lint(track_config): improve concepts check

src/lint/track_config.nim

@@ -215,8 +215,16 @@ type
   Exercises = object
     `concept`: seq[ConceptExercise]
 
+  Concept = object
+    name: string
+    slug: string
+    uuid: string
+
+  Concepts = seq[Concept]
+
   TrackConfig = object
     exercises: Exercises
+    concepts: Concepts
 
 proc hasValidPrerequisites(s: string; path: Path): bool =
   # TODO: Add the missing checks to this proc.
@@ -230,6 +238,10 @@ proc hasValidPrerequisites(s: string; path: Path): bool =
       for conceptTaught in conceptExercise.concepts:
         conceptsTaught.incl conceptTaught
 
+  var conceptSlugs = initHashSet[string]()
+  for con in trackConfig.concepts:
+    conceptSlugs.incl con.slug
+
   # Require that every prerequisite is taught by different Concept Exercise
   for conceptExercise in trackConfig.exercises.`concept`:
     if conceptExercise.status in [sBeta, sActive]:
@@ -244,6 +256,12 @@ proc hasValidPrerequisites(s: string; path: Path): bool =
                     "`concepts` array of any other user-facing Concept Exercise"
           result.setFalseAndPrint(msg, path)
 
+        if prereq notin conceptSlugs:
+            let msg = &"The Concept Exercise {q conceptExercise.slug} has " &
+                      &"{q preReq} in its `prerequisites`, which is not a `slug` " &
+                      "in the list of `concepts`"
+            result.setFalseAndPrint(msg, path)
+
 proc isValidTrackConfig(data: JsonNode; path: Path): bool =
   if isObject(data, jsonRoot, path):
     let checks = [