cmd/vet: include the errors.As check from upstream x/tools

This change revendors golang.org/x/tools to include the check and
modifies cmd/vet to add it to the command.

CL 179977 will enable the check by default for 'go test'.

Commands run (starting in GOROOT/src):
	cd cmd
	emacs vet/main.go
	go get -u=patch golang.org/x/tools/go/analysis/passes/errorsas@latest
	go mod tidy
	go mod vendor
	cd ..
	./make.bash
	go test all

Updates #31213

Change-Id: Ic2ba9bd2d31c4c5fd9e7c42ca14e8dc38520c93b
Reviewed-on: https://go-review.googlesource.com/c/go/+/181717
Reviewed-by: Jonathan Amsterdam <jba@google.com>

src/cmd/go.mod

@@ -8,5 +8,5 @@ require (
 	golang.org/x/arch v0.0.0-20181203225421-5a4828bb7045
 	golang.org/x/crypto v0.0.0-20190325154230-a5d413f7728c
 	golang.org/x/sys v0.0.0-20190502175342-a43fa875dd82 // indirect
-	golang.org/x/tools v0.0.0-20190602112858-2de7f9bf822c
+	golang.org/x/tools v0.0.0-20190611154301-25a4f137592f
 )

src/cmd/go.sum

@@ -13,5 +13,5 @@ golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5h
 golang.org/x/sys v0.0.0-20190502175342-a43fa875dd82 h1:vsphBvatvfbhlb4PO1BYSr9dzugGxJ/SQHoNufZJq1w=
 golang.org/x/sys v0.0.0-20190502175342-a43fa875dd82/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
-golang.org/x/tools v0.0.0-20190602112858-2de7f9bf822c h1:8QARbM77BTyoVvSaGaoQPCYgZlVROYX1uKApKK98b+8=
-golang.org/x/tools v0.0.0-20190602112858-2de7f9bf822c/go.mod h1:/rFqwRUd4F7ZHNgwSSTFct+R/Kf4OFW1sUzUTQQTgfc=
+golang.org/x/tools v0.0.0-20190611154301-25a4f137592f h1:6awn5JC4pwVI5HiBqs7MDtRxnwV9PpO5iSA9v6P09pA=
+golang.org/x/tools v0.0.0-20190611154301-25a4f137592f/go.mod h1:/rFqwRUd4F7ZHNgwSSTFct+R/Kf4OFW1sUzUTQQTgfc=

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

@@ -0,0 +1,75 @@
+// Copyright 2018 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.
+
+// The errorsas package defines an Analyzer that checks that the second arugment to
+// errors.As is a pointer to a type implementing error.
+package errorsas
+
+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"
+)
+
+const doc = `report passing non-pointer or non-error values to errors.As
+
+The errorsas analysis reports calls to errors.As where the type
+of the second argument is not a pointer to a type implementing error.`
+
+var Analyzer = &analysis.Analyzer{
+	Name:     "errorsas",
+	Doc:      doc,
+	Requires: []*analysis.Analyzer{inspect.Analyzer},
+	Run:      run,
+}
+
+func run(pass *analysis.Pass) (interface{}, error) {
+	switch pass.Pkg.Path() {
+	case "errors", "errors_test":
+		// These packages know how to use their own APIs.
+		// Sometimes they are testing what happens to incorrect programs.
+		return nil, nil
+	}
+
+	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 := typeutil.StaticCallee(pass.TypesInfo, call)
+		if fn == nil {
+			return // not a static call
+		}
+		if len(call.Args) < 2 {
+			return // not enough arguments, e.g. called with return values of another function
+		}
+		if fn.FullName() == "errors.As" && !pointerToInterfaceOrError(pass, call.Args[1]) {
+			pass.Reportf(call.Pos(), "second argument to errors.As must be a pointer to an interface or a type implementing error")
+		}
+	})
+	return nil, nil
+}
+
+var errorType = types.Universe.Lookup("error").Type().Underlying().(*types.Interface)
+
+// pointerToInterfaceOrError reports whether the type of e is a pointer to an interface or a type implementing error,
+// or is the empty interface.
+func pointerToInterfaceOrError(pass *analysis.Pass, e ast.Expr) bool {
+	t := pass.TypesInfo.Types[e].Type
+	if it, ok := t.Underlying().(*types.Interface); ok && it.NumMethods() == 0 {
+		return true
+	}
+	pt, ok := t.Underlying().(*types.Pointer)
+	if !ok {
+		return false
+	}
+	_, ok = pt.Elem().Underlying().(*types.Interface)
+	return ok || types.Implements(pt.Elem(), errorType)
+}

src/cmd/vendor/modules.txt

@@ -26,7 +26,7 @@ golang.org/x/crypto/ssh/terminal
 # golang.org/x/sys v0.0.0-20190502175342-a43fa875dd82
 golang.org/x/sys/unix
 golang.org/x/sys/windows
-# golang.org/x/tools v0.0.0-20190602112858-2de7f9bf822c
+# golang.org/x/tools v0.0.0-20190611154301-25a4f137592f
 golang.org/x/tools/go/analysis
 golang.org/x/tools/go/analysis/internal/analysisflags
 golang.org/x/tools/go/analysis/internal/facts
@@ -39,6 +39,7 @@ golang.org/x/tools/go/analysis/passes/cgocall
 golang.org/x/tools/go/analysis/passes/composite
 golang.org/x/tools/go/analysis/passes/copylock
 golang.org/x/tools/go/analysis/passes/ctrlflow
+golang.org/x/tools/go/analysis/passes/errorsas
 golang.org/x/tools/go/analysis/passes/httpresponse
 golang.org/x/tools/go/analysis/passes/inspect
 golang.org/x/tools/go/analysis/passes/internal/analysisutil

src/cmd/vet/main.go

@@ -13,6 +13,7 @@ import (
 	"golang.org/x/tools/go/analysis/passes/cgocall"
 	"golang.org/x/tools/go/analysis/passes/composite"
 	"golang.org/x/tools/go/analysis/passes/copylock"
+	"golang.org/x/tools/go/analysis/passes/errorsas"
 	"golang.org/x/tools/go/analysis/passes/httpresponse"
 	"golang.org/x/tools/go/analysis/passes/loopclosure"
 	"golang.org/x/tools/go/analysis/passes/lostcancel"
@@ -40,6 +41,7 @@ func main() {
 		cgocall.Analyzer,
 		composite.Analyzer,
 		copylock.Analyzer,
+		errorsas.Analyzer,
 		httpresponse.Analyzer,
 		loopclosure.Analyzer,
 		lostcancel.Analyzer,