checker: disallow struct init mut a: b, when b is a const array

vlib/crypto/blake3/blake3_chunk_test.v

@@ -51,7 +51,7 @@ const test_cases = [
 		}
 		results: Chunk{
 			chunk_number:   0
-			chaining_value: iv
+			chaining_value: iv.clone()
 			block_words:    [u32(0), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
 			flags:          0x0b
 		}
@@ -69,7 +69,7 @@ const test_cases = [
 		}
 		results: Chunk{
 			chunk_number:   0
-			chaining_value: iv
+			chaining_value: iv.clone()
 			block_words:    [u32(0x00000041), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
 			flags:          0x0b
 		}
@@ -87,7 +87,7 @@ const test_cases = [
 		}
 		results: Chunk{
 			chunk_number:   0
-			chaining_value: iv
+			chaining_value: iv.clone()
 			block_words:    [u32(0x00636261), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
 			flags:          0x0b
 		}
@@ -105,7 +105,7 @@ const test_cases = [
 		}
 		results: Chunk{
 			chunk_number:   0
-			chaining_value: iv
+			chaining_value: iv.clone()
 			block_words:    [u32(0x44434241), 0x48474645, 0x4c4b4a49, 0x504f4e4d, 0x54535251,
 				0x58575655, 0x61205a59, 0x65646362, 0x69686766, 0x6d6c6b6a, 0x71706f6e, 0x75747372,
 				0x79787776, 0x3130207a, 0x35343332, 0x00383736]
@@ -125,7 +125,7 @@ const test_cases = [
 		}
 		results: Chunk{
 			chunk_number:   0
-			chaining_value: iv
+			chaining_value: iv.clone()
 			block_words:    [u32(0x44434241), 0x48474645, 0x4c4b4a49, 0x504f4e4d, 0x54535251,
 				0x58575655, 0x61205a59, 0x65646362, 0x69686766, 0x6d6c6b6a, 0x71706f6e, 0x75747372,
 				0x79787776, 0x3130207a, 0x35343332, 0x39383736]
@@ -269,7 +269,7 @@ const test_cases = [
 		}
 		results: Chunk{
 			chunk_number:   1
-			chaining_value: iv
+			chaining_value: iv.clone()
 			block_words:    [u32(0x0000004f), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
 			flags:          0x0b
 		}

vlib/v/checker/struct.v

@@ -745,6 +745,13 @@ or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.',
 						got_type = c.expr(mut right)
 						node.init_fields[i].expr = right
 					}
+					// disallow `mut a: b`, when b is const array
+					if field_info.is_mut
+						&& (init_field.expr is ast.Ident && init_field.expr.obj is ast.ConstField)
+						&& !c.inside_unsafe {
+						c.error('cannot assign a const array to mut struct field, call `clone` method (or use `unsafe`)',
+							init_field.expr.pos())
+					}
 				}
 				if exp_type_sym.kind == .interface {
 					if c.type_implements(got_type, exp_type, init_field.pos) {

vlib/v/checker/tests/struct_arr_mut_field_arr_assign_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/struct_arr_mut_field_arr_assign_err.vv:10:6: error: cannot assign a const array to mut struct field, call `clone` method (or use `unsafe`)
+    8 | fn main() {
+    9 |     mut muta := Muta{
+   10 |         a: arr
+      |            ~~~
+   11 |     }
+   12 |     muta.a[0] = 52

vlib/v/checker/tests/struct_arr_mut_field_arr_assign_err.vv

@@ -0,0 +1,14 @@
+const arr = [1, 2, 3]
+
+struct Muta {
+mut:
+	a []int
+}
+
+fn main() {
+	mut muta := Muta{
+		a: arr
+	}
+	muta.a[0] = 52
+	println(arr) // [52, 2, 3]
+}