cmd/vet: update buildtag check for //go:build lines

Brings in golang.org/x/tools@2363391a
and adjusts, adds cmd/vet tests accordingly.

Part of //go:build change (#41184).
See https://golang.org/design/draft-gobuild

This brings in the new //go:build checks in cmd/vet.

Change-Id: I8a9735cc014171691012b307ec30e94c81aadfe1
Reviewed-on: https://go-review.googlesource.com/c/go/+/240609
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go.mod

@@ -8,5 +8,5 @@ require (
 	golang.org/x/crypto v0.0.0-20201016220609-9e8e0b390897
 	golang.org/x/mod v0.4.1
 	golang.org/x/sys v0.0.0-20210218145245-beda7e5e158e // indirect
-	golang.org/x/tools v0.0.0-20210107193943-4ed967dd8eff
+	golang.org/x/tools v0.1.1-0.20210220032852-2363391a5b2f
 )

src/cmd/go.sum

@@ -13,7 +13,6 @@ golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8U
 golang.org/x/crypto v0.0.0-20200622213623-75b288015ac9/go.mod h1:LzIPMQfyMNhhGPhUkYOs5KpL4U8rLKemX1yGLhDgUto=
 golang.org/x/crypto v0.0.0-20201016220609-9e8e0b390897 h1:pLI5jrR7OSLijeIDcmRxNmw2api+jEfxLoykJVice/E=
 golang.org/x/crypto v0.0.0-20201016220609-9e8e0b390897/go.mod h1:LzIPMQfyMNhhGPhUkYOs5KpL4U8rLKemX1yGLhDgUto=
-golang.org/x/mod v0.3.0/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
 golang.org/x/mod v0.4.1 h1:Kvvh58BN8Y9/lBi7hTekvtMpm07eUZ0ck5pRHpsMWrY=
 golang.org/x/mod v0.4.1/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
 golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
@@ -25,14 +24,15 @@ golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5h
 golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20191204072324-ce4227a45e2e/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20200930185726-fdedc70b468f/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20210124154548-22da62e12c0c/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210218145245-beda7e5e158e h1:f5mksnk+hgXHnImpZoWj64ja99j9zV7YUgrVG95uFE4=
 golang.org/x/sys v0.0.0-20210218145245-beda7e5e158e/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
 golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
-golang.org/x/tools v0.0.0-20210107193943-4ed967dd8eff h1:6EkB024TP1fu6cmQqeCNw685zYDVt5g8N1BXh755SQM=
-golang.org/x/tools v0.0.0-20210107193943-4ed967dd8eff/go.mod h1:emZCQorbCU4vsT4fOWvOPXz4eW1wZW4PmDk9uLelYpA=
+golang.org/x/tools v0.1.1-0.20210220032852-2363391a5b2f h1:R8L2zr6nSvQoIIw/EiaPP6HfmxeiArf+Nh/CWTC60wQ=
+golang.org/x/tools v0.1.1-0.20210220032852-2363391a5b2f/go.mod h1:9bzcO0MWcOuT0tm1iBGzDVPshzfwoVvREIui8C+MHqU=
 golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 h1:go1bK/D/BFZV2I8cIQd1NKEZ+0owSTG1fDTci4IqFcE=

src/cmd/vendor/golang.org/x/tools/go/analysis/passes/buildtag/buildtag.go

@@ -2,14 +2,17 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.16
+// +build go1.16
+
 // Package buildtag defines an Analyzer that checks build tags.
 package buildtag
 
 import (
-	"bytes"
-	"fmt"
 	"go/ast"
+	"go/build/constraint"
 	"go/parser"
+	"go/token"
 	"strings"
 	"unicode"
 
@@ -52,118 +55,313 @@ func runBuildTag(pass *analysis.Pass) (interface{}, error) {
 }
 
 func checkGoFile(pass *analysis.Pass, f *ast.File) {
-	pastCutoff := false
+	var check checker
+	check.init(pass)
+	defer check.finish()
+
 	for _, group := range f.Comments {
 		// A +build comment is ignored after or adjoining the package declaration.
 		if group.End()+1 >= f.Package {
-			pastCutoff = true
+			check.plusBuildOK = false
 		}
-
-		// "+build" is ignored within or after a /*...*/ comment.
-		if !strings.HasPrefix(group.List[0].Text, "//") {
-			pastCutoff = true
-			continue
+		// A //go:build comment is ignored after the package declaration
+		// (but adjoining it is OK, in contrast to +build comments).
+		if group.Pos() >= f.Package {
+			check.goBuildOK = false
 		}
 
 		// Check each line of a //-comment.
 		for _, c := range group.List {
-			if !strings.Contains(c.Text, "+build") {
-				continue
-			}
-			if err := checkLine(c.Text, pastCutoff); err != nil {
-				pass.Reportf(c.Pos(), "%s", err)
+			// "+build" is ignored within or after a /*...*/ comment.
+			if !strings.HasPrefix(c.Text, "//") {
+				check.plusBuildOK = false
 			}
+			check.comment(c.Slash, c.Text)
 		}
 	}
 }
 
 func checkOtherFile(pass *analysis.Pass, filename string) error {
+	var check checker
+	check.init(pass)
+	defer check.finish()
+
+	// We cannot use the Go parser, since this may not be a Go source file.
+	// Read the raw bytes instead.
 	content, tf, err := analysisutil.ReadFile(pass.Fset, filename)
 	if err != nil {
 		return err
 	}
 
-	// We must look at the raw lines, as build tags may appear in non-Go
-	// files such as assembly files.
-	lines := bytes.SplitAfter(content, nl)
+	check.file(token.Pos(tf.Base()), string(content))
+	return nil
+}
 
+type checker struct {
+	pass         *analysis.Pass
+	plusBuildOK  bool            // "+build" lines still OK
+	goBuildOK    bool            // "go:build" lines still OK
+	crossCheck   bool            // cross-check go:build and +build lines when done reading file
+	inStar       bool            // currently in a /* */ comment
+	goBuildPos   token.Pos       // position of first go:build line found
+	plusBuildPos token.Pos       // position of first "+build" line found
+	goBuild      constraint.Expr // go:build constraint found
+	plusBuild    constraint.Expr // AND of +build constraints found
+}
+
+func (check *checker) init(pass *analysis.Pass) {
+	check.pass = pass
+	check.goBuildOK = true
+	check.plusBuildOK = true
+	check.crossCheck = true
+}
+
+func (check *checker) file(pos token.Pos, text string) {
 	// Determine cutpoint where +build comments are no longer valid.
 	// They are valid in leading // comments in the file followed by
 	// a blank line.
 	//
 	// This must be done as a separate pass because of the
 	// requirement that the comment be followed by a blank line.
-	var cutoff int
-	for i, line := range lines {
-		line = bytes.TrimSpace(line)
-		if !bytes.HasPrefix(line, slashSlash) {
-			if len(line) > 0 {
-				break
+	var plusBuildCutoff int
+	fullText := text
+	for text != "" {
+		i := strings.Index(text, "\n")
+		if i < 0 {
+			i = len(text)
+		} else {
+			i++
+		}
+		offset := len(fullText) - len(text)
+		line := text[:i]
+		text = text[i:]
+		line = strings.TrimSpace(line)
+		if !strings.HasPrefix(line, "//") && line != "" {
+			break
+		}
+		if line == "" {
+			plusBuildCutoff = offset
+		}
+	}
+
+	// Process each line.
+	// Must stop once we hit goBuildOK == false
+	text = fullText
+	check.inStar = false
+	for text != "" {
+		i := strings.Index(text, "\n")
+		if i < 0 {
+			i = len(text)
+		} else {
+			i++
+		}
+		offset := len(fullText) - len(text)
+		line := text[:i]
+		text = text[i:]
+		check.plusBuildOK = offset < plusBuildCutoff
+
+		if strings.HasPrefix(line, "//") {
+			check.comment(pos+token.Pos(offset), line)
+			continue
+		}
+
+		// Keep looking for the point at which //go:build comments
+		// stop being allowed. Skip over, cut out any /* */ comments.
+		for {
+			line = strings.TrimSpace(line)
+			if check.inStar {
+				i := strings.Index(line, "*/")
+				if i < 0 {
+					line = ""
+					break
+				}
+				line = line[i+len("*/"):]
+				check.inStar = false
+				continue
 			}
-			cutoff = i
+			if strings.HasPrefix(line, "/*") {
+				check.inStar = true
+				line = line[len("/*"):]
+				continue
+			}
+			break
+		}
+		if line != "" {
+			// Found non-comment non-blank line.
+			// Ends space for valid //go:build comments,
+			// but also ends the fraction of the file we can
+			// reliably parse. From this point on we might
+			// incorrectly flag "comments" inside multiline
+			// string constants or anything else (this might
+			// not even be a Go program). So stop.
+			break
 		}
 	}
-
-	for i, line := range lines {
-		line = bytes.TrimSpace(line)
-		if !bytes.HasPrefix(line, slashSlash) {
-			continue
-		}
-		if !bytes.Contains(line, []byte("+build")) {
-			continue
-		}
-		if err := checkLine(string(line), i >= cutoff); err != nil {
-			pass.Reportf(analysisutil.LineStart(tf, i+1), "%s", err)
-			continue
-		}
-	}
-	return nil
 }
 
-// checkLine checks a line that starts with "//" and contains "+build".
-func checkLine(line string, pastCutoff bool) error {
-	line = strings.TrimPrefix(line, "//")
-	line = strings.TrimSpace(line)
+func (check *checker) comment(pos token.Pos, text string) {
+	if strings.HasPrefix(text, "//") {
+		if strings.Contains(text, "+build") {
+			check.plusBuildLine(pos, text)
+		}
+		if strings.Contains(text, "//go:build") {
+			check.goBuildLine(pos, text)
+		}
+	}
+	if strings.HasPrefix(text, "/*") {
+		if i := strings.Index(text, "\n"); i >= 0 {
+			// multiline /* */ comment - process interior lines
+			check.inStar = true
+			i++
+			pos += token.Pos(i)
+			text = text[i:]
+			for text != "" {
+				i := strings.Index(text, "\n")
+				if i < 0 {
+					i = len(text)
+				} else {
+					i++
+				}
+				line := text[:i]
+				if strings.HasPrefix(line, "//") {
+					check.comment(pos, line)
+				}
+				pos += token.Pos(i)
+				text = text[i:]
+			}
+			check.inStar = false
+		}
+	}
+}
 
-	if strings.HasPrefix(line, "+build") {
-		fields := strings.Fields(line)
-		if fields[0] != "+build" {
-			// Comment is something like +buildasdf not +build.
-			return fmt.Errorf("possible malformed +build comment")
-		}
-		if pastCutoff {
-			return fmt.Errorf("+build comment must appear before package clause and be followed by a blank line")
-		}
-		if err := checkArguments(fields); err != nil {
-			return err
+func (check *checker) goBuildLine(pos token.Pos, line string) {
+	if !constraint.IsGoBuild(line) {
+		if !strings.HasPrefix(line, "//go:build") && constraint.IsGoBuild("//"+strings.TrimSpace(line[len("//"):])) {
+			check.pass.Reportf(pos, "malformed //go:build line (space between // and go:build)")
 		}
+		return
+	}
+	if !check.goBuildOK || check.inStar {
+		check.pass.Reportf(pos, "misplaced //go:build comment")
+		check.crossCheck = false
+		return
+	}
+
+	if check.goBuildPos == token.NoPos {
+		check.goBuildPos = pos
 	} else {
-		// Comment with +build but not at beginning.
-		if !pastCutoff {
-			return fmt.Errorf("possible malformed +build comment")
-		}
+		check.pass.Reportf(pos, "unexpected extra //go:build line")
+		check.crossCheck = false
+	}
+
+	// testing hack: stop at // ERROR
+	if i := strings.Index(line, " // ERROR "); i >= 0 {
+		line = line[:i]
+	}
+
+	x, err := constraint.Parse(line)
+	if err != nil {
+		check.pass.Reportf(pos, "%v", err)
+		check.crossCheck = false
+		return
+	}
+
+	if check.goBuild == nil {
+		check.goBuild = x
 	}
-	return nil
 }
 
-func checkArguments(fields []string) error {
+func (check *checker) plusBuildLine(pos token.Pos, line string) {
+	line = strings.TrimSpace(line)
+	if !constraint.IsPlusBuild(line) {
+		// Comment with +build but not at beginning.
+		// Only report early in file.
+		if check.plusBuildOK && !strings.HasPrefix(line, "// want") {
+			check.pass.Reportf(pos, "possible malformed +build comment")
+		}
+		return
+	}
+	if !check.plusBuildOK { // inStar implies !plusBuildOK
+		check.pass.Reportf(pos, "misplaced +build comment")
+		check.crossCheck = false
+	}
+
+	if check.plusBuildPos == token.NoPos {
+		check.plusBuildPos = pos
+	}
+
+	// testing hack: stop at // ERROR
+	if i := strings.Index(line, " // ERROR "); i >= 0 {
+		line = line[:i]
+	}
+
+	fields := strings.Fields(line[len("//"):])
+	// IsPlusBuildConstraint check above implies fields[0] == "+build"
 	for _, arg := range fields[1:] {
 		for _, elem := range strings.Split(arg, ",") {
 			if strings.HasPrefix(elem, "!!") {
-				return fmt.Errorf("invalid double negative in build constraint: %s", arg)
+				check.pass.Reportf(pos, "invalid double negative in build constraint: %s", arg)
+				check.crossCheck = false
+				continue
 			}
 			elem = strings.TrimPrefix(elem, "!")
 			for _, c := range elem {
 				if !unicode.IsLetter(c) && !unicode.IsDigit(c) && c != '_' && c != '.' {
-					return fmt.Errorf("invalid non-alphanumeric build constraint: %s", arg)
+					check.pass.Reportf(pos, "invalid non-alphanumeric build constraint: %s", arg)
+					check.crossCheck = false
+					break
 				}
 			}
 		}
 	}
-	return nil
+
+	if check.crossCheck {
+		y, err := constraint.Parse(line)
+		if err != nil {
+			// Should never happen - constraint.Parse never rejects a // +build line.
+			// Also, we just checked the syntax above.
+			// Even so, report.
+			check.pass.Reportf(pos, "%v", err)
+			check.crossCheck = false
+			return
+		}
+		if check.plusBuild == nil {
+			check.plusBuild = y
+		} else {
+			check.plusBuild = &constraint.AndExpr{X: check.plusBuild, Y: y}
+		}
+	}
 }
 
-var (
-	nl         = []byte("\n")
-	slashSlash = []byte("//")
-)
+func (check *checker) finish() {
+	if !check.crossCheck || check.plusBuildPos == token.NoPos || check.goBuildPos == token.NoPos {
+		return
+	}
+
+	// Have both //go:build and // +build,
+	// with no errors found (crossCheck still true).
+	// Check they match.
+	var want constraint.Expr
+	lines, err := constraint.PlusBuildLines(check.goBuild)
+	if err != nil {
+		check.pass.Reportf(check.goBuildPos, "%v", err)
+		return
+	}
+	for _, line := range lines {
+		y, err := constraint.Parse(line)
+		if err != nil {
+			// Definitely should not happen, but not the user's fault.
+			// Do not report.
+			return
+		}
+		if want == nil {
+			want = y
+		} else {
+			want = &constraint.AndExpr{X: want, Y: y}
+		}
+	}
+	if want.String() != check.plusBuild.String() {
+		check.pass.Reportf(check.plusBuildPos, "+build lines do not match //go:build condition")
+		return
+	}
+}

src/cmd/vendor/golang.org/x/tools/go/analysis/passes/buildtag/buildtag_old.go

@@ -0,0 +1,174 @@
+// Copyright 2013 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.
+
+// TODO(rsc): Delete this file once Go 1.17 comes out and we can retire Go 1.15 support.
+
+//go:build !go1.16
+// +build !go1.16
+
+// Package buildtag defines an Analyzer that checks build tags.
+package buildtag
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"strings"
+	"unicode"
+
+	"golang.org/x/tools/go/analysis"
+	"golang.org/x/tools/go/analysis/passes/internal/analysisutil"
+)
+
+const Doc = "check that +build tags are well-formed and correctly located"
+
+var Analyzer = &analysis.Analyzer{
+	Name: "buildtag",
+	Doc:  Doc,
+	Run:  runBuildTag,
+}
+
+func runBuildTag(pass *analysis.Pass) (interface{}, error) {
+	for _, f := range pass.Files {
+		checkGoFile(pass, f)
+	}
+	for _, name := range pass.OtherFiles {
+		if err := checkOtherFile(pass, name); err != nil {
+			return nil, err
+		}
+	}
+	for _, name := range pass.IgnoredFiles {
+		if strings.HasSuffix(name, ".go") {
+			f, err := parser.ParseFile(pass.Fset, name, nil, parser.ParseComments)
+			if err != nil {
+				// Not valid Go source code - not our job to diagnose, so ignore.
+				return nil, nil
+			}
+			checkGoFile(pass, f)
+		} else {
+			if err := checkOtherFile(pass, name); err != nil {
+				return nil, err
+			}
+		}
+	}
+	return nil, nil
+}
+
+func checkGoFile(pass *analysis.Pass, f *ast.File) {
+	pastCutoff := false
+	for _, group := range f.Comments {
+		// A +build comment is ignored after or adjoining the package declaration.
+		if group.End()+1 >= f.Package {
+			pastCutoff = true
+		}
+
+		// "+build" is ignored within or after a /*...*/ comment.
+		if !strings.HasPrefix(group.List[0].Text, "//") {
+			pastCutoff = true
+			continue
+		}
+
+		// Check each line of a //-comment.
+		for _, c := range group.List {
+			if !strings.Contains(c.Text, "+build") {
+				continue
+			}
+			if err := checkLine(c.Text, pastCutoff); err != nil {
+				pass.Reportf(c.Pos(), "%s", err)
+			}
+		}
+	}
+}
+
+func checkOtherFile(pass *analysis.Pass, filename string) error {
+	content, tf, err := analysisutil.ReadFile(pass.Fset, filename)
+	if err != nil {
+		return err
+	}
+
+	// We must look at the raw lines, as build tags may appear in non-Go
+	// files such as assembly files.
+	lines := bytes.SplitAfter(content, nl)
+
+	// Determine cutpoint where +build comments are no longer valid.
+	// They are valid in leading // comments in the file followed by
+	// a blank line.
+	//
+	// This must be done as a separate pass because of the
+	// requirement that the comment be followed by a blank line.
+	var cutoff int
+	for i, line := range lines {
+		line = bytes.TrimSpace(line)
+		if !bytes.HasPrefix(line, slashSlash) {
+			if len(line) > 0 {
+				break
+			}
+			cutoff = i
+		}
+	}
+
+	for i, line := range lines {
+		line = bytes.TrimSpace(line)
+		if !bytes.HasPrefix(line, slashSlash) {
+			continue
+		}
+		if !bytes.Contains(line, []byte("+build")) {
+			continue
+		}
+		if err := checkLine(string(line), i >= cutoff); err != nil {
+			pass.Reportf(analysisutil.LineStart(tf, i+1), "%s", err)
+			continue
+		}
+	}
+	return nil
+}
+
+// checkLine checks a line that starts with "//" and contains "+build".
+func checkLine(line string, pastCutoff bool) error {
+	line = strings.TrimPrefix(line, "//")
+	line = strings.TrimSpace(line)
+
+	if strings.HasPrefix(line, "+build") {
+		fields := strings.Fields(line)
+		if fields[0] != "+build" {
+			// Comment is something like +buildasdf not +build.
+			return fmt.Errorf("possible malformed +build comment")
+		}
+		if pastCutoff {
+			return fmt.Errorf("+build comment must appear before package clause and be followed by a blank line")
+		}
+		if err := checkArguments(fields); err != nil {
+			return err
+		}
+	} else {
+		// Comment with +build but not at beginning.
+		if !pastCutoff {
+			return fmt.Errorf("possible malformed +build comment")
+		}
+	}
+	return nil
+}
+
+func checkArguments(fields []string) error {
+	for _, arg := range fields[1:] {
+		for _, elem := range strings.Split(arg, ",") {
+			if strings.HasPrefix(elem, "!!") {
+				return fmt.Errorf("invalid double negative in build constraint: %s", arg)
+			}
+			elem = strings.TrimPrefix(elem, "!")
+			for _, c := range elem {
+				if !unicode.IsLetter(c) && !unicode.IsDigit(c) && c != '_' && c != '.' {
+					return fmt.Errorf("invalid non-alphanumeric build constraint: %s", arg)
+				}
+			}
+		}
+	}
+	return nil
+}
+
+var (
+	nl         = []byte("\n")
+	slashSlash = []byte("//")
+)

src/cmd/vendor/golang.org/x/tools/go/analysis/passes/loopclosure/loopclosure.go

@@ -8,22 +8,14 @@ package loopclosure
 
 import (
 	"go/ast"
+	"go/types"
 
 	"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"
 )
 
-// TODO(adonovan): also report an error for the following structure,
-// which is often used to ensure that deferred calls do not accumulate
-// in a loop:
-//
-//	for i, x := range c {
-//		func() {
-//			...reference to i or x...
-//		}()
-//	}
-
 const Doc = `check references to loop variables from within nested functions
 
 This analyzer checks for references to loop variables from within a
@@ -95,16 +87,19 @@ func run(pass *analysis.Pass) (interface{}, error) {
 		if len(body.List) == 0 {
 			return
 		}
-		var last *ast.CallExpr
+		// The function invoked in the last return statement.
+		var fun ast.Expr
 		switch s := body.List[len(body.List)-1].(type) {
 		case *ast.GoStmt:
-			last = s.Call
+			fun = s.Call.Fun
 		case *ast.DeferStmt:
-			last = s.Call
-		default:
-			return
+			fun = s.Call.Fun
+		case *ast.ExprStmt: // check for errgroup.Group.Go()
+			if call, ok := s.X.(*ast.CallExpr); ok {
+				fun = goInvokes(pass.TypesInfo, call)
+			}
 		}
-		lit, ok := last.Fun.(*ast.FuncLit)
+		lit, ok := fun.(*ast.FuncLit)
 		if !ok {
 			return
 		}
@@ -128,3 +123,43 @@ func run(pass *analysis.Pass) (interface{}, error) {
 	})
 	return nil, nil
 }
+
+// goInvokes returns a function expression that would be called asynchronously
+// (but not awaited) in another goroutine as a consequence of the call.
+// For example, given the g.Go call below, it returns the function literal expression.
+//
+//   import "sync/errgroup"
+//   var g errgroup.Group
+//   g.Go(func() error { ... })
+//
+// Currently only "golang.org/x/sync/errgroup.Group()" is considered.
+func goInvokes(info *types.Info, call *ast.CallExpr) ast.Expr {
+	f := typeutil.StaticCallee(info, call)
+	// Note: Currently only supports: golang.org/x/sync/errgroup.Go.
+	if f == nil || f.Name() != "Go" {
+		return nil
+	}
+	recv := f.Type().(*types.Signature).Recv()
+	if recv == nil {
+		return nil
+	}
+	rtype, ok := recv.Type().(*types.Pointer)
+	if !ok {
+		return nil
+	}
+	named, ok := rtype.Elem().(*types.Named)
+	if !ok {
+		return nil
+	}
+	if named.Obj().Name() != "Group" {
+		return nil
+	}
+	pkg := f.Pkg()
+	if pkg == nil {
+		return nil
+	}
+	if pkg.Path() != "golang.org/x/sync/errgroup" {
+		return nil
+	}
+	return call.Args[0]
+}

src/cmd/vendor/golang.org/x/tools/go/analysis/unitchecker/unitchecker.go

@@ -97,7 +97,7 @@ func Main(analyzers ...*analysis.Analyzer) {
 
 Usage of %[1]s:
 	%.16[1]s unit.cfg	# execute analysis specified by config file
-	%.16[1]s help    	# general help
+	%.16[1]s help    	# general help, including listing analyzers and flags
 	%.16[1]s help name	# help on specific analyzer and its flags
 `, progname)
 		os.Exit(1)

src/cmd/vendor/golang.org/x/tools/go/analysis/unitchecker/unitchecker112.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.12
 // +build go1.12
 
 package unitchecker

src/cmd/vendor/modules.txt

@@ -44,7 +44,7 @@ golang.org/x/mod/zip
 golang.org/x/sys/internal/unsafeheader
 golang.org/x/sys/unix
 golang.org/x/sys/windows
-# golang.org/x/tools v0.0.0-20210107193943-4ed967dd8eff
+# golang.org/x/tools v0.1.1-0.20210220032852-2363391a5b2f
 ## explicit
 golang.org/x/tools/go/analysis
 golang.org/x/tools/go/analysis/internal/analysisflags

src/cmd/vet/testdata/asm/asm1.s

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build amd64
 // +build amd64
 
 TEXT ·arg1(SB),0,$0-2

src/cmd/vet/testdata/buildtag/buildtag.go

@@ -4,12 +4,14 @@
 
 // This file contains tests for the buildtag checker.
 
-// +builder // ERROR "possible malformed \+build comment"
+// ERRORNEXT "possible malformed [+]build comment"
+// +builder
 // +build !ignore
 
 package testdata
 
-// +build toolate // ERROR "build comment must appear before package clause and be followed by a blank line$"
+// ERRORNEXT "misplaced \+build comment"
+// +build toolate
 
 var _ = 3
 

src/cmd/vet/testdata/buildtag/buildtag2.go

@@ -0,0 +1,22 @@
+// Copyright 2013 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 buildtag checker.
+
+// ERRORNEXT "possible malformed [+]build comment"
+// +builder
+// +build !ignore
+
+package testdata
+
+// ERRORNEXT "misplaced \+build comment"
+// +build toolate
+// ERRORNEXT "misplaced //go:build comment"
+//go:build toolate
+
+var _ = 3
+
+var _ = `
+// +build notacomment
+`

src/cmd/vet/testdata/buildtag/buildtag3.go

@@ -0,0 +1,15 @@
+// Copyright 2020 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 buildtag checker.
+
+//go:build good
+// ERRORNEXT "[+]build lines do not match //go:build condition"
+// +build bad
+
+package testdata
+
+var _ = `
+// +build notacomment
+`

src/cmd/vet/testdata/buildtag/buildtag4.go

@@ -0,0 +1,11 @@
+// Copyright 2020 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 buildtag checker.
+
+//go:build !(bad || worse)
+// +build !bad
+// +build !worse
+
+package testdata

src/cmd/vet/testdata/buildtag/buildtag5.go

@@ -0,0 +1,11 @@
+// Copyright 2020 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 buildtag checker.
+
+//go:build !(bad || worse)
+
+package testdata
+
+// +build other // ERROR "misplaced \+build comment"

src/cmd/vet/testdata/buildtag/buildtag6.s

@@ -0,0 +1,9 @@
+// Copyright 2020 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.
+
+#include "go_asm.h"
+
+// ok because we cannot parse assembly files.
+// +build no
+

src/cmd/vet/testdata/buildtag/buildtag7.s

@@ -0,0 +1,11 @@
+// Copyright 2020 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.
+
+// +build ignore
+
+#include "go_asm.h"
+
+// ok because we cannot parse assembly files
+// the assembler would complain if we did assemble this file.
+//go:build no

src/cmd/vet/testdata/tagtest/file1.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build testtag
 // +build testtag
 
 package main

src/cmd/vet/testdata/tagtest/file2.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !testtag
 // +build !testtag
 
 package main

src/cmd/vet/vet_test.go

@@ -334,8 +334,8 @@ type wantedError struct {
 }
 
 var (
-	errRx       = regexp.MustCompile(`// (?:GC_)?ERROR (.*)`)
-	errAutoRx   = regexp.MustCompile(`// (?:GC_)?ERRORAUTO (.*)`)
+	errRx       = regexp.MustCompile(`// (?:GC_)?ERROR(NEXT)? (.*)`)
+	errAutoRx   = regexp.MustCompile(`// (?:GC_)?ERRORAUTO(NEXT)? (.*)`)
 	errQuotesRx = regexp.MustCompile(`"([^"]*)"`)
 	lineRx      = regexp.MustCompile(`LINE(([+-])([0-9]+))?`)
 )
@@ -364,7 +364,10 @@ func wantedErrors(file, short string) (errs []wantedError) {
 		if m == nil {
 			continue
 		}
-		all := m[1]
+		if m[1] == "NEXT" {
+			lineNum++
+		}
+		all := m[2]
 		mm := errQuotesRx.FindAllStringSubmatch(all, -1)
 		if mm == nil {
 			log.Fatalf("%s:%d: invalid errchk line: %s", file, lineNum, line)