builtin,checker: add array.flags. Allow changing it in `unsafe {}` blocks

vlib/builtin/array.v

@@ -5,19 +5,26 @@ module builtin
 
 import strings
 
-// array is a struct used for denoting array types in V
+// `array` is a struct, used for denoting all array types in V.
+// `.data` is a void pointer to the backing heap memory block,
+// which avoids using generics and thus without generating extra
+// code for every type.
 pub struct array {
 pub:
 	element_size int // size in bytes of one element in the array.
 pub mut:
 	data   voidptr
 	offset int // in bytes (should be `usize`)
-	len    int // length of the array.
+	len    int // length of the array in elements.
-	cap    int // capacity of the array.
+	cap    int // capacity of the array in elements.
+	flags  ArrayFlags
+}
+
+[flag]
+pub enum ArrayFlags {
+	noslices
 }
 
-// array.data uses a void pointer, which allows implementing arrays without generics and without generating
-// extra code for every type.
 // Internal function, used by V (`nums := []int`)
 fn __new_array(mylen int, cap int, elm_size int) array {
 	cap_ := if cap < mylen { mylen } else { cap }
@@ -104,6 +111,11 @@ fn (mut a array) ensure_cap(required int) {
 	if a.data != voidptr(0) {
 		unsafe { vmemcpy(new_data, a.data, a.len * a.element_size) }
 		// TODO: the old data may be leaked when no GC is used (ref-counting?)
+		if a.flags.has(.noslices) {
+			unsafe {
+				free(a.data)
+			}
+		}
 	}
 	a.data = new_data
 	a.offset = 0

vlib/builtin/builtin.c.v

@@ -510,7 +510,7 @@ pub fn memdup_noscan(src voidptr, sz int) voidptr {
 		return vcalloc_noscan(1)
 	}
 	unsafe {
-		mem := vcalloc_noscan(sz)
+		mem := malloc_noscan(sz)
 		return C.memcpy(mem, src, sz)
 	}
 }

vlib/v/checker/checker.v

@@ -1663,8 +1663,9 @@ fn (mut c Checker) fail_if_immutable(expr ast.Expr) (string, token.Position) {
 					c.fail_if_immutable(expr.expr)
 				}
 				.array, .string {
-					// This should only happen in `builtin`
+					// should only happen in `builtin` and unsafe blocks
-					if c.file.mod.name != 'builtin' {
+					inside_builtin := c.file.mod.name == 'builtin'
+					if !inside_builtin && !c.inside_unsafe {
 						c.error('`$typ_sym.kind` can not be modified', expr.pos)
 						return '', expr.pos
 					}