Fix suggestion-causes-error of manual_swap (#14978)

Fixes: rust-lang/rust-clippy#14931

changelog: Fix [`manual_swap`]'s suggestion-causes-error when the
variable is mutable or as loop variable.

Author: samueltardieu

clippy_lints/src/swap.rs

@@ -3,7 +3,7 @@ use clippy_utils::source::{snippet_indent, snippet_with_context};
 use clippy_utils::sugg::Sugg;
 use clippy_utils::ty::is_type_diagnostic_item;
 
-use clippy_utils::{can_mut_borrow_both, eq_expr_value, is_in_const_context, std_or_core};
+use clippy_utils::{can_mut_borrow_both, eq_expr_value, is_in_const_context, path_to_local, std_or_core};
 use itertools::Itertools;
 
 use rustc_data_structures::fx::FxIndexSet;
@@ -350,12 +350,21 @@ impl<'tcx> IndexBinding<'_, 'tcx> {
                 format!("{lhs_snippet}{rhs_snippet}")
             },
             ExprKind::Path(QPath::Resolved(_, path)) => {
-                let init = self.cx.expr_or_init(expr);
-
                 let Some(first_segment) = path.segments.first() else {
                     return String::new();
                 };
-                if !self.suggest_span.contains(init.span) || !self.is_used_other_than_swapping(first_segment.ident) {
+
+                let init = self.cx.expr_or_init(expr);
+
+                // We skip suggesting a variable binding in any of these cases:
+                // - Variable initialization is outside the suggestion span
+                // - Variable declaration is outside the suggestion span
+                // - Variable is not used as an index or elsewhere later
+                if !self.suggest_span.contains(init.span)
+                    || path_to_local(expr)
+                        .is_some_and(|hir_id| !self.suggest_span.contains(self.cx.tcx.hir_span(hir_id)))
+                    || !self.is_used_other_than_swapping(first_segment.ident)
+                {
                     return String::new();
                 }
 

tests/ui/manual_swap_auto_fix.fixed

@@ -55,3 +55,14 @@ fn swap8() {
     let i2 = 1;
     v.swap(i1 + i2, i2);
 }
+
+fn issue_14931() {
+    let mut v = [1, 2, 3, 4];
+
+    let mut i1 = 0;
+    for i2 in 0..4 {
+        v.swap(i1, i2);
+
+        i1 += 2;
+    }
+}

tests/ui/manual_swap_auto_fix.rs

@@ -78,3 +78,17 @@ fn swap8() {
     v[i1 + i2] = v[i2];
     v[i2] = tmp;
 }
+
+fn issue_14931() {
+    let mut v = [1, 2, 3, 4];
+
+    let mut i1 = 0;
+    for i2 in 0..4 {
+        let tmp = v[i1];
+        //~^ manual_swap
+        v[i1] = v[i2];
+        v[i2] = tmp;
+
+        i1 += 2;
+    }
+}

tests/ui/manual_swap_auto_fix.stderr

@@ -92,5 +92,14 @@ LL | |     v[i1 + i2] = v[i2];
 LL | |     v[i2] = tmp;
    | |________________^ help: try: `v.swap(i1 + i2, i2);`
 
-error: aborting due to 8 previous errors
+error: this looks like you are swapping elements of `v` manually
+  --> tests/ui/manual_swap_auto_fix.rs:87:9
+   |
+LL | /         let tmp = v[i1];
+LL | |
+LL | |         v[i1] = v[i2];
+LL | |         v[i2] = tmp;
+   | |____________________^ help: try: `v.swap(i1, i2);`
+
+error: aborting due to 9 previous errors