internal/core/adt: add lifetime check for closedness

Once a counter was decremented to zero, it is no longer
allowed to be incremented.

Issue #2884

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: I82e0883467babd474ae8aaecb729418365181929
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1174013
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@gmail.com>

internal/core/adt/fields.go

@@ -195,6 +195,9 @@ type closeContext struct {
 	// values.
 	isTotal bool
 
+	// done is true if all dependencies have been decremented.
+	done bool
+
 	// needsCloseInSchedule is non-nil if a closeContext that was created
 	// as an arc still needs to be decremented. It points to the creating arc
 	// for reporting purposes.
@@ -425,6 +428,13 @@ func (c *closeContext) incDependent(kind depKind, dependant *closeContext) (debu
 
 	debug = c.addDependent(kind, dependant)
 
+	if c.done {
+		ctx := c.src.state.ctx
+		openDebugGraph(ctx, c.src, "incDependent: already checked")
+
+		panic(fmt.Sprintf("incDependent: already closed: %p", c))
+	}
+
 	c.conjunctCount++
 	return debug
 }
@@ -446,6 +456,8 @@ func (c *closeContext) decDependent(ctx *OpContext, kind depKind, dependant *clo
 		return
 	}
 
+	c.done = true
+
 	p := c.parent
 
 	if c.isDef {