go/tools: add vet check for time formats with 2006-02-01

yyyy-dd-mm is a time format that isn't really used anywhere [1]. It is
much more likely that the user intended to use yyyy-mm-dd instead and
made a mistake. This happens quite often [2] because of the unusual way
to handle time formatting and parsing in Go. Since the mistake is Go
specific and happens so often a vet check will be useful.

1. https://stackoverflow.com/questions/2254014/are-there-locales-or-common-programs-that-use-yyyy-dd-mm-as-the-date-format
2. https://github.com/search?l=&p=1&q=%222006-02-01%22+language%3AGo&type=Code

Author: erikdubbelboer

go/analysis/passes/timeformat/testdata/src/a/a.go

@@ -0,0 +1,22 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains tests for the timeformat checker.
+
+package a
+
+import (
+	"time"
+)
+
+func hasError() {
+	a, _ := time.Parse("2006-02-01 15:04:05", "2021-01-01 00:00:00") // want `2006-02-01 should be 2006-01-02`
+	a.Format("2006-02-01")                                           // want `2006-02-01 should be 2006-01-02`
+	a.Format("2006-02-01 15:04:05")                                  // want `2006-02-01 should be 2006-01-02`
+}
+
+func notHasError() {
+	a, _ := time.Parse("2006-01-02 15:04:05", "2021-01-01 00:00:00")
+	a.Format("2006-01-02")
+}

go/analysis/passes/timeformat/timeformat.go

@@ -0,0 +1,66 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package timeformat defines an Analyzer that checks for the use
+// of time.Format or time.Parse calls with a bad format.
+package timeformat
+
+import (
+	"go/ast"
+	"go/constant"
+	"go/types"
+	"strings"
+
+	"golang.org/x/tools/go/analysis"
+	"golang.org/x/tools/go/analysis/passes/inspect"
+	"golang.org/x/tools/go/ast/inspector"
+	"golang.org/x/tools/go/types/typeutil"
+)
+
+const Doc = `check for calls of (time.Time).Format or time.Parse with 2006-02-01
+
+The timeformat checker looks time formats with the bad 2006-02-01 format
+which should be replaced by the intended 2006-01-02 format.
+`
+
+var Analyzer = &analysis.Analyzer{
+	Name:     "timeformat",
+	Doc:      Doc,
+	Requires: []*analysis.Analyzer{inspect.Analyzer},
+	Run:      run,
+}
+
+func run(pass *analysis.Pass) (interface{}, error) {
+	inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
+
+	nodeFilter := []ast.Node{
+		(*ast.CallExpr)(nil),
+	}
+	inspect.Preorder(nodeFilter, func(n ast.Node) {
+		call := n.(*ast.CallExpr)
+		fn, ok := typeutil.Callee(pass.TypesInfo, call).(*types.Func)
+		if !ok {
+			return
+		}
+		if (fn.FullName() == "(time.Time).Format" || fn.FullName() == "time.Parse") && isBadFormat(pass, call.Args[0]) {
+			pass.ReportRangef(call, "2006-02-01 should be 2006-01-02")
+		}
+	})
+	return nil, nil
+}
+
+// isBadFormat return true when e is a string containing 2006-02-01.
+func isBadFormat(pass *analysis.Pass, e ast.Expr) bool {
+	tv, ok := pass.TypesInfo.Types[e]
+	if !ok { // no type info, assume good
+		return false
+	}
+
+	t, ok := tv.Type.(*types.Basic)
+	if !ok || t.Info()&types.IsString == 0 {
+		return false
+	}
+
+	return strings.Contains(constant.StringVal(tv.Value), "2006-02-01")
+}

go/analysis/passes/timeformat/timeformat_test.go

@@ -0,0 +1,17 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package timeformat_test
+
+import (
+	"testing"
+
+	"golang.org/x/tools/go/analysis/analysistest"
+	"golang.org/x/tools/go/analysis/passes/timeformat"
+)
+
+func Test(t *testing.T) {
+	testdata := analysistest.TestData()
+	analysistest.Run(t, testdata, timeformat.Analyzer, "a")
+}

gopls/doc/analyzers.md

@@ -483,6 +483,15 @@ for the conventions that are enforced for Tests, Benchmarks, and Examples.
 
 **Enabled by default.**
 
+## **timeformat**
+
+check for calls of (time.Time).Format or time.Parse with 2006-02-01
+
+The timeformat checker looks time formats with the bad 2006-02-01 format
+which should be replaced by the intended 2006-01-02 format.
+
+**Enabled by default.**
+
 ## **unmarshal**
 
 report passing non-pointer or non-interface values to unmarshal

internal/lsp/source/api_json.go

@@ -43,6 +43,7 @@ import (
 	"golang.org/x/tools/go/analysis/passes/structtag"
 	"golang.org/x/tools/go/analysis/passes/testinggoroutine"
 	"golang.org/x/tools/go/analysis/passes/tests"
+	"golang.org/x/tools/go/analysis/passes/timeformat"
 	"golang.org/x/tools/go/analysis/passes/unmarshal"
 	"golang.org/x/tools/go/analysis/passes/unreachable"
 	"golang.org/x/tools/go/analysis/passes/unsafeptr"
@@ -1305,6 +1306,7 @@ func defaultAnalyzers() map[string]*Analyzer {
 		unusedwrite.Analyzer.Name:      {Analyzer: unusedwrite.Analyzer, Enabled: false},
 		useany.Analyzer.Name:           {Analyzer: useany.Analyzer, Enabled: false},
 		infertypeargs.Analyzer.Name:    {Analyzer: infertypeargs.Analyzer, Enabled: true},
+		timeformat.Analyzer.Name:       {Analyzer: timeformat.Analyzer, Enabled: true},
 
 		// gofmt -s suite:
 		simplifycompositelit.Analyzer.Name: {