Allow manual swap in const fn

clippy_lints/src/swap.rs

@@ -2,7 +2,7 @@ use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_and_then};
 use clippy_utils::source::snippet_with_applicability;
 use clippy_utils::sugg::Sugg;
 use clippy_utils::ty::is_type_diagnostic_item;
-use clippy_utils::{can_mut_borrow_both, eq_expr_value, std_or_core};
+use clippy_utils::{can_mut_borrow_both, eq_expr_value, in_constant, std_or_core};
 use if_chain::if_chain;
 use rustc_errors::Applicability;
 use rustc_hir::{BinOpKind, Block, Expr, ExprKind, PatKind, QPath, Stmt, StmtKind};
@@ -16,6 +16,8 @@ declare_clippy_lint! {
     /// ### What it does
     /// Checks for manual swapping.
     ///
+    /// Note that the lint will not be emitted in const blocks, as the suggestion would not be applicable.
+    ///
     /// ### Why is this bad?
     /// The `std::mem::swap` function exposes the intent better
     /// without deinitializing or copying either variable.
@@ -138,6 +140,10 @@ fn generate_swap_warning(cx: &LateContext<'_>, e1: &Expr<'_>, e2: &Expr<'_>, spa
 
 /// Implementation of the `MANUAL_SWAP` lint.
 fn check_manual_swap(cx: &LateContext<'_>, block: &Block<'_>) {
+    if in_constant(cx, block.hir_id) {
+        return;
+    }
+
     for w in block.stmts.windows(3) {
         if_chain! {
             // let t = foo();

tests/ui/swap.fixed

@@ -155,3 +155,12 @@ fn issue_8154() {
     let s = S3(&mut s);
     std::mem::swap(&mut s.0.x, &mut s.0.y);
 }
+
+const fn issue_9864(mut u: u32) -> u32 {
+    let mut v = 10;
+
+    let temp = u;
+    u = v;
+    v = temp;
+    u + v
+}

tests/ui/swap.rs

@@ -179,3 +179,12 @@ fn issue_8154() {
     s.0.x = s.0.y;
     s.0.y = t;
 }
+
+const fn issue_9864(mut u: u32) -> u32 {
+    let mut v = 10;
+
+    let temp = u;
+    u = v;
+    v = temp;
+    u + v
+}