imports: prefer paths imported by sibling files.

Adds an Imports field to packageInfo with the imports used by sibling
files, and uses it preferentially if it matches a missing import.

Example: if foo/foo.go imports "local/log", it's a reasonable assumption
that foo/bar.go will also want "local/log" instead of "log".

Change-Id: Ifb504ed5e00ff18459f19d8598cc2c94099ae563
Reviewed-on: https://go-review.googlesource.com/43454
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

imports/fix.go

@@ -68,9 +68,16 @@ func importGroup(importPath string) int {
 	return 0
 }
 
+// importInfo is a summary of information about one import.
+type importInfo struct {
+	Path  string // full import path (e.g. "crypto/rand")
+	Alias string // import alias, if present (e.g. "crand")
+}
+
 // packageInfo is a summary of features found in a package.
 type packageInfo struct {
-	Globals map[string]bool // symbol => true
+	Globals map[string]bool       // symbol => true
+	Imports map[string]importInfo // pkg base name or alias => info
 }
 
 // dirPackageInfo exposes the dirPackageInfoFile function so that it can be overridden.
@@ -94,7 +101,7 @@ func dirPackageInfoFile(pkgName, srcDir, filename string) (*packageInfo, error)
 		return nil, err
 	}
 
-	info := &packageInfo{Globals: make(map[string]bool)}
+	info := &packageInfo{Globals: make(map[string]bool), Imports: make(map[string]importInfo)}
 	for _, fi := range packageFileInfos {
 		if fi.Name() == fileBase || !strings.HasSuffix(fi.Name(), ".go") {
 			continue
@@ -123,6 +130,16 @@ func dirPackageInfoFile(pkgName, srcDir, filename string) (*packageInfo, error)
 				info.Globals[valueSpec.Names[0].Name] = true
 			}
 		}
+
+		for _, imp := range root.Imports {
+			impInfo := importInfo{Path: strings.Trim(imp.Path.Value, `"`)}
+			name := path.Base(impInfo.Path)
+			if imp.Name != nil {
+				name = strings.Trim(imp.Name.Name, `"`)
+				impInfo.Alias = name
+			}
+			info.Imports[name] = impInfo
+		}
 	}
 	return info, nil
 }
@@ -217,6 +234,16 @@ func fixImports(fset *token.FileSet, f *ast.File, filename string) (added []stri
 		}
 	}
 
+	// Fast path, all references already imported.
+	if len(refs) == 0 {
+		return nil, nil
+	}
+
+	// Can assume this will be necessary in all cases now.
+	if !loadedPackageInfo {
+		packageInfo, _ = dirPackageInfo(f.Name.Name, srcDir, filename)
+	}
+
 	// Search for imports matching potential package references.
 	searches := 0
 	type result struct {
@@ -227,6 +254,11 @@ func fixImports(fset *token.FileSet, f *ast.File, filename string) (added []stri
 	results := make(chan result)
 	for pkgName, symbols := range refs {
 		go func(pkgName string, symbols map[string]bool) {
+			sibling := packageInfo.Imports[pkgName]
+			if sibling.Path != "" {
+				results <- result{ipath: sibling.Path, name: sibling.Alias}
+				return
+			}
 			ipath, rename, err := findImport(pkgName, symbols, filename)
 			r := result{ipath: ipath, err: err}
 			if rename {

imports/fix_test.go

@@ -1536,6 +1536,57 @@ func TestGlobalImports(t *testing.T) {
 	})
 }
 
+// Tests that sibling files - other files in the same package - can provide an
+// import that may not be the default one otherwise.
+func TestSiblingImports(t *testing.T) {
+
+	// provide is the sibling file that provides the desired import.
+	const provide = `package siblingimporttest
+
+import "local/log"
+
+func LogSomething() {
+	log.Print("Something")
+}
+`
+
+	// need is the file being tested that needs the import.
+	const need = `package siblingimporttest
+
+func LogSomethingElse() {
+	log.Print("Something else")
+}
+`
+
+	// want is the expected result file
+	const want = `package siblingimporttest
+
+import "local/log"
+
+func LogSomethingElse() {
+	log.Print("Something else")
+}
+`
+
+	const pkg = "siblingimporttest"
+	const siblingFile = pkg + "/needs_import.go"
+	testConfig{
+		gopathFiles: map[string]string{
+			siblingFile:                 need,
+			pkg + "/provides_import.go": provide,
+		},
+	}.test(t, func(t *goimportTest) {
+		buf, err := Process(
+			t.gopath+"/src/"+siblingFile, []byte(need), nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if string(buf) != want {
+			t.Errorf("wrong output.\ngot:\n%q\nwant:\n%q\n", buf, want)
+		}
+	})
+}
+
 func strSet(ss []string) map[string]bool {
 	m := make(map[string]bool)
 	for _, s := range ss {