cmd/cue/cmd: fmt rewrites alias to let clauses

Issue #339

Change-Id: I0bcb634251e54300f8d84f8a822c2081902a85dd
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5685
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cmd/cue/cmd/fix.go

@@ -25,6 +25,29 @@ import (
 )
 
 func fix(f *ast.File) *ast.File {
+	// Rewrite an old-style alias to a let clause.
+	ast.Walk(f, func(n ast.Node) bool {
+		var decls []ast.Decl
+		switch x := n.(type) {
+		case *ast.StructLit:
+			decls = x.Elts
+		case *ast.File:
+			decls = x.Decls
+		}
+		for i, d := range decls {
+			if a, ok := d.(*ast.Alias); ok {
+				x := &ast.LetClause{
+					Ident: a.Ident,
+					Equal: a.Equal,
+					Expr:  a.Expr,
+				}
+				astutil.CopyMeta(x, a)
+				decls[i] = x
+			}
+		}
+		return true
+	}, nil)
+
 	// Rewrite block comments to regular comments.
 	ast.Walk(f, func(n ast.Node) bool {
 		switch x := n.(type) {

cmd/cue/cmd/fix_test.go

@@ -61,6 +61,13 @@ a: {
 		`,
 		out: `y: [1, 2, 3, 4]
 a: [ for x in y { x } ]
+`,
+	}, {
+		in: `
+		y = foo
+		`,
+		out: `
+let y = foo
 `,
 		// 	}, {
 		// 		name: "slice",

cmd/cue/cmd/testdata/script/legacy.txt

@@ -11,6 +11,10 @@ cmp foo.cue bar.cue
 -- foo.cue --
 a: [1]
 b: [ x for x in a ]
+
+X = foo
 -- bar.cue --
 a: [1]
 b: [ for x in a { x } ]
+
+let X = foo