cmd/gofmt: make gofmt -s simplify slices in presence of dot-imports

A dot-import cannot possibly introduce a `len` function since that
function would not be exported (it's lowercase). Furthermore, the
existing code already (incorrectly) assumed that there was no other
`len` function in another file of the package. Since this has been
an ok assumption for years, let's leave it, but remove the dot-import
restriction.

Fixes #15153.

Change-Id: I18fbb27acc5a5668833b4b4aead0cca540862b52
Reviewed-on: https://go-review.googlesource.com/21613Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/gofmt/simplify.go

@@ -10,11 +10,9 @@ import (
 	"reflect"
 )
 
-type simplifier struct {
-	hasDotImport bool // package file contains: import . "some/import/path"
-}
+type simplifier struct{}
 
-func (s *simplifier) Visit(node ast.Node) ast.Visitor {
+func (s simplifier) Visit(node ast.Node) ast.Visitor {
 	switch n := node.(type) {
 	case *ast.CompositeLit:
 		// array, slice, and map composite literals may be simplified
@@ -68,10 +66,13 @@ func (s *simplifier) Visit(node ast.Node) ast.Visitor {
 		// a slice expression of the form: s[a:len(s)]
 		// can be simplified to: s[a:]
 		// if s is "simple enough" (for now we only accept identifiers)
-		if n.Max != nil || s.hasDotImport {
+		//
+		// Note: This may not be correct because len may have been redeclared in another
+		//       file belonging to the same package. However, this is extremely unlikely
+		//       and so far (April 2016, after years of supporting this rewrite feature)
+		//       has never come up, so let's keep it working as is (see also #15153).
+		if n.Max != nil {
 			// - 3-index slices always require the 2nd and 3rd index
-			// - if dot imports are present, we cannot be certain that an
-			//   unresolved "len" identifier refers to the predefined len()
 			break
 		}
 		if s, _ := n.X.(*ast.Ident); s != nil && s.Obj != nil {
@@ -118,20 +119,11 @@ func isBlank(x ast.Expr) bool {
 }
 
 func simplify(f *ast.File) {
-	var s simplifier
-
-	// determine if f contains dot imports
-	for _, imp := range f.Imports {
-		if imp.Name != nil && imp.Name.Name == "." {
-			s.hasDotImport = true
-			break
-		}
-	}
-
 	// remove empty declarations such as "const ()", etc
 	removeEmptyDeclGroups(f)
 
-	ast.Walk(&s, f)
+	var s simplifier
+	ast.Walk(s, f)
 }
 
 func removeEmptyDeclGroups(f *ast.File) {

src/cmd/gofmt/testdata/slices2.golden

-//gofmt -s
-
-// Test cases for slice expression simplification.
-// Because of a dot import, these slices must remain untouched.
-package p
-
-import . "math"
-
-var (
-	a [10]byte
-	b [20]float32
-	s []int
-	t struct {
-		s []byte
-	}
-
-	_ = a[0:]
-	_ = a[1:10]
-	_ = a[2:len(a)]
-	_ = a[3:(len(a))]
-	_ = a[len(a) : len(a)-1]
-	_ = a[0:len(b)]
-
-	_ = a[:]
-	_ = a[:10]
-	_ = a[:len(a)]
-	_ = a[:(len(a))]
-	_ = a[:len(a)-1]
-	_ = a[:len(b)]
-
-	_ = s[0:]
-	_ = s[1:10]
-	_ = s[2:len(s)]
-	_ = s[3:(len(s))]
-	_ = s[len(a) : len(s)-1]
-	_ = s[0:len(b)]
-
-	_ = s[:]
-	_ = s[:10]
-	_ = s[:len(s)]
-	_ = s[:(len(s))]
-	_ = s[:len(s)-1]
-	_ = s[:len(b)]
-
-	_ = t.s[0:]
-	_ = t.s[1:10]
-	_ = t.s[2:len(t.s)]
-	_ = t.s[3:(len(t.s))]
-	_ = t.s[len(a) : len(t.s)-1]
-	_ = t.s[0:len(b)]
-
-	_ = t.s[:]
-	_ = t.s[:10]
-	_ = t.s[:len(t.s)]
-	_ = t.s[:(len(t.s))]
-	_ = t.s[:len(t.s)-1]
-	_ = t.s[:len(b)]
-)
-
-func _() {
-	s := s[0:len(s)]
-	_ = s
-}

src/cmd/gofmt/testdata/slices2.input

-//gofmt -s
-
-// Test cases for slice expression simplification.
-// Because of a dot import, these slices must remain untouched.
-package p
-
-import . "math"
-
-var (
-	a [10]byte
-	b [20]float32
-	s []int
-	t struct {
-		s []byte
-	}
-
-	_ = a[0:]
-	_ = a[1:10]
-	_ = a[2:len(a)]
-	_ = a[3:(len(a))]
-	_ = a[len(a) : len(a)-1]
-	_ = a[0:len(b)]
-
-	_ = a[:]
-	_ = a[:10]
-	_ = a[:len(a)]
-	_ = a[:(len(a))]
-	_ = a[:len(a)-1]
-	_ = a[:len(b)]
-
-	_ = s[0:]
-	_ = s[1:10]
-	_ = s[2:len(s)]
-	_ = s[3:(len(s))]
-	_ = s[len(a) : len(s)-1]
-	_ = s[0:len(b)]
-
-	_ = s[:]
-	_ = s[:10]
-	_ = s[:len(s)]
-	_ = s[:(len(s))]
-	_ = s[:len(s)-1]
-	_ = s[:len(b)]
-
-	_ = t.s[0:]
-	_ = t.s[1:10]
-	_ = t.s[2:len(t.s)]
-	_ = t.s[3:(len(t.s))]
-	_ = t.s[len(a) : len(t.s)-1]
-	_ = t.s[0:len(b)]
-
-	_ = t.s[:]
-	_ = t.s[:10]
-	_ = t.s[:len(t.s)]
-	_ = t.s[:(len(t.s))]
-	_ = t.s[:len(t.s)-1]
-	_ = t.s[:len(b)]
-)
-
-func _() {
-	s := s[0:len(s)]
-	_ = s
-}