checkers: add stringXbytes checker

Fixes go-critic#427

checkers/stringXbytes_checker.go

@@ -0,0 +1,47 @@
+package checkers
+
+import (
+	"go/ast"
+	"strings"
+
+	"github.com/go-lintpack/lintpack"
+	"github.com/go-lintpack/lintpack/astwalk"
+	"github.com/go-toolsmith/astfmt"
+	"github.com/go-toolsmith/typep"
+)
+
+func init() {
+	var info lintpack.CheckerInfo
+	info.Name = "stringXbytes"
+	info.Tags = []string{"diagnostic", "experimental"}
+	info.Summary = "Detects redundant conversions between string and []byte"
+	info.Before = `copy(b, []byte(s))`
+	info.After = `copy(b, s)`
+
+	collection.AddChecker(&info, func(ctx *lintpack.CheckerContext) lintpack.FileWalker {
+		return astwalk.WalkerForExpr(&stringXbytes{ctx: ctx})
+	})
+}
+
+type stringXbytes struct {
+	astwalk.WalkHandler
+	ctx *lintpack.CheckerContext
+}
+
+func (c *stringXbytes) VisitExpr(expr ast.Expr) {
+	if x, ok := expr.(*ast.CallExpr); ok && qualifiedName(x.Fun) == "copy" {
+		src := x.Args[1]
+
+		if byteCast, ok := src.(*ast.CallExpr); ok &&
+			typep.IsTypeExpr(c.ctx.TypesInfo, byteCast.Fun) &&
+			typep.HasStringProp(c.ctx.TypesInfo.TypeOf(byteCast.Args[0])) {
+
+			c.warn(byteCast, strings.TrimSuffix(strings.TrimPrefix(astfmt.Sprint(byteCast), "[]byte("), ")"))
+		}
+	}
+}
+
+func (c *stringXbytes) warn(cause *ast.CallExpr, suggestion string) {
+	c.ctx.Warn(cause, "can simplify `%s` to `%s`",
+		cause, suggestion)
+}

checkers/testdata/stringXbytes/negative_tests.go

@@ -0,0 +1,8 @@
+package checker_test
+
+func noWarnings() {
+	var b []byte
+	var s string
+
+	copy(b, s)
+}

checkers/testdata/stringXbytes/positive_tests.go

@@ -0,0 +1,9 @@
+package checker_test
+
+func warnings() {
+	var b []byte
+	var s string
+
+	/*! can simplify `[]byte(s)` to `s` */
+	copy(b, []byte(s))
+}