Emit errors when seeing a matrix with row/col count of 1

Such matrices aren't well supported by our targets, and we currently don't transform such
types to supported ones.
Therefore, generate an error rather than outputting invalid target code.

This closes shader-slang#5987.

Author: aleino-nv

source/slang/slang-check-decl.cpp

@@ -11876,6 +11876,20 @@ void SemanticsDeclAttributesVisitor::checkVarDeclCommon(VarDeclBase* varDecl)
             getSink()->diagnose(varDecl, Diagnostics::pushOrSpecializationConstantCannotBeStatic);
         }
     }
+
+    if (auto matrixType = as<MatrixExpressionType>(varDecl->getType()))
+    {
+        auto rowCountVal = matrixType->getRowCount();
+        auto colCountVal = matrixType->getColumnCount();
+        ConstantIntVal* rowCountConst = as<ConstantIntVal>(rowCountVal);
+        ConstantIntVal* colCountConst = as<ConstantIntVal>(colCountVal);
+        IntegerLiteralValue rowCount = rowCountConst ? rowCountConst->getValue() : -1;
+        IntegerLiteralValue colCount = colCountConst ? colCountConst->getValue() : -1;
+        if (rowCount == 1 || colCount == 1)
+        {
+            getSink()->diagnose(varDecl, Diagnostics::matrixColumnOrRowCountIsOne, matrixType);
+        }
+    }
 }
 
 void SemanticsDeclAttributesVisitor::checkForwardDerivativeOfAttribute(

source/slang/slang-diagnostic-defs.h

@@ -1658,6 +1658,12 @@ DIAGNOSTIC(
     overloadedParameterToHigherOrderFunction,
     "passing overloaded functions to higher order functions is not supported")
 
+DIAGNOSTIC(
+    39999,
+    Error,
+    matrixColumnOrRowCountIsOne,
+    "matrix of type '$0' has column or row count of 1")
+
 // 38xxx
 
 DIAGNOSTIC(