Warn on unused return values in guidance code, resolves #2686.

https://github.com/Project-OSRM/osrm-backend/pull/2685/files

fixes an issue where we did

    elongate(fstStep, sndStep);

instead of

    newStep = elongate(fstStep, sndStep);

we didn't get any warnings.

The only way to trigger a warning here is to use

```cpp
__attribute__((warn_unused_result))
```

This changeset does exactly that: for the new guidance code prone to
these kind of issue we add such an attribute to the declaration.

include/engine/guidance/lane_processing.hpp

@@ -4,6 +4,7 @@
 #include <vector>
 
 #include "engine/guidance/route_step.hpp"
+#include "util/attributes.hpp"
 
 namespace osrm
 {
@@ -17,10 +18,12 @@ namespace guidance
 // we anticipate lane changes emitting only matching lanes early on.
 // the second parameter describes the duration that we feel two segments need to be apart to count
 // as separate maneuvers.
+OSRM_ATTR_WARN_UNUSED
 std::vector<RouteStep> anticipateLaneChange(std::vector<RouteStep> steps,
                                             const double min_duration_needed_for_lane_change = 15);
 
 // Remove all lane information from roundabouts. See #2626.
+OSRM_ATTR_WARN_UNUSED
 std::vector<RouteStep> removeLanesFromRoundabouts(std::vector<RouteStep> steps);
 
 } // namespace guidance

include/engine/guidance/post_processing.hpp

@@ -4,6 +4,7 @@
 #include "engine/guidance/leg_geometry.hpp"
 #include "engine/guidance/route_step.hpp"
 #include "engine/phantom_node.hpp"
+#include "util/attributes.hpp"
 
 #include <vector>
 
@@ -15,6 +16,7 @@ namespace guidance
 {
 
 // passed as none-reference to modify in-place and move out again
+OSRM_ATTR_WARN_UNUSED
 std::vector<RouteStep> postProcess(std::vector<RouteStep> steps);
 
 // Multiple possible reasons can result in unnecessary/confusing instructions
@@ -22,6 +24,7 @@ std::vector<RouteStep> postProcess(std::vector<RouteStep> steps);
 // intersection would result in two instructions to turn left.
 // Collapsing such turns into a single turn instruction, we give a clearer
 // set of instructionst that is not cluttered by unnecessary turns/name changes.
+OSRM_ATTR_WARN_UNUSED
 std::vector<RouteStep> collapseTurns(std::vector<RouteStep> steps);
 
 // trim initial/final segment of very short length.
@@ -32,15 +35,18 @@ std::vector<RouteStep> collapseTurns(std::vector<RouteStep> steps);
 void trimShortSegments(std::vector<RouteStep> &steps, LegGeometry &geometry);
 
 // assign relative locations to depart/arrive instructions
+OSRM_ATTR_WARN_UNUSED
 std::vector<RouteStep> assignRelativeLocations(std::vector<RouteStep> steps,
                                                const LegGeometry &geometry,
                                                const PhantomNode &source_node,
                                                const PhantomNode &target_node);
 
 // collapse suppressed instructions remaining into intersections array
+OSRM_ATTR_WARN_UNUSED
 std::vector<RouteStep> buildIntersections(std::vector<RouteStep> steps);
 
 // remove steps invalidated by post-processing
+OSRM_ATTR_WARN_UNUSED
 std::vector<RouteStep> removeNoTurnInstructions(std::vector<RouteStep> steps);
 
 // remove use lane information that is not actually a turn. For post-processing, we need to
@@ -50,6 +56,7 @@ std::vector<RouteStep> removeNoTurnInstructions(std::vector<RouteStep> steps);
 // FIXME this is currently only a heuristic. We need knowledge on which lanes actually might become
 // turn lanes. If a straight lane becomes a turn lane, this might be something to consider. Right
 // now we bet on lane-anticipation to catch this.
+OSRM_ATTR_WARN_UNUSED
 std::vector<RouteStep> collapseUseLane(std::vector<RouteStep> steps);
 
 // postProcess will break the connection between the leg geometry
@@ -57,6 +64,7 @@ std::vector<RouteStep> collapseUseLane(std::vector<RouteStep> steps);
 // between routing maneuvers and the route steps itself.
 // If required, we can get both in sync again using this function.
 // Move in LegGeometry for modification in place.
+OSRM_ATTR_WARN_UNUSED
 LegGeometry resyncGeometry(LegGeometry leg_geometry, const std::vector<RouteStep> &steps);
 
 } // namespace guidance