internal/core/export: fix bug 473

Ensure that exporter always exports a struct literal,
and not an arbitrary expression.

TODO: this can be enforced at compile time by
changing the type of the Value field to *ast.StructLit.
This is a breaking change in the API though. At some
point this will be useful, as it will help prevent users
of the API to make similar errors.

Fixes #473

Change-Id: I26d1590c9b793ad5c82b02f69af47795a4bbd1a4
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6950
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>

cmd/cue/cmd/testdata/script/issue473.txt

@@ -0,0 +1,36 @@
+cue eval --out json x.cue
+
+-- cue.mod/module.cue --
+module: "mod.com"
+-- x.cue --
+package x
+
+#Guide: {
+        Terminals: [string]: #Terminal
+
+        Steps: [string]: #Step
+
+        #TerminalName: or([ for k, _ in Terminals {k}])
+
+        #Step: {
+                Terminal: #TerminalName
+                Cmd:      string
+        }
+
+        #Terminal: {
+                Image: string
+        }
+}
+
+g: #Guide & {
+        Terminals: client: {
+                Image: "golang"
+        }
+
+        Steps: {
+                list: {
+                        Terminal: "client"
+                        Cmd:      "ls"
+                }
+        }
+}

internal/core/compile/compile.go

@@ -663,6 +663,7 @@ func (c *compiler) comprehension(x *ast.Comprehension) adt.Elem {
 		prev = next
 	}
 
+	// TODO: make x.Value an *ast.StructLit and this is redundant.
 	if y, ok := x.Value.(*ast.StructLit); !ok {
 		return c.errf(x.Value,
 			"comprehension value must be struct, found %T", y)

internal/core/export/adt.go

@@ -398,7 +398,11 @@ func (e *exporter) comprehension(y adt.Yielder) ast.Expr {
 			y = x.Dst
 
 		case *adt.ValueClause:
-			c.Value = e.expr(x.StructLit)
+			v := e.expr(x.StructLit)
+			if _, ok := v.(*ast.StructLit); !ok {
+				v = ast.NewStruct(ast.Embed(v))
+			}
+			c.Value = v
 			return c
 
 		default: