cmd/fiximports: do not assume go list -json unmarshals into build.Package

Work planned for golang/go#56986 adds fields to go list -json
and build.Package of different types, but this program
assumes that you can unmarshal go list -json output
into a build.Package. We've never promised that,
and I'm unaware of any other programs that do it,
so fix this program. (If many programs made this
assumption, we'd want to consider whether they need
to stay compatible, but if it's just this old program,
it's not worth fixing.)

Change-Id: I8b402bb3e3db7ecff961e1f033f6c5cf967b8578
Reviewed-on: https://go-review.googlesource.com/c/tools/+/461658
Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>

cmd/fiximports/main.go

@@ -72,11 +72,9 @@ import (
 	"flag"
 	"fmt"
 	"go/ast"
-	"go/build"
 	"go/format"
 	"go/parser"
 	"go/token"
-	exec "golang.org/x/sys/execabs"
 	"io"
 	"io/ioutil"
 	"log"
@@ -86,6 +84,8 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+
+	exec "golang.org/x/sys/execabs"
 )
 
 // flags
@@ -137,16 +137,16 @@ type canonicalName struct{ path, name string }
 // Invariant: a false result implies an error was already printed.
 func fiximports(packages ...string) bool {
 	// importedBy is the transpose of the package import graph.
-	importedBy := make(map[string]map[*build.Package]bool)
+	importedBy := make(map[string]map[*listPackage]bool)
 
 	// addEdge adds an edge to the import graph.
-	addEdge := func(from *build.Package, to string) {
+	addEdge := func(from *listPackage, to string) {
 		if to == "C" || to == "unsafe" {
 			return // fake
 		}
 		pkgs := importedBy[to]
 		if pkgs == nil {
-			pkgs = make(map[*build.Package]bool)
+			pkgs = make(map[*listPackage]bool)
 			importedBy[to] = pkgs
 		}
 		pkgs[from] = true
@@ -162,7 +162,7 @@ func fiximports(packages ...string) bool {
 	// packageName maps each package's path to its name.
 	packageName := make(map[string]string)
 	for _, p := range pkgs {
-		packageName[p.ImportPath] = p.Package.Name
+		packageName[p.ImportPath] = p.Name
 	}
 
 	// canonical maps each non-canonical package path to
@@ -207,21 +207,21 @@ func fiximports(packages ...string) bool {
 		}
 
 		for _, imp := range p.Imports {
-			addEdge(&p.Package, imp)
+			addEdge(p, imp)
 		}
 		for _, imp := range p.TestImports {
-			addEdge(&p.Package, imp)
+			addEdge(p, imp)
 		}
 		for _, imp := range p.XTestImports {
-			addEdge(&p.Package, imp)
+			addEdge(p, imp)
 		}
 
 		// Does package have an explicit import comment?
 		if p.ImportComment != "" {
 			if p.ImportComment != p.ImportPath {
 				canonical[p.ImportPath] = canonicalName{
-					path: p.Package.ImportComment,
-					name: p.Package.Name,
+					path: p.ImportComment,
+					name: p.Name,
 				}
 			}
 		} else {
@@ -273,7 +273,7 @@ func fiximports(packages ...string) bool {
 
 	// Find all clients (direct importers) of canonical packages.
 	// These are the packages that need fixing up.
-	clients := make(map[*build.Package]bool)
+	clients := make(map[*listPackage]bool)
 	for path := range canonical {
 		for client := range importedBy[path] {
 			clients[client] = true
@@ -350,7 +350,7 @@ func fiximports(packages ...string) bool {
 }
 
 // Invariant: false result => error already printed.
-func rewritePackage(client *build.Package, canonical map[string]canonicalName) bool {
+func rewritePackage(client *listPackage, canonical map[string]canonicalName) bool {
 	ok := true
 
 	used := make(map[string]bool)
@@ -450,11 +450,20 @@ func rewriteFile(filename string, canonical map[string]canonicalName, used map[s
 	return nil
 }
 
-// listPackage is a copy of cmd/go/list.Package.
-// It has more fields than build.Package and we need some of them.
+// listPackage corresponds to the output of go list -json,
+// but only the fields we need.
 type listPackage struct {
-	build.Package
-	Error *packageError // error loading package
+	Name          string
+	Dir           string
+	ImportPath    string
+	GoFiles       []string
+	TestGoFiles   []string
+	XTestGoFiles  []string
+	Imports       []string
+	TestImports   []string
+	XTestImports  []string
+	ImportComment string
+	Error         *packageError // error loading package
 }
 
 // A packageError describes an error loading information about a package.