checker: fix assigning array slice in struct init (#19150)

vlib/v/checker/assign.v

@@ -483,7 +483,8 @@ fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 							}
 						}
 					}
-				} else if mut left is ast.Ident && right is ast.IndexExpr {
+				} else if mut left is ast.Ident && left.kind != .blank_ident
+					&& right is ast.IndexExpr {
 					if (right as ast.IndexExpr).left is ast.Ident
 						&& (right as ast.IndexExpr).index is ast.RangeExpr
 						&& ((right as ast.IndexExpr).left.is_mut() || left.is_mut())

vlib/v/checker/struct.v

@@ -566,6 +566,30 @@ fn (mut c Checker) struct_init(mut node ast.StructInit, is_field_zero_struct_ini
 					&& !got_type.is_ptr() {
 					c.error('allocate on the heap for use in other functions', init_field.pos)
 				}
+				if exp_type_sym.kind == .array && got_type_sym.kind == .array {
+					if init_field.expr is ast.IndexExpr
+						&& (init_field.expr as ast.IndexExpr).left is ast.Ident
+						&& ((init_field.expr as ast.IndexExpr).left.is_mut()
+						|| field_info.is_mut) && init_field.expr.index is ast.RangeExpr
+						&& !c.inside_unsafe {
+						// `a: arr[..]` auto add clone() -> `a: arr[..].clone()`
+						c.add_error_detail_with_pos('To silence this notice, use either an explicit `a[..].clone()`,
+or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.',
+							init_field.expr.pos())
+						c.note('an implicit clone of the slice was done here', init_field.expr.pos())
+						mut right := ast.CallExpr{
+							name: 'clone'
+							left: init_field.expr
+							left_type: got_type
+							is_method: true
+							receiver_type: got_type.ref()
+							return_type: got_type
+							scope: c.fn_scope
+						}
+						got_type = c.expr(mut right)
+						node.init_fields[i].expr = right
+					}
+				}
 				if exp_type_sym.kind == .interface_ {
 					if c.type_implements(got_type, exp_type, init_field.pos) {
 						if !c.inside_unsafe && got_type_sym.kind != .interface_

vlib/v/checker/tests/slice_clone_in_struct_init_notice.out

@@ -0,0 +1,15 @@
+vlib/v/checker/tests/slice_clone_in_struct_init_notice.vv:9:7: notice: an implicit clone of the slice was done here
+    7 |
+    8 |     _ := AA{
+    9 |         a: a[0..5]
+      |             ~~~~~~
+   10 |     }
+   11 |     _ := a[0..5]
+Details: vlib/v/checker/tests/slice_clone_in_struct_init_notice.vv:9:7: details: To silence this notice, use either an explicit `a[..].clone()`,
+or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.
+    7 |
+    8 |     _ := AA{
+    9 |         a: a[0..5]
+      |             ~~~~~~
+   10 |     }
+   11 |     _ := a[0..5]

vlib/v/checker/tests/slice_clone_in_struct_init_notice.vv

@@ -0,0 +1,12 @@
+struct AA {
+	a []int
+}
+
+fn main() {
+	mut a := []int{len: 10}
+
+	_ := AA{
+		a: a[0..5]
+	}
+	_ := a[0..5]
+}