builtin: add .nogrow and .nofree flags to array (#16661)

vlib/builtin/array.v

@@ -24,6 +24,8 @@ pub mut:
 pub enum ArrayFlags {
 	noslices // when <<, `.noslices` will free the old data block immediately (you have to be sure, that there are *no slices* to that specific array). TODO: integrate with reference counting/compiler support for the static cases.
 	noshrink // when `.noslices` and `.noshrink` are *both set*, .delete(x) will NOT allocate new memory and free the old. It will just move the elements in place, and adjust .len.
+	nogrow // the array will never be allowed to grow past `.cap`. set `.nogrow` and `.noshrink` for a truly fixed heap array
+	nofree // `.data` will never be freed
 }
 
 // Internal function, used by V (`nums := []int`)
@@ -133,6 +135,9 @@ fn (mut a array) ensure_cap(required int) {
 	if required <= a.cap {
 		return
 	}
+	if a.flags.has(.nogrow) {
+		panic('array.ensure_cap: array with the flag `.nogrow` cannot grow in size, array required new size: ${required}')
+	}
 	mut cap := if a.cap > 0 { a.cap } else { 2 }
 	for required > cap {
 		cap *= 2
@@ -698,6 +703,9 @@ pub fn (a &array) free() {
 	// if a.is_slice {
 	// return
 	// }
+	if a.flags.has(.nofree) {
+		return
+	}
 	mblock_ptr := &u8(u64(a.data) - u64(a.offset))
 	unsafe { free(mblock_ptr) }
 }

vlib/builtin/array_flags_test.v

@@ -55,3 +55,40 @@ fn test_array_cap_shrinkage_after_deletion() {
 	assert a.len == 12
 	assert a.cap == 20
 }
+
+fn fixed_array_on_the_heap(len int, size int) []u8 {
+	data := vcalloc(size)
+	println(ptr_str(data))
+	return unsafe {
+		array{
+			element_size: 1
+			len: len
+			cap: size
+			data: data
+			flags: .noshrink | .nogrow | .nofree
+		}
+	}
+}
+
+fn test_array_fixed_growth() {
+	mut x := fixed_array_on_the_heap(0, 10)
+	println(ptr_str(x.data))
+	x << 5
+	x << 10
+	x << 15
+	x << 20
+	dump(x)
+	dump(x.flags)
+	assert x[2] == 15
+	assert x.flags == .noshrink | .nogrow | .nofree
+}
+
+fn test_array_fixed() {
+	mut x := fixed_array_on_the_heap(10, 10)
+	println(ptr_str(x.data))
+	x[2] = 5
+	dump(x)
+	dump(x.flags)
+	assert x[2] == 5
+	assert x.flags == .noshrink | .nogrow | .nofree
+}