       "Unnecessary type conversions can make the code less readable and may slightly impact performance. They are safe to remove when the expression already has the desired type.",
+	}
+
+	snippet := &internal.SourceCode{
+		Lines: []string{
+			"package main",
+			"",
+			"func main() {",
+			"    myInt := 42",
+			"    result := int(myInt)",
+			"}",
+		},
+	}
+
+	expected := `  |
+5 |     result := int(myInt)
+  |          ^ unnecessary type conversion
+
+Suggestion:
+5 | Remove the type conversion. Change ` + "`int(myInt)`" + ` to just ` + "`myInt`" + `.
+
+Note: Unnecessary type conversions can make the code less readable and may slightly impact performance. They are safe to remove when the expression already has the desired type.
+
+`
+
+	result := formatter.Format(issue, snippet)
+
+	assert.Equal(t, expected, result, "Formatted output should match expected output")
+}

formatter/unnecessary_type_conv.go

@@ -0,0 +1,35 @@
+package formatter
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/gnoswap-labs/lint/internal"
+)
+
+type UnnecessaryTypeConversionFormatter struct{}
+
+func (f *UnnecessaryTypeConversionFormatter) Format(
+	issue internal.Issue,
+	snippet *internal.SourceCode,
+) string {
+	var result strings.Builder
+
+	lineNumberStr := fmt.Sprintf("%d", issue.Start.Line)
+	padding := strings.Repeat(" ", len(lineNumberStr)-1)
+	result.WriteString(lineStyle.Sprintf("  %s|\n", padding))
+
+	line := expandTabs(snippet.Lines[issue.Start.Line-1])
+	result.WriteString(lineStyle.Sprintf("%d | ", issue.Start.Line))
+	result.WriteString(line + "\n")
+
+	visualColumn := calculateVisualColumn(line, issue.Start.Column)
+	result.WriteString(lineStyle.Sprintf("  %s| ", padding))
+	result.WriteString(strings.Repeat(" ", visualColumn))
+	result.WriteString(messageStyle.Sprintf("^ %s\n\n", issue.Message))
+
+	buildSuggestion(&result, issue, lineStyle, suggestionStyle)
+	buildNote(&result, issue, suggestionStyle)
+
+	return result.String()
+}

internal/engine.go

@@ -33,6 +33,7 @@ func (e *Engine) registerDefaultRules() {
 		&UnnecessaryElseRule{},
 		&UnusedFunctionRule{},
 		&SimplifySliceExprRule{},
+		&UnnecessaryConversionRule{},
 	)
 }
 

internal/lint.go

@@ -4,8 +4,10 @@ import (
 	"encoding/json"
 	"fmt"
 	"go/ast"
+	"go/importer"
 	"go/parser"
 	"go/token"
+	"go/types"
 	"os/exec"
 )
 
@@ -226,3 +228,219 @@ func (e *Engine) detectUnnecessarySliceLength(filename string) ([]Issue, error)
 
 	return issues, nil
 }
+
+type UnnecessaryConversionRule struct{}
+
+func (r *UnnecessaryConversionRule) Check(filename string) ([]Issue, error) {
+	engine := &Engine{}
+	return engine.detectUnnecessaryConversions(filename)
+}
+
+func (e *Engine) detectUnnecessaryConversions(filename string) ([]Issue, error) {
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, filename, nil, parser.ParseComments)
+	if err != nil {
+		return nil, err
+	}
+
+	info := &types.Info{
+		Types: make(map[ast.Expr]types.TypeAndValue),
+		Uses:  make(map[*ast.Ident]types.Object),
+		Defs:  make(map[*ast.Ident]types.Object),
+	}
+
+	conf := types.Config{Importer: importer.Default()}
+	//! DO NOT CHECK ERROR HERE.
+	//! error check may broke the lint formatting process.
+	conf.Check("", fset, []*ast.File{f}, info)
+
+	var issues []Issue
+	varDecls := make(map[*types.Var]ast.Node)
+
+	// First pass: collect variable declarations
+	ast.Inspect(f, func(n ast.Node) bool {
+		switch node := n.(type) {
+		case *ast.ValueSpec:
+			for _, name := range node.Names {
+				if obj := info.Defs[name]; obj != nil {
+					if v, ok := obj.(*types.Var); ok {
+						varDecls[v] = node
+					}
+				}
+			}
+		case *ast.AssignStmt:
+			for _, lhs := range node.Lhs {
+				if id, ok := lhs.(*ast.Ident); ok {
+					if obj := info.Defs[id]; obj != nil {
+						if v, ok := obj.(*types.Var); ok {
+							varDecls[v] = node
+						}
+					}
+				}
+			}
+		}
+		return true
+	})
+
+	// Second pass: check for unnecessary conversions
+	ast.Inspect(f, func(n ast.Node) bool {
+		call, ok := n.(*ast.CallExpr)
+		if !ok || len(call.Args) != 1 {
+			return true
+		}
+
+		ft, ok := info.Types[call.Fun]
+		if !ok || !ft.IsType() {
+			return true
+		}
+
+		at, ok := info.Types[call.Args[0]]
+		if !ok {
+			return true
+		}
+
+		if types.Identical(ft.Type, at.Type) && !isUntypedValue(call.Args[0], info) {
+			var memo, suggestion string
+
+			// find parent node and retrieve the entire assignment statement
+			var parent ast.Node
+			ast.Inspect(f, func(node ast.Node) bool {
+				if node == n {
+					return false
+				}
+				if contains(node, n) {
+					parent = node
+					return false
+				}
+				return true
+			})
+
+			if assignStmt, ok := parent.(*ast.AssignStmt); ok {
+				if len(assignStmt.Lhs) > 0 {
+					lhs := types.ExprString(assignStmt.Lhs[0])
+					rhs := types.ExprString(call.Args[0])
+					suggestion = fmt.Sprintf("%s := %s", lhs, rhs)
+				}
+			} else {
+				// not an assignment statement
+				// keep maintaining the original code
+				suggestion = types.ExprString(call.Args[0])
+			}
+
+			if id, ok := call.Args[0].(*ast.Ident); ok {
+				if obj, ok := info.Uses[id].(*types.Var); ok {
+					if _, exists := varDecls[obj]; exists {
+						declType := obj.Type().String()
+						memo = fmt.Sprintf(
+							"The variable '%s' is declared as type '%s'. This type conversion appears unnecessary.",
+							id.Name, declType,
+						)
+					}
+				}
+			}
+
+			issues = append(issues, Issue{
+				Rule:       "unnecessary-type-conversion",
+				Filename:   filename,
+				Start:      fset.Position(call.Pos()),
+				End:        fset.Position(call.End()),
+				Message:    "unnecessary type conversion",
+				Suggestion: suggestion,
+				Note:       memo,
+			})
+		}
+
+		return true
+	})
+
+	return issues, nil
+}
+
+// ref: https://github.com/mdempsky/unconvert/blob/master/unconvert.go#L570
+func isUntypedValue(n ast.Expr, info *types.Info) (res bool) {
+	switch n := n.(type) {
+	case *ast.BinaryExpr:
+		switch n.Op {
+		case token.SHL, token.SHR:
+			// Shifts yield an untyped value if their LHS is untyped.
+			return isUntypedValue(n.X, info)
+		case token.EQL, token.NEQ, token.LSS, token.GTR, token.LEQ, token.GEQ:
+			// Comparisons yield an untyped boolean value.
+			return true
+		case token.ADD, token.SUB, token.MUL, token.QUO, token.REM,
+			token.AND, token.OR, token.XOR, token.AND_NOT,
+			token.LAND, token.LOR:
+			return isUntypedValue(n.X, info) && isUntypedValue(n.Y, info)
+		}
+	case *ast.UnaryExpr:
+		switch n.Op {
+		case token.ADD, token.SUB, token.NOT, token.XOR:
+			return isUntypedValue(n.X, info)
+		}
+	case *ast.BasicLit:
+		// Basic literals are always untyped.
+		return true
+	case *ast.ParenExpr:
+		return isUntypedValue(n.X, info)
+	case *ast.SelectorExpr:
+		return isUntypedValue(n.Sel, info)
+	case *ast.Ident:
+		if obj, ok := info.Uses[n]; ok {
+			if obj.Pkg() == nil && obj.Name() == "nil" {
+				// The universal untyped zero value.
+				return true
+			}
+			if b, ok := obj.Type().(*types.Basic); ok && b.Info()&types.IsUntyped != 0 {
+				// Reference to an untyped constant.
+				return true
+			}
+		}
+	case *ast.CallExpr:
+		if b, ok := asBuiltin(n.Fun, info); ok {
+			switch b.Name() {
+			case "real", "imag":
+				return isUntypedValue(n.Args[0], info)
+			case "complex":
+				return isUntypedValue(n.Args[0], info) && isUntypedValue(n.Args[1], info)
+			}
+		}
+	}
+
+	return false
+}
+
+func asBuiltin(n ast.Expr, info *types.Info) (*types.Builtin, bool) {
+	for {
+		paren, ok := n.(*ast.ParenExpr)
+		if !ok {
+			break
+		}
+		n = paren.X
+	}
+
+	ident, ok := n.(*ast.Ident)
+	if !ok {
+		return nil, false
+	}
+
+	obj, ok := info.Uses[ident]
+	if !ok {
+		return nil, false
+	}
+
+	b, ok := obj.(*types.Builtin)
+	return b, ok
+}
+
+// contains checks if parent contains child node
+func contains(parent, child ast.Node) bool {
+	found := false
+	ast.Inspect(parent, func(n ast.Node) bool {
+		if n == child {
+			found = true
+			return false
+		}
+		return true
+	})
+	return found
+}