GL: make calls to Mesh::setCount() and MeshView::setCount() mandatory.

Helps a lot with black screen of death and should have been here ages
ago.

doc/changelog.dox

@@ -57,6 +57,14 @@ See also:
 
 @subsection changelog-latest-changes Changes and improvements
 
+@subsubsection changelog-latest-changes-gl GL library
+
+-   To prevent nothing being rendered by accident, @ref GL::Mesh::setCount()
+    and @ref GL::MeshView::setCount() now has always to be called, even just to
+    set the count to @cpp 0 @ce.
+
+@subsubsection changelog-latest-changes-platform Platform libraries
+
 -   @ref Platform::GlfwApplication now behaves the same as
     @ref Platform::Sdl2Application when creating an OpenGL context: first it
     attempts to create a forward-compatible core OpenGL 3.2+ context and if

src/Magnum/GL/CMakeLists.txt

@@ -34,7 +34,6 @@ set(MagnumGL_SRCS
     Context.cpp
     DefaultFramebuffer.cpp
     Framebuffer.cpp
-    MeshView.cpp
     OpenGL.cpp
     Renderbuffer.cpp
     Renderer.cpp
@@ -56,6 +55,7 @@ set(MagnumGL_SRCS
 
 set(MagnumGL_GracefulAssert_SRCS
     Mesh.cpp
+    MeshView.cpp
     PixelFormat.cpp
     Sampler.cpp)
 

src/Magnum/GL/Mesh.cpp

@@ -221,7 +221,7 @@ Mesh::~Mesh() {
     (this->*Context::current().state().mesh->destroyImplementation)();
 }
 
-Mesh::Mesh(Mesh&& other) noexcept: _id(other._id), _primitive(other._primitive), _flags{other._flags}, _count(other._count), _baseVertex{other._baseVertex}, _instanceCount{other._instanceCount},
+Mesh::Mesh(Mesh&& other) noexcept: _id(other._id), _primitive(other._primitive), _flags{other._flags}, _countSet{other._countSet}, _count(other._count), _baseVertex{other._baseVertex}, _instanceCount{other._instanceCount},
     #ifndef MAGNUM_TARGET_GLES
     _baseInstance{other._baseInstance},
     #endif
@@ -239,6 +239,7 @@ Mesh& Mesh::operator=(Mesh&& other) noexcept {
     swap(_id, other._id);
     swap(_flags, other._flags);
     swap(_primitive, other._primitive);
+    swap(_countSet, other._countSet);
     swap(_count, other._count);
     swap(_baseVertex, other._baseVertex);
     swap(_instanceCount, other._instanceCount);
@@ -345,6 +346,8 @@ Mesh& Mesh::setIndexBuffer(Buffer& buffer, GLintptr offset, MeshIndexType type,
 }
 
 void Mesh::draw(AbstractShaderProgram& shader) {
+    CORRADE_ASSERT(_countSet, "GL::Mesh::draw(): setCount() was never called, probably a mistake?", );
+
     /* Nothing to draw, exit without touching any state */
     if(!_count || !_instanceCount) return;
 

src/Magnum/GL/Mesh.h

@@ -545,10 +545,15 @@ class MAGNUM_GL_EXPORT Mesh: public AbstractObject {
          * otherwise the value is vertex count. If set to @cpp 0 @ce, no draw
          * commands are issued when calling @ref draw(AbstractShaderProgram&).
          * Ignored when calling @ref draw(AbstractShaderProgram&, TransformFeedback&, UnsignedInt).
-         * Default is @cpp 0 @ce.
+         *
+         * @attention To prevent nothing being rendered by accident, this
+         *      function has to be always called, even to just set the count to
+         *      @cpp 0 @ce.
+         *
          * @see @ref isIndexed(), @ref setBaseVertex(), @ref setInstanceCount()
          */
         Mesh& setCount(Int count) {
+            _countSet = true;
             _count = count;
             return *this;
         }
@@ -1035,6 +1040,9 @@ class MAGNUM_GL_EXPORT Mesh: public AbstractObject {
         GLuint _id;
         MeshPrimitive _primitive;
         ObjectFlags _flags;
+        /* using a separate bool instead of Optional to make use of the 3-byte
+           gap after _flags */
+        bool _countSet{};
         Int _count{}, _baseVertex{}, _instanceCount{1};
         #ifndef MAGNUM_TARGET_GLES
         UnsignedInt _baseInstance{};

src/Magnum/GL/MeshView.cpp

@@ -146,6 +146,8 @@ MeshView& MeshView::setIndexRange(Int first) {
 }
 
 void MeshView::draw(AbstractShaderProgram& shader) {
+    CORRADE_ASSERT(_countSet, "GL::MeshView::draw(): setCount() was never called, probably a mistake?", );
+
     /* Nothing to draw, exit without touching any state */
     if(!_count || !_instanceCount) return;
 

src/Magnum/GL/MeshView.h

@@ -125,9 +125,13 @@ class MAGNUM_GL_EXPORT MeshView {
          * @return Reference to self (for method chaining)
          *
          * Ignored when calling @ref draw(AbstractShaderProgram&, TransformFeedback&, UnsignedInt).
-         * Default is @cpp 0 @ce.
+         *
+         * @attention To prevent nothing being rendered by accident, this
+         *      function has to be always called, even to just set the count to
+         *      @cpp 0 @ce.
          */
         MeshView& setCount(Int count) {
+            _countSet = true;
             _count = count;
             return *this;
         }
@@ -286,6 +290,7 @@ class MAGNUM_GL_EXPORT MeshView {
 
         std::reference_wrapper<Mesh> _original;
 
+        bool _countSet{};
         Int _count{}, _baseVertex{}, _instanceCount{1};
         #ifndef MAGNUM_TARGET_GLES
         UnsignedInt _baseInstance{};

src/Magnum/GL/Test/MeshTest.cpp

@@ -28,15 +28,22 @@
 #include <Corrade/Utility/Configuration.h>
 
 #include "Magnum/Mesh.h"
+#include "Magnum/GL/AbstractShaderProgram.h"
 #include "Magnum/GL/Mesh.h"
+#include "Magnum/GL/MeshView.h"
 
 namespace Magnum { namespace GL { namespace Test {
 
+/* Tests MeshView as well */
+
 struct MeshTest: TestSuite::Tester {
     explicit MeshTest();
 
     void constructNoCreate();
 
+    void drawCountNotSet();
+    void drawViewCountNotSet();
+
     #ifdef MAGNUM_BUILD_DEPRECATED
     void indexSizeDeprecated();
     #endif
@@ -56,6 +63,9 @@ struct MeshTest: TestSuite::Tester {
 MeshTest::MeshTest() {
     addTests({&MeshTest::constructNoCreate,
 
+              &MeshTest::drawCountNotSet,
+              &MeshTest::drawViewCountNotSet,
+
               #ifdef MAGNUM_BUILD_DEPRECATED
               &MeshTest::indexSizeDeprecated,
               #endif
@@ -81,6 +91,35 @@ void MeshTest::constructNoCreate() {
     CORRADE_VERIFY(true);
 }
 
+namespace {
+    struct Shader: AbstractShaderProgram {
+        explicit Shader(NoCreateT): AbstractShaderProgram{NoCreate} {}
+    };
+}
+
+void MeshTest::drawCountNotSet() {
+    std::ostringstream out;
+    Error redirectError{&out};
+
+    Mesh mesh{NoCreate};
+    mesh.draw(Shader{NoCreate});
+
+    CORRADE_COMPARE(out.str(),
+        "GL::Mesh::draw(): setCount() was never called, probably a mistake?\n");
+}
+
+void MeshTest::drawViewCountNotSet() {
+    std::ostringstream out;
+    Error redirectError{&out};
+
+    Mesh mesh{NoCreate};
+    MeshView view{mesh};
+    view.draw(Shader{NoCreate});
+
+    CORRADE_COMPARE(out.str(),
+        "GL::MeshView::draw(): setCount() was never called, probably a mistake?\n");
+}
+
 #ifdef MAGNUM_BUILD_DEPRECATED
 void MeshTest::indexSizeDeprecated() {
     CORRADE_IGNORE_DEPRECATED_PUSH