1 files changed, 169 insertions, 0 deletions
diff --git a/src/cmd/gofmt/simplify.go b/src/cmd/gofmt/simplify.go
new file mode 100644
index 0000000..eb55daa
--- /dev/null
+++ b/src/cmd/gofmt/simplify.go
@@ -0,0 +1,169 @@
+// 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 main
+
+import (
+	"go/ast"
+	"go/token"
+	"reflect"
+)
+
+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
+		//       the same package. However, this is extremely unlikely and so far
+		//       (April 2022, after years of supporting this rewrite feature)
+		//       has never come up, so let's keep it working as is (see also #15153).
+		//
+		// Also note that this code used to use go/ast's object tracking,
+		// which was removed in exchange for go/parser.Mode.SkipObjectResolution.
+		// False positives are extremely unlikely as described above,
+		// and go/ast's object tracking is incomplete in any case.
+		if n.Max != nil {
+			// - 3-index slices always require the 2nd and 3rd index
+			break
+		}
+		if s, _ := n.X.(*ast.Ident); s != nil {
+			// the array/slice object is a single 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" {
+					// the function called is "len"
+					if arg, _ := call.Args[0].(*ast.Ident); arg != nil && arg.Name == s.Name {
+						// 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
+}