Fix FP for same_item_push

Don't emit a lint when `pushed_item` was declared as mutable variable.

Author: giraffate

clippy_lints/src/loops.rs

@@ -1141,11 +1141,36 @@ fn detect_same_item_push<'tcx>(
     if same_item_push_visitor.should_lint {
         if let Some((vec, pushed_item)) = same_item_push_visitor.vec_push {
             // Make sure that the push does not involve possibly mutating values
-            if mutated_variables(pushed_item, cx).map_or(false, |mutvars| mutvars.is_empty()) {
-                if let PatKind::Wild = pat.kind {
-                    let vec_str = snippet_with_macro_callsite(cx, vec.span, "");
-                    let item_str = snippet_with_macro_callsite(cx, pushed_item.span, "");
-
+            if let PatKind::Wild = pat.kind {
+                let vec_str = snippet_with_macro_callsite(cx, vec.span, "");
+                let item_str = snippet_with_macro_callsite(cx, pushed_item.span, "");
+                if let ExprKind::Path(ref qpath) = pushed_item.kind {
+                    if let Res::Local(hir_id) = qpath_res(cx, qpath, pushed_item.hir_id) {
+                        let node = cx.tcx.hir().get(hir_id);
+                        if_chain! {
+                            if let Node::Binding(pat) = node;
+                            if let PatKind::Binding(bind_ann, ..) = pat.kind;
+                            then {
+                                match bind_ann {
+                                    BindingAnnotation::RefMut | BindingAnnotation::Mutable => {},
+                                    _ => {
+                                        span_lint_and_help(
+                                            cx,
+                                            SAME_ITEM_PUSH,
+                                            vec.span,
+                                            "it looks like the same item is being pushed into this Vec",
+                                            None,
+                                            &format!(
+                                                "try using vec![{};SIZE] or {}.resize(NEW_SIZE, {})",
+                                                item_str, vec_str, item_str
+                                            ),
+                                        )
+                                    }
+                                }
+                            }
+                        }
+                    }
+                } else if mutated_variables(pushed_item, cx).map_or(false, |mutvars| mutvars.is_empty()) {
                     span_lint_and_help(
                         cx,
                         SAME_ITEM_PUSH,

tests/ui/same_item_push.rs

@@ -86,4 +86,12 @@ fn main() {
     for a in vec_a {
         vec12.push(2u8.pow(a.kind));
     }
+
+    // Fix #5902
+    let mut vec13: Vec<u8> = Vec::new();
+    let mut item = 0;
+    for _ in 0..10 {
+        vec13.push(item);
+        item += 10;
+    }
 }