cmd/vet: refactor to support alternative importers

Instead of constructing the importer in init, do it lazily as needed.
This lets us select the importer using a command line flag.
The addition of the command line flag will come in a follow-up CL.

Change-Id: Ieb3a5f01a34fb5bd220a95086daf5d6b37e83bb5
Reviewed-on: https://go-review.googlesource.com/37669Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/vet/httpresponse.go

@@ -12,29 +12,17 @@ import (
 	"go/types"
 )
 
-var (
-	httpResponseType types.Type
-	httpClientType   types.Type
-)
-
 func init() {
-	if typ := importType("net/http", "Response"); typ != nil {
-		httpResponseType = typ
-	}
-	if typ := importType("net/http", "Client"); typ != nil {
-		httpClientType = typ
-	}
-	// if http.Response or http.Client are not defined don't register this check.
-	if httpResponseType == nil || httpClientType == nil {
-		return
-	}
-
 	register("httpresponse",
 		"check errors are checked before using an http Response",
 		checkHTTPResponse, callExpr)
 }
 
 func checkHTTPResponse(f *File, node ast.Node) {
+	// If http.Response or http.Client are not defined, skip this check.
+	if httpResponseType == nil || httpClientType == nil {
+		return
+	}
 	call := node.(*ast.CallExpr)
 	if !isHTTPFuncOrMethodOnClient(f, call) {
 		return // the function call is not related to this check.

src/cmd/vet/types.go

@@ -14,26 +14,32 @@ import (
 )
 
 // stdImporter is the importer we use to import packages.
-// It is created during initialization so that all packages
-// are imported by the same importer.
-var stdImporter = importer.Default()
+// It is shared so that all packages are imported by the same importer.
+var stdImporter types.Importer
 
 var (
 	errorType        *types.Interface
 	stringerType     *types.Interface // possibly nil
 	formatterType    *types.Interface // possibly nil
+	httpResponseType types.Type       // possibly nil
+	httpClientType   types.Type       // possibly nil
 )
 
-func init() {
+func inittypes() {
 	errorType = types.Universe.Lookup("error").Type().Underlying().(*types.Interface)
 
 	if typ := importType("fmt", "Stringer"); typ != nil {
 		stringerType = typ.Underlying().(*types.Interface)
 	}
-
 	if typ := importType("fmt", "Formatter"); typ != nil {
 		formatterType = typ.Underlying().(*types.Interface)
 	}
+	if typ := importType("net/http", "Response"); typ != nil {
+		httpResponseType = typ
+	}
+	if typ := importType("net/http", "Client"); typ != nil {
+		httpClientType = typ
+	}
 }
 
 // importType returns the type denoted by the qualified identifier
@@ -54,6 +60,10 @@ func importType(path, name string) types.Type {
 }
 
 func (pkg *Package) check(fs *token.FileSet, astFiles []*ast.File) error {
+	if stdImporter == nil {
+		stdImporter = importer.Default()
+		inittypes()
+	}
 	pkg.defs = make(map[*ast.Ident]types.Object)
 	pkg.uses = make(map[*ast.Ident]types.Object)
 	pkg.selectors = make(map[*ast.SelectorExpr]*types.Selection)