gofmt: simplify "x, _ = range y" to "x = range y"

(inspired by CL 3529041 by hitchmanr@gmail.com)

R=rsc
CC=golang-dev
https://golang.org/cl/3527042

diff --git a/src/cmd/gofmt/simplify.go b/src/cmd/gofmt/simplify.go
index de135f3f684a49ba101ba4ce898e491ebab0fb9d..b166f04d8eb6bef7e7dbf2f87f3f72f7f33d6f71 100644 (file)
--- a/src/cmd/gofmt/simplify.go
+++ b/src/cmd/gofmt/simplify.go
@@ -10,11 +10,13 @@ import (
 )
 
 
-type compositeLitFinder struct{}
+type simplifier struct{}
 
-func (f *compositeLitFinder) Visit(node interface{}) ast.Visitor {
-       if outer, ok := node.(*ast.CompositeLit); ok {
+func (s *simplifier) Visit(node interface{}) ast.Visitor {
+       switch n := node.(type) {
+       case *ast.CompositeLit:
                // array, slice, and map composite literals may be simplified
+               outer := n
                var eltType ast.Expr
                switch typ := outer.Type.(type) {
                case *ast.ArrayType:
@@ -41,17 +43,25 @@ func (f *compositeLitFinder) Visit(node interface{}) ast.Visitor {
                                }
                        }
 
-                       // node was simplified - stop walk
+                       // node was simplified - stop walk (there are no subnodes to simplify)
                        return nil
                }
+
+       case *ast.RangeStmt:
+               // range of the form: for x, _ = range v {...}
+               // can be simplified to: for x = range v {...}
+               if n.Value != nil {
+                       if ident, ok := n.Value.(*ast.Ident); ok && ident.Name == "_" {
+                               n.Value = nil
+                       }
+               }
        }
 
-       // not a composite literal or not simplified - continue walk
-       return f
+       return s
 }
 
 
 func simplify(node interface{}) {
-       var f compositeLitFinder
-       ast.Walk(&f, node)
+       var s simplifier
+       ast.Walk(&s, node)
 }