[clang] HLSL availability diagnostics design doc (PR #92207)

[Cooper Partin via cfe-commits]

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+=============================
+HLSL Availability Diagnostics
+=============================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+HLSL availability diagnostics emits errors or warning when unavailable shader APIs are used. Unavailable shader APIs are APIs that are exposed in HLSL code but are not available in the target shader stage or shader model version.
+
+There are three modes of HLSL availability diagnostic:
+
+#. **Default mode** - compiler emits an error when an unavailable shader API is found in a code that is reachable from the shader entry point function or from an exported library function (when compiling a shader library)
+
+#. **Relaxed mode** - same as default mode except the compiler emits a warning. This mode is enabled by ``-Wno-error=hlsl-availability``.
+
+#. **Strict mode** - compiler emits an error when when an unavailable API is found in parsed code regardless of whether it can be reached from the shader entry point or exported functions, or not. This mode is enabled by ``-fhlsl-strict-diagnostics``.
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coopp wrote:

Extra "when" in Strict mode description.

https://github.com/llvm/llvm-project/pull/92207