Add staticcheck tool

vendor/honnef.co/go/tools/staticcheck/sa4006/sa4006.go

@@ -0,0 +1,155 @@
+package sa4006
+
+import (
+	"fmt"
+	"go/ast"
+
+	"honnef.co/go/tools/analysis/code"
+	"honnef.co/go/tools/analysis/facts/generated"
+	"honnef.co/go/tools/analysis/lint"
+	"honnef.co/go/tools/analysis/report"
+	"honnef.co/go/tools/go/ir"
+	"honnef.co/go/tools/go/ir/irutil"
+	"honnef.co/go/tools/internal/passes/buildir"
+
+	"golang.org/x/tools/go/analysis"
+)
+
+var SCAnalyzer = lint.InitializeAnalyzer(&lint.Analyzer{
+	Analyzer: &analysis.Analyzer{
+		Name:     "SA4006",
+		Run:      run,
+		Requires: []*analysis.Analyzer{buildir.Analyzer, generated.Analyzer},
+	},
+	Doc: &lint.RawDocumentation{
+		Title:    `A value assigned to a variable is never read before being overwritten. Forgotten error check or dead code?`,
+		Since:    "2017.1",
+		Severity: lint.SeverityWarning,
+		MergeIf:  lint.MergeIfAll,
+	},
+})
+
+var Analyzer = SCAnalyzer.Analyzer
+
+func run(pass *analysis.Pass) (any, error) {
+	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
+		if irutil.IsExample(fn) {
+			continue
+		}
+		node := fn.Source()
+		if node == nil {
+			continue
+		}
+		if gen, ok := code.Generator(pass, node.Pos()); ok && gen == generated.Goyacc {
+			// Don't flag unused values in code generated by goyacc.
+			// There may be hundreds of those due to the way the state
+			// machine is constructed.
+			continue
+		}
+
+		switchTags := map[ir.Value]struct{}{}
+		ast.Inspect(node, func(node ast.Node) bool {
+			s, ok := node.(*ast.SwitchStmt)
+			if !ok {
+				return true
+			}
+			v, _ := fn.ValueForExpr(s.Tag)
+			switchTags[v] = struct{}{}
+			return true
+		})
+
+		// OPT(dh): don't use a map, possibly use a bitset
+		var hasUse func(v ir.Value, seen map[ir.Value]struct{}) bool
+		hasUse = func(v ir.Value, seen map[ir.Value]struct{}) bool {
+			if _, ok := seen[v]; ok {
+				return false
+			}
+			if _, ok := switchTags[v]; ok {
+				return true
+			}
+			refs := v.Referrers()
+			if refs == nil {
+				// TODO investigate why refs can be nil
+				return true
+			}
+			for _, ref := range *refs {
+				switch ref := ref.(type) {
+				case *ir.DebugRef:
+				case *ir.Sigma:
+					if seen == nil {
+						seen = map[ir.Value]struct{}{}
+					}
+					seen[v] = struct{}{}
+					if hasUse(ref, seen) {
+						return true
+					}
+				case *ir.Phi:
+					if seen == nil {
+						seen = map[ir.Value]struct{}{}
+					}
+					seen[v] = struct{}{}
+					if hasUse(ref, seen) {
+						return true
+					}
+				default:
+					return true
+				}
+			}
+			return false
+		}
+
+		ast.Inspect(node, func(node ast.Node) bool {
+			assign, ok := node.(*ast.AssignStmt)
+			if !ok {
+				return true
+			}
+			if len(assign.Lhs) > 1 && len(assign.Rhs) == 1 {
+				// Either a function call with multiple return values,
+				// or a comma-ok assignment
+
+				val, _ := fn.ValueForExpr(assign.Rhs[0])
+				if val == nil {
+					return true
+				}
+				refs := val.Referrers()
+				if refs == nil {
+					return true
+				}
+				for _, ref := range *refs {
+					ex, ok := ref.(*ir.Extract)
+					if !ok {
+						continue
+					}
+					if !hasUse(ex, nil) {
+						lhs := assign.Lhs[ex.Index]
+						if ident, ok := lhs.(*ast.Ident); !ok || ok && ident.Name == "_" {
+							continue
+						}
+						report.Report(pass, assign, fmt.Sprintf("this value of %s is never used", lhs))
+					}
+				}
+				return true
+			}
+			for i, lhs := range assign.Lhs {
+				rhs := assign.Rhs[i]
+				if ident, ok := lhs.(*ast.Ident); !ok || ok && ident.Name == "_" {
+					continue
+				}
+				val, _ := fn.ValueForExpr(rhs)
+				if val == nil {
+					continue
+				}
+
+				if _, ok := val.(*ir.Const); ok {
+					// a zero-valued constant, for example in 'foo := []string(nil)'
+					continue
+				}
+				if !hasUse(val, nil) {
+					report.Report(pass, assign, fmt.Sprintf("this value of %s is never used", lhs))
+				}
+			}
+			return true
+		})
+	}
+	return nil, nil
+}