cuego: remove deprecated cue.Instance usage

*cue.Instance is deprecated and being replaced by cue.Value. This
CL replaces the usage of a *cue.Instance by the equivalent
cue.Context method.

Updates #2480.

Signed-off-by: Noam Dolovich <[email protected]>
Change-Id: Idb1c4a3dab4144fe62bcb1f58deb926fb46082e0
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194691
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: NoamTD

cuego/cuego.go

@@ -140,7 +140,7 @@ func (c *Context) Constrain(x interface{}, constraints string) error {
 		return err
 	}
 
-	v := instance.Eval(expr)
+	v := runtime.BuildExpr(expr)
 	if v.Err() != nil {
 		return err
 	}
@@ -158,19 +158,10 @@ func (c *Context) Constrain(x interface{}, constraints string) error {
 }
 
 var (
-	mutex    sync.Mutex
-	instance *cue.Instance
-	runtime  = cuecontext.New()
+	mutex   sync.Mutex
+	runtime = cuecontext.New()
 )
 
-func init() {
-	var err error
-	instance, err = value.ConvertToRuntime(runtime).Compile("<cuego>", "{}")
-	if err != nil {
-		panic(err)
-	}
-}
-
 // fromGoValue converts a Go value to CUE
 func fromGoValue(x interface{}, nilIsNull bool) (v cue.Value, err error) {
 	// TODO: remove the need to have a lock here. We could use a new index (new

cuego/doc.go

@@ -54,7 +54,7 @@
 // A Complete will implicitly validate a struct.
 package cuego // import "cuelang.org/go/cuego"
 
-// The first goal of this packages is to get the semantics right. After that,
+// The first goal of this package is to get the semantics right. After that,
 // there are a lot of performance gains to be made:
 // - cache the type info extracted during value (as opposed to type) conversion
 // - remove the usage of mutex for value conversions