Fix #521

Author: CppCXY

crates/emmylua_code_analysis/src/compilation/test/array_test.rs

@@ -50,4 +50,22 @@ mod test {
         let t_expected = ws.ty("integer[]");
         assert_eq!(t, t_expected)
     }
+
+    #[test]
+    fn test_array_for_flow() {
+        let mut ws = VirtualWorkspace::new();
+        assert!(ws.check_code_for(
+            DiagnosticCode::NeedCheckNil,
+            r#"
+        --- @param _x string
+        local function foo(_x) end
+
+        local list = {} --- @type string[]
+
+        for i = #list, 1, -1 do
+            foo(list[i])
+        end
+        "#,
+        ));
+    }
 }

crates/emmylua_code_analysis/src/semantic/infer/infer_index.rs

@@ -285,18 +285,42 @@ fn check_iter_var_range(
     let token = decl_syntax_id.to_token_from_root(&root)?;
     let parent_node = token.parent()?;
     let for_stat = LuaForStat::cast(parent_node)?;
-    // get second expr
-    let test_len_expr = for_stat.get_iter_expr().skip(1).next()?;
-    let LuaExpr::UnaryExpr(unary_expr) = test_len_expr else {
-        return None;
+    let iter_exprs = for_stat.get_iter_expr().collect::<Vec<_>>();
+    let test_len_expr = match iter_exprs.len() {
+        2 => {
+            let LuaExpr::UnaryExpr(unary_expr) = iter_exprs[1].clone() else {
+                return None;
+            };
+            unary_expr
+        }
+        3 => {
+            let step_type = infer_expr(db, cache, iter_exprs[2].clone()).ok()?;
+            let LuaType::IntegerConst(step_value) = step_type else {
+                return None;
+            };
+            if step_value > 0 {
+                let LuaExpr::UnaryExpr(unary_expr) = iter_exprs[1].clone() else {
+                    return None;
+                };
+                unary_expr
+            } else if step_value < 0 {
+                let LuaExpr::UnaryExpr(unary_expr) = iter_exprs[0].clone() else {
+                    return None;
+                };
+                unary_expr
+            } else {
+                return None;
+            }
+        }
+        _ => return None,
     };
 
-    let op = unary_expr.get_op_token()?;
+    let op = test_len_expr.get_op_token()?;
     if op.get_op() != UnaryOperator::OpLen {
         return None;
     }
 
-    let len_expr = unary_expr.get_expr()?;
+    let len_expr = test_len_expr.get_expr()?;
     let len_expr_var_ref_id = get_var_expr_var_ref_id(db, cache, len_expr)?;
     let prefix_expr_var_ref_id = get_var_expr_var_ref_id(db, cache, prefix_expr)?;