Handle possibility of bad types in varying input/output signature.

Fixes #160

If the front-end runs into a type it doesn't understand in the parameter list of an entry point, it will create an `ErrorType` for that parameter, but then the parameter binding/layout rules will fail to create a `TypeLayout` for the prameter (and return `NULL`).
There were some places where the code was expecting that operation to succeed unconditionally, and so would crash when there was a bad type.

The specific case in the bug report was when the return type of a shader entry point was bad:

    // `vec4` is not an HLSL type
    vec4 main(...) { ... }

Note that the specific case in the buf report only manifests in "rewriter" mode (when the Slang compiler isn't allowed to issue error messages from the front-end), but the same basic thing would happen if the varying parameter/output had used a type that is invalid for varying input/output:

    Texture2D main(...) { ... }

I'm not 100% happy with just adding more `NULL` checks for this, because there is no easy way to tell if they are exhaustive.
A better solution in the longer term might be to construct a kind of `ErrorTypeLayout` to represent cases where we wanted a type layout, but none could be constructed.

2 files changed, 16 insertions, 0 deletions

diff --git a/tests/rewriter/gh-160.hlsl b/tests/rewriter/gh-160.hlsl
new file mode 100644
index 000000000..46ef6a92c
--- /dev/null
+++ b/tests/rewriter/gh-160.hlsl
@@ -0,0 +1,13 @@
+//TEST:COMPARE_HLSL: -no-checking -target dxbc-assembly -profile ps_4_0 -entry main
+
+#ifdef __SLANG__
+__import gh_160;
+#endif
+
+vec4 main(VS_OUT vOut) : SV_TARGET
+{
+	float3 color = float3(1,0,0);
+
+    vec4 finalColor = vec4(color, 1.f);
+    return finalColor;
+}
\ No newline at end of file
diff --git a/tests/rewriter/gh-160.slang b/tests/rewriter/gh-160.slang
new file mode 100644
index 000000000..c86f63c19
--- /dev/null
+++ b/tests/rewriter/gh-160.slang
@@ -0,0 +1,3 @@
+//TEST_IGNORE_FILE:
+
+// This file only exists so that `gh-160.hlsl` can import it.