Fix false positive of needless_range_loop when meeting multidimensional array

Author: relaxcn

clippy_lints/src/loops/needless_range_loop.rs

@@ -252,13 +252,31 @@ struct VarVisitor<'a, 'tcx> {
 }
 
 impl<'tcx> VarVisitor<'_, 'tcx> {
-    fn check(&mut self, idx: &'tcx Expr<'_>, seqexpr: &'tcx Expr<'_>, expr: &'tcx Expr<'_>) -> bool {
-        let index_used_directly = matches!(idx.kind, ExprKind::Path(_));
+    fn check<'a>(&mut self, idxs: &mut Vec<&'tcx Expr<'a>>, seqexpr: &'tcx Expr<'a>, expr: &'tcx Expr<'_>) -> bool {
+        // Handle multi-dimensional array
+        if let ExprKind::Index(seqexpr, idx, _) = seqexpr.kind {
+            idxs.push(idx);
+            // Recursive call
+            return self.check(idxs, seqexpr, expr);
+        }
+
+        // Ensure idx contains the target index; otherwise, it's meaningless.
+        let used_idxs: Vec<_> = idxs
+            .iter()
+            .filter(|&&idx| is_local_used(self.cx, idx, self.var))
+            .collect();
+
+        match used_idxs.len() {
+            0 => return true,
+            n if n > 1 => self.nonindex = true, // Optimize code like `a[i][i]`
+            _ => {},
+        }
+
+        let index_used_directly = used_idxs.iter().all(|idx| matches!(idx.kind, ExprKind::Path(_)));
         if let ExprKind::Path(ref seqpath) = seqexpr.kind
             // the indexed container is referenced by a name
             && let QPath::Resolved(None, seqvar) = *seqpath
             && seqvar.segments.len() == 1
-            && is_local_used(self.cx, idx, self.var)
         {
             if self.prefer_mutable {
                 self.indexed_mut.insert(seqvar.segments[0].ident.name);
@@ -320,16 +338,20 @@ impl<'tcx> Visitor<'tcx> for VarVisitor<'_, 'tcx> {
                 .and_then(|def_id| self.cx.tcx.trait_of_assoc(def_id))
             && ((meth.ident.name == sym::index && self.cx.tcx.lang_items().index_trait() == Some(trait_id))
                 || (meth.ident.name == sym::index_mut && self.cx.tcx.lang_items().index_mut_trait() == Some(trait_id)))
-            && !self.check(args_1, args_0, expr)
         {
-            return;
+            let mut idxs: Vec<&Expr<'_>> = vec![args_1];
+            if !self.check(&mut idxs, args_0, expr) {
+                return;
+            }
         }
 
-        if let ExprKind::Index(seqexpr, idx, _) = expr.kind
+        if let ExprKind::Index(seqexpr, idx, _) = expr.kind {
+            // Only handle the top `Index` expr
+            let mut idxs: Vec<&Expr<'_>> = vec![idx];
             // an index op
-            && !self.check(idx, seqexpr, expr)
-        {
-            return;
+            if !self.check(&mut idxs, seqexpr, expr) {
+                return;
+            }
         }
 
         if let ExprKind::Path(QPath::Resolved(None, path)) = expr.kind

tests/ui/needless_range_loop.rs

@@ -210,3 +210,35 @@ fn needless_loop() {
         black_box([1, 2, 3, 4, 5, 6, 7, 8][i]);
     }
 }
+
+fn issue_15068() {
+    let a = vec![vec![0u8; MAX_LEN]; MAX_LEN];
+    let b = vec![0u8; MAX_LEN];
+
+    for i in 0..MAX_LEN {
+        // no error
+        let _ = a[0][i];
+        let _ = b[i];
+    }
+
+    for i in 0..MAX_LEN {
+        // no error
+        let _ = a[i][0];
+        let _ = b[i];
+    }
+
+    for i in 0..MAX_LEN {
+        // no error
+        let _ = a[i][b[i] as usize];
+    }
+
+    for i in 0..MAX_LEN {
+        //~^ needless_range_loop
+        let _ = a[i][i];
+    }
+
+    for i in 0..MAX_LEN {
+        //~^ needless_range_loop
+        let _ = a[0][i];
+    }
+}

tests/ui/needless_range_loop.stderr

@@ -168,5 +168,29 @@ LL -     for i in 0..vec.len() {
 LL +     for (i, <item>) in vec.iter_mut().enumerate() {
    |
 
-error: aborting due to 14 previous errors
+error: the loop variable `i` is used to index `a`
+  --> tests/ui/needless_range_loop.rs:235:14
+   |
+LL |     for i in 0..MAX_LEN {
+   |              ^^^^^^^^^^
+   |
+help: consider using an iterator and enumerate()
+   |
+LL -     for i in 0..MAX_LEN {
+LL +     for (i, <item>) in a.iter().enumerate().take(MAX_LEN) {
+   |
+
+error: the loop variable `i` is only used to index `a`
+  --> tests/ui/needless_range_loop.rs:240:14
+   |
+LL |     for i in 0..MAX_LEN {
+   |              ^^^^^^^^^^
+   |
+help: consider using an iterator
+   |
+LL -     for i in 0..MAX_LEN {
+LL +     for <item> in a.iter().take(MAX_LEN) {
+   |
+
+error: aborting due to 16 previous errors