Add SimplifyAST option to match 'gofmt -s'

imports.go

@@ -36,7 +36,8 @@ type Options struct {
 	TabIndent bool // Use tabs for indent (true if nil *Options provided)
 	TabWidth  int  // Tab width (8 if nil *Options provided)
 
-	FormatOnly bool // Disable the insertion and deletion of imports
+	FormatOnly  bool // Disable the insertion and deletion of imports
+	SimplifyAST bool // simplify code
 }
 
 // Process formats and adjusts imports for the provided file.
@@ -93,6 +94,9 @@ func process(filename string, src []byte, opt *Options, env *fixEnv) ([]byte, er
 		}
 
 	}
+	if opt.SimplifyAST {
+		simplify(file)
+	}
 
 	printerMode := printer.UseSpaces
 	if opt.TabIndent {

simplify.go

@@ -0,0 +1,270 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package imports
+
+import (
+	"go/ast"
+	"go/token"
+	"reflect"
+	"unicode"
+	"unicode/utf8"
+)
+
+type simplifier struct{}
+
+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
+		outer := n
+		var keyType, eltType ast.Expr
+		switch typ := outer.Type.(type) {
+		case *ast.ArrayType:
+			eltType = typ.Elt
+		case *ast.MapType:
+			keyType = typ.Key
+			eltType = typ.Value
+		}
+
+		if eltType != nil {
+			var ktyp reflect.Value
+			if keyType != nil {
+				ktyp = reflect.ValueOf(keyType)
+			}
+			typ := reflect.ValueOf(eltType)
+			for i, x := range outer.Elts {
+				px := &outer.Elts[i]
+				// look at value of indexed/named elements
+				if t, ok := x.(*ast.KeyValueExpr); ok {
+					if keyType != nil {
+						s.simplifyLiteral(ktyp, keyType, t.Key, &t.Key)
+					}
+					x = t.Value
+					px = &t.Value
+				}
+				s.simplifyLiteral(typ, eltType, x, px)
+			}
+			// node was simplified - stop walk (there are no subnodes to simplify)
+			return nil
+		}
+
+	case *ast.SliceExpr:
+		// 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)
+		//
+		// 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
+			break
+		}
+		if s, _ := n.X.(*ast.Ident); s != nil && s.Obj != nil {
+			// the array/slice object is a single, resolved identifier
+			if call, _ := n.High.(*ast.CallExpr); call != nil && len(call.Args) == 1 && !call.Ellipsis.IsValid() {
+				// the high expression is a function call with a single argument
+				if fun, _ := call.Fun.(*ast.Ident); fun != nil && fun.Name == "len" && fun.Obj == nil {
+					// the function called is "len" and it is not locally defined; and
+					// because we don't have dot imports, it must be the predefined len()
+					if arg, _ := call.Args[0].(*ast.Ident); arg != nil && arg.Obj == s.Obj {
+						// the len argument is the array/slice object
+						n.High = nil
+					}
+				}
+			}
+		}
+		// Note: We could also simplify slice expressions of the form s[0:b] to s[:b]
+		//       but we leave them as is since sometimes we want to be very explicit
+		//       about the lower bound.
+		// An example where the 0 helps:
+		//       x, y, z := b[0:2], b[2:4], b[4:6]
+		// An example where it does not:
+		//       x, y := b[:n], b[n:]
+
+	case *ast.RangeStmt:
+		// - a range of the form: for x, _ = range v {...}
+		// can be simplified to: for x = range v {...}
+		// - a range of the form: for _ = range v {...}
+		// can be simplified to: for range v {...}
+		if isBlank(n.Value) {
+			n.Value = nil
+		}
+		if isBlank(n.Key) && n.Value == nil {
+			n.Key = nil
+		}
+	}
+
+	return s
+}
+
+func (s simplifier) simplifyLiteral(typ reflect.Value, astType, x ast.Expr, px *ast.Expr) {
+	ast.Walk(s, x) // simplify x
+
+	// if the element is a composite literal and its literal type
+	// matches the outer literal's element type exactly, the inner
+	// literal type may be omitted
+	if inner, ok := x.(*ast.CompositeLit); ok {
+		if match(nil, typ, reflect.ValueOf(inner.Type)) {
+			inner.Type = nil
+		}
+	}
+	// if the outer literal's element type is a pointer type *T
+	// and the element is & of a composite literal of type T,
+	// the inner &T may be omitted.
+	if ptr, ok := astType.(*ast.StarExpr); ok {
+		if addr, ok := x.(*ast.UnaryExpr); ok && addr.Op == token.AND {
+			if inner, ok := addr.X.(*ast.CompositeLit); ok {
+				if match(nil, reflect.ValueOf(ptr.X), reflect.ValueOf(inner.Type)) {
+					inner.Type = nil // drop T
+					*px = inner      // drop &
+				}
+			}
+		}
+	}
+}
+
+func isBlank(x ast.Expr) bool {
+	ident, ok := x.(*ast.Ident)
+	return ok && ident.Name == "_"
+}
+
+func simplify(f *ast.File) {
+	// remove empty declarations such as "const ()", etc
+	removeEmptyDeclGroups(f)
+
+	var s simplifier
+	ast.Walk(s, f)
+}
+
+func removeEmptyDeclGroups(f *ast.File) {
+	i := 0
+	for _, d := range f.Decls {
+		if g, ok := d.(*ast.GenDecl); !ok || !isEmpty(f, g) {
+			f.Decls[i] = d
+			i++
+		}
+	}
+	f.Decls = f.Decls[:i]
+}
+
+func isEmpty(f *ast.File, g *ast.GenDecl) bool {
+	if g.Doc != nil || g.Specs != nil {
+		return false
+	}
+
+	for _, c := range f.Comments {
+		// if there is a comment in the declaration, it is not considered empty
+		if g.Pos() <= c.Pos() && c.End() <= g.End() {
+			return false
+		}
+	}
+
+	return true
+}
+
+// -----------------------------------------------------------------------------
+// rewrite.go logic
+
+// Values/types for special cases.
+var (
+	identType     = reflect.TypeOf((*ast.Ident)(nil))
+	objectPtrType = reflect.TypeOf((*ast.Object)(nil))
+	positionType  = reflect.TypeOf(token.NoPos)
+	callExprType  = reflect.TypeOf((*ast.CallExpr)(nil))
+)
+
+func isWildcard(s string) bool {
+	rune, size := utf8.DecodeRuneInString(s)
+	return size == len(s) && unicode.IsLower(rune)
+}
+
+// match reports whether pattern matches val,
+// recording wildcard submatches in m.
+// If m == nil, match checks whether pattern == val.
+func match(m map[string]reflect.Value, pattern, val reflect.Value) bool {
+	// Wildcard matches any expression. If it appears multiple
+	// times in the pattern, it must match the same expression
+	// each time.
+	if m != nil && pattern.IsValid() && pattern.Type() == identType {
+		name := pattern.Interface().(*ast.Ident).Name
+		if isWildcard(name) && val.IsValid() {
+			// wildcards only match valid (non-nil) expressions.
+			if _, ok := val.Interface().(ast.Expr); ok && !val.IsNil() {
+				if old, ok := m[name]; ok {
+					return match(nil, old, val)
+				}
+				m[name] = val
+				return true
+			}
+		}
+	}
+
+	// Otherwise, pattern and val must match recursively.
+	if !pattern.IsValid() || !val.IsValid() {
+		return !pattern.IsValid() && !val.IsValid()
+	}
+	if pattern.Type() != val.Type() {
+		return false
+	}
+
+	// Special cases.
+	switch pattern.Type() {
+	case identType:
+		// For identifiers, only the names need to match
+		// (and none of the other *ast.Object information).
+		// This is a common case, handle it all here instead
+		// of recursing down any further via reflection.
+		p := pattern.Interface().(*ast.Ident)
+		v := val.Interface().(*ast.Ident)
+		return p == nil && v == nil || p != nil && v != nil && p.Name == v.Name
+	case objectPtrType, positionType:
+		// object pointers and token positions always match
+		return true
+	case callExprType:
+		// For calls, the Ellipsis fields (token.Position) must
+		// match since that is how f(x) and f(x...) are different.
+		// Check them here but fall through for the remaining fields.
+		p := pattern.Interface().(*ast.CallExpr)
+		v := val.Interface().(*ast.CallExpr)
+		if p.Ellipsis.IsValid() != v.Ellipsis.IsValid() {
+			return false
+		}
+	}
+
+	p := reflect.Indirect(pattern)
+	v := reflect.Indirect(val)
+	if !p.IsValid() || !v.IsValid() {
+		return !p.IsValid() && !v.IsValid()
+	}
+
+	switch p.Kind() {
+	case reflect.Slice:
+		if p.Len() != v.Len() {
+			return false
+		}
+		for i := 0; i < p.Len(); i++ {
+			if !match(m, p.Index(i), v.Index(i)) {
+				return false
+			}
+		}
+		return true
+
+	case reflect.Struct:
+		for i := 0; i < p.NumField(); i++ {
+			if !match(m, p.Field(i), v.Field(i)) {
+				return false
+			}
+		}
+		return true
+
+	case reflect.Interface:
+		return match(m, p.Elem(), v.Elem())
+	}
+
+	// Handle token integers, etc.
+	return p.Interface() == v.Interface()
+}