Fix legalization crash when processing metal parameter blocks. (#8591)

Closes #7606.

When Slang compile for a bindful target, we will run the resource type
legalization pass to hoist resource typed struct fields outside of the
struct type and define them as global parameters and passing them around
via dedicated function parameters.
When we compile for a bindless target, we don't run this pass.
However, Metal is a hybrid bindful and bindless target. We need to run
type legalization for the constant buffer, but skip type legalization
for parameter block.

The previous attempt to support this behavior is to hack the type
legalization pass to return `LegalVal::simple` when it sees a
`ParameterBlock<T>`. However, whenever the code is accessing
`parameterBlock.someNestedField`, the type of the nested field may get a
`LegalType::tuple`, and now we will run into inconsistent scenarios
where we have a `LegalVal::simple` on the operand val, and but the
legalization logic is expecting that val to be a `LegalType::tuple`.
This breaks a lot of assumptions and invariants in the type legalization
pass, resulting unstable/fragile behavior.

To systematically solve this problem, this change generalizes the
existing legalize buffer element type pass to translate
`ParameterBlock<Texture2D>` (and similar cases) to
`ParameterBlock<Texture2D.Handle>`. So that such parameter block will
always be legalized to `LegalType:::simple` during type legalization,
and we will never run into any inconsistent cases. This allowed us to
get rid of the hacky logic in the type legalization pass to try to
workaround the inconsistencies.

1 files changed, 1 insertions, 0 deletions

diff --git a/source/slang/slang-emit-c-like.cpp b/source/slang/slang-emit-c-like.cpp
index 77c45a6d9..d69485cce 100644
--- a/source/slang/slang-emit-c-like.cpp
+++ b/source/slang/slang-emit-c-like.cpp
@@ -2543,6 +2543,7 @@ void CLikeSourceEmitter::defaultEmitInstExpr(IRInst* inst, const EmitOpInfo& inO
     case kIROp_CastDescriptorHandleToUInt2:
     case kIROp_CastUInt2ToDescriptorHandle:
     case kIROp_CastDescriptorHandleToResource:
+    case kIROp_CastResourceToDescriptorHandle:
     case kIROp_CastUInt64ToDescriptorHandle:
     case kIROp_CastDescriptorHandleToUInt64:
         emitOperand(inst->getOperand(0), outerPrec);