checker: disallow array append in an expression (#9265)

vlib/v/ast/ast.v

@@ -581,6 +581,7 @@ pub struct InfixExpr {
 pub:
 	op      token.Kind
 	pos     token.Position
+	is_stmt bool
 pub mut:
 	left        Expr
 	right       Expr

vlib/v/checker/check_types.v

@@ -155,7 +155,7 @@ fn (mut c Checker) check_shift(left_type table.Type, right_type table.Type, left
 			// allow `bool << 2` in translated C code
 			return table.int_type
 		}
-		c.error('invalid operation: shift of type `$sym.name`', left_pos)
+		c.error('invalid operation: shift on type `$sym.name`', left_pos)
 		return table.void_type
 	} else if !right_type.is_int() {
 		c.error('cannot shift non-integer type `${c.table.get_type_symbol(right_type).name}` into type `${c.table.get_type_symbol(left_type).name}`',

vlib/v/checker/checker.v

@@ -875,6 +875,9 @@ pub fn (mut c Checker) infix_expr(mut infix_expr ast.InfixExpr) table.Type {
 		}
 		.left_shift {
 			if left_final.kind == .array {
+				if !infix_expr.is_stmt {
+					c.error('array append cannot be used in an expression', infix_expr.pos)
+				}
 				// `array << elm`
 				infix_expr.auto_locked, _ = c.fail_if_immutable(infix_expr.left)
 				left_value_type := c.table.value_type(left_type)

vlib/v/checker/tests/append_err.out

@@ -0,0 +1,20 @@
+vlib/v/checker/tests/append_err.vv:3:7: error: cannot append `string` to `[]int`
+    1 | fn main() {
+    2 |     mut l := []int{}
+    3 |     l << 'test'
+      |          ~~~~~~
+    4 |     
+    5 |     _ = l << 3
+vlib/v/checker/tests/append_err.vv:5:8: error: array append cannot be used in an expression
+    3 |     l << 'test'
+    4 |     
+    5 |     _ = l << 3
+      |           ~~
+    6 |     _ = (l << 3).len
+    7 | }
+vlib/v/checker/tests/append_err.vv:6:9: error: array append cannot be used in an expression
+    4 |     
+    5 |     _ = l << 3
+    6 |     _ = (l << 3).len
+      |            ~~
+    7 | }

vlib/v/checker/tests/append_err.vv

@@ -1,4 +1,7 @@
 fn main() {
 	mut l := []int{}
 	l << 'test'
+	
+	_ = l << 3
+	_ = (l << 3).len
 }

vlib/v/checker/tests/array_literal_modify_err.out

@@ -1,7 +1,7 @@
-vlib/v/checker/tests/array_literal_modify_err.vv:2:14: error: array literal can not be modified
+vlib/v/checker/tests/array_literal_modify_err.vv:2:24: error: array append cannot be used in an expression
     1 | fn main() {
     2 |     mut nums := [1, 2, 3] << 4
-      |                 ~~~~~~~~~
+      |                           ~~
     3 |     println(nums)
     4 | }
 vlib/v/checker/tests/array_literal_modify_err.vv:3:2: error: `println` can not print void expressions

vlib/v/checker/tests/left_shift_err.out

@@ -1,6 +0,0 @@
-vlib/v/checker/tests/left_shift_err.vv:3:7: error: cannot append `string` to `[]int`
-    1 | fn main() {
-    2 |     mut l := []int{}
-    3 |     l << 'test'
-      |          ~~~~~~
-    4 | }

vlib/v/checker/tests/rshift_op_wrong_left_type_err.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/rshift_op_wrong_left_type_err.vv:2:10: error: invalid operation: shift of type `float literal`
+vlib/v/checker/tests/rshift_op_wrong_left_type_err.vv:2:10: error: invalid operation: shift on type `float literal`
     1 | fn main() {
     2 |     println(0.5 >> 1)
       |             ~~~

vlib/v/checker/tests/shift_op_wrong_left_type_err.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/shift_op_wrong_left_type_err.vv:2:10: error: invalid operation: shift of type `float literal`
+vlib/v/checker/tests/shift_op_wrong_left_type_err.vv:2:10: error: invalid operation: shift on type `float literal`
     1 | fn main() {
     2 |     println(0.5 << 1)
       |             ~~~

vlib/v/parser/pratt.v

@@ -377,6 +377,7 @@ pub fn (mut p Parser) expr_with_left(left ast.Expr, precedence int, is_stmt_iden
 				right: right
 				op: tok.kind
 				pos: pos
+				is_stmt: true
 			}
 		} else if p.tok.kind.is_infix() {
 			if p.tok.kind.is_prefix() && p.tok.line_nr != p.prev_tok.line_nr {
@@ -473,6 +474,7 @@ fn (mut p Parser) infix_expr(left ast.Expr) ast.Expr {
 		right: right
 		op: op
 		pos: pos
+		is_stmt: p.is_stmt_ident
 		or_block: ast.OrExpr{
 			stmts: or_stmts
 			kind: or_kind