MSL: Don't output depth and stencil values with explicit early fragment tests.

Fragment shaders that require explicit early fragment tests are incompatible
with specifying depth and stencil values within the shader. If explicit early
fragment tests is specified, remove the depth and stencil outputs from the
output structure, and replace them with dummy local variables.

Add CompilerMSL:uses_explicit_early_fragment_test() function to consolidate
testing for whether early fragment tests are required.

Add two unit tests for depth-out with, and without, early fragment tests.

2 files changed, 38 insertions, 0 deletions

diff --git a/reference/opt/shaders-msl/frag/depth-out-early-frag-tests.frag b/reference/opt/shaders-msl/frag/depth-out-early-frag-tests.frag
new file mode 100644
index 00000000..21884d81
--- /dev/null
+++ b/reference/opt/shaders-msl/frag/depth-out-early-frag-tests.frag
@@ -0,0 +1,19 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+struct main0_out
+{
+    float4 color_out [[color(0)]];
+};
+
+[[ early_fragment_tests ]] fragment main0_out main0()
+{
+    float gl_FragDepth;
+    main0_out out = {};
+    out.color_out = float4(1.0, 0.0, 0.0, 1.0);
+    gl_FragDepth = 0.699999988079071044921875;
+    return out;
+}
+
diff --git a/reference/opt/shaders-msl/frag/depth-out-no-early-frag-tests.frag b/reference/opt/shaders-msl/frag/depth-out-no-early-frag-tests.frag
new file mode 100644
index 00000000..57d810fa
--- /dev/null
+++ b/reference/opt/shaders-msl/frag/depth-out-no-early-frag-tests.frag
@@ -0,0 +1,19 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+struct main0_out
+{
+    float4 color_out [[color(0)]];
+    float gl_FragDepth [[depth(less)]];
+};
+
+fragment main0_out main0()
+{
+    main0_out out = {};
+    out.color_out = float4(1.0, 0.0, 0.0, 1.0);
+    out.gl_FragDepth = 0.699999988079071044921875;
+    return out;
+}
+