shader: Implement cap function

Fixes #1361

internal/shader/expr.go

@@ -220,13 +220,13 @@ func (cs *compileState) parseExpr(block *block, expr ast.Expr, markLocalVariable
 		// For built-in functions, we can call this in this position. Return an expression for the function
 		// call.
 		if callee.Type == shaderir.BuiltinFuncExpr {
-			if callee.BuiltinFunc == shaderir.Len {
+			if callee.BuiltinFunc == shaderir.Len || callee.BuiltinFunc == shaderir.Cap {
 				if len(args) != 1 {
-					cs.addError(e.Pos(), fmt.Sprintf("number of len's arguments must be 1 but %d", len(args)))
+					cs.addError(e.Pos(), fmt.Sprintf("number of %s's arguments must be 1 but %d", callee.BuiltinFunc, len(args)))
 					return nil, nil, nil, false
 				}
 				if argts[0].Main != shaderir.Array {
-					cs.addError(e.Pos(), fmt.Sprintf("len takes an array but %s", argts[0].String()))
+					cs.addError(e.Pos(), fmt.Sprintf("%s takes an array but %s", callee.BuiltinFunc, argts[0].String()))
 					return nil, nil, nil, false
 				}
 				return []shaderir.Expr{

internal/shader/testdata/len.expected.vs

@@ -1,4 +1,5 @@
 void F0(in vec2 l0, out int l1);
+void F1(in vec2 l0, out int l1);
 
 void F0(in vec2 l0, out int l1) {
 	int l2[2];
@@ -7,3 +8,12 @@ void F0(in vec2 l0, out int l1) {
 	l1 = 2;
 	return;
 }
+
+void F1(in vec2 l0, out int l1) {
+	int l2[3];
+	l2[0] = 0;
+	l2[1] = 0;
+	l2[2] = 0;
+	l1 = 3;
+	return;
+}

internal/shader/testdata/len.go

@@ -4,3 +4,8 @@ func Foo(foo vec2) int {
 	var a [2]int
 	return len(a)
 }
+
+func Bar(foo vec2) int {
+	var a [3]int
+	return cap(a)
+}

internal/shaderir/program.go

@@ -203,6 +203,7 @@ type BuiltinFunc string
 
 const (
 	Len         BuiltinFunc = "len"
+	Cap         BuiltinFunc = "cap"
 	BoolF       BuiltinFunc = "bool"
 	IntF        BuiltinFunc = "int"
 	FloatF      BuiltinFunc = "float"
@@ -257,6 +258,7 @@ const (
 func ParseBuiltinFunc(str string) (BuiltinFunc, bool) {
 	switch BuiltinFunc(str) {
 	case Len,
+		Cap,
 		BoolF,
 		IntF,
 		FloatF,