checker: add error for ast.PrefixExpr on the left side of decl_assign (#6660)

vlib/v/checker/checker.v

@@ -1975,6 +1975,9 @@ pub fn (mut c Checker) assign_stmt(mut assign_stmt ast.AssignStmt) {
 					c.error('modifying variables via dereferencing can only be done in `unsafe` blocks',
 						assign_stmt.pos)
 				}
+				if is_decl {
+					c.error('non-name on the left side of `:=`', left.pos)
+				}
 			}
 			else {
 				if is_decl {

vlib/v/checker/tests/prefix_expr_decl_assign_err.out

@@ -0,0 +1,11 @@
+vlib/v/checker/tests/prefix_expr_decl_assign_err.vv:2:5: error: non-name on the left side of `:=`
+    1 | fn main() {
+    2 |     &a := 12
+      |     ^
+    3 |     (*d) := 14
+    4 | }
+vlib/v/checker/tests/prefix_expr_decl_assign_err.vv:1:1: error: non-name `(*d)` on left side of `:=`
+    1 | fn main() {
+      | ^
+    2 |     &a := 12
+    3 |     (*d) := 14

vlib/v/checker/tests/prefix_expr_decl_assign_err.vv

@@ -0,0 +1,4 @@
+fn main() {
+    &a := 12
+    (*d) := 14
+}

vlib/v/parser/pratt.v

@@ -60,7 +60,7 @@ pub fn (mut p Parser) expr(precedence int) ast.Expr {
 			p.next()
 		}
 		.minus, .amp, .mul, .not, .bit_not, .arrow {
-			// -1, -a, !x, &x, ~x
+			// -1, -a, !x, &x, ~x, <-a
 			node = p.prefix_expr()
 		}
 		.key_true, .key_false {