doc/ref/spec.md: allow ... in an ast.File

This is consistent with allowing ... in non-schema.

Other benefits:
- code generation can generate []Decl regardless of
whether it is used for a File or StructLit.
- once we have topological sort, `...` could be used
to hint insertion points when allowed at an arbitrary
position in the Decls list (although [string]: foo could
be used for that too).
- when adopting JSON Schema semantics, ...T may
has meaning at the file level as well.

Change-Id: Icfb299f951f5531119d33aeb5d89237eb64e33a1
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5940
Reviewed-by: roger peppe <rogpeppe@gmail.com>

cue/ast.go

@@ -219,14 +219,19 @@ func (v *astVisitor) walk(astNode ast.Node) (ret value) {
 			astState: v.astState,
 			object:   obj,
 		}
-		for _, e := range n.Decls {
+		for i, e := range n.Decls {
 			switch x := e.(type) {
 			case *ast.EmbedDecl:
 				if v1.object.emit == nil {
 					v1.object.emit = v1.walk(x.Expr)
 				} else {
 					v1.object.emit = mkBin(v.ctx(), token.NoPos, opUnify, v1.object.emit, v1.walk(x.Expr))
 				}
+			case *ast.Ellipsis:
+				if i != len(n.Decls)-1 {
+					return v1.walk(x.Type) // Generate an error
+				}
+
 			default:
 				v1.walk(e)
 			}

doc/ref/spec.md

@@ -2859,8 +2859,9 @@ If the result of the unification of all embedded values is not a struct,
 it will be output instead of its enclosing file when exporting CUE
 to a data format
 
+<!-- TODO: allow ... anywhere in SourceFile and struct. -->
 ```
-SourceFile      = [ PackageClause "," ] { ImportDecl "," } { Declaration "," } .
+SourceFile = [ PackageClause "," ] { ImportDecl "," } { Declaration "," }  [ "..." ] .
 ```
 
 ```