warn on rejected line geometry

src/mbgl/renderer/buckets/line_bucket.cpp

@@ -86,7 +86,19 @@ void LineBucket::addGeometry(const GeometryCoordinates& coordinates,
     }();
 
     // Ignore invalid geometry.
-    if (len < (options.type == FeatureType::Polygon ? 3 : 2)) {
+    const std::size_t minLen = (options.type == FeatureType::Polygon ? 3 : 2);
+    if (len < minLen) {
+        // Warn once, but only if the source geometry is invalid, not if de-duplication made it invalid.
+        // This happens, e.g., when attempting to use a GeoJSON `MultiPoint`
+        // or `MLNPointCollectionFeature` as the source for a line layer.
+        // Unfortunately, we cannot show the layer or source name from here.
+        if (coordinates.size() < minLen) {
+            static bool warned = false; // not thread-safe, there's a small chance of warning more than once
+            if (!warned) {
+                warned = true;
+                Log::Warning(Event::General, "Invalid geometry in line layer");
+            }
+        }
         return;
     }