clippy_lints: Do not warn against Box parameter in C FFI

Fixes #5542.

When using C FFI, to handle pointers in parameters it is needed to
declare them as `Box` in its Rust-side signature. However, the current
linter warns against the usage of Box stating that "local variable
doesn't need to be boxed here".

This commit fixes it by ignoring functions whose Abi is Cdecl.

clippy_lints/src/escape.rs

@@ -6,6 +6,7 @@
 use rustc_session::{declare_tool_lint, impl_lint_pass};
 use rustc_span::source_map::Span;
 use rustc_target::abi::LayoutOf;
+use rustc_target::spec::abi::Abi;
 use rustc_typeck::expr_use_visitor::{ConsumeMode, Delegate, ExprUseVisitor, PlaceBase, PlaceWithHirId};
 
 use crate::utils::span_lint;
@@ -60,12 +61,18 @@ impl<'tcx> LateLintPass<'tcx> for BoxedLocal {
     fn check_fn(
         &mut self,
         cx: &LateContext<'tcx>,
-        _: intravisit::FnKind<'tcx>,
+        fn_kind: intravisit::FnKind<'tcx>,
         _: &'tcx FnDecl<'_>,
         body: &'tcx Body<'_>,
         _: Span,
         hir_id: HirId,
     ) {
+        if let Some(header) = fn_kind.header() {
+            if header.abi == Abi::Cdecl {
+                return;
+            }
+        }
+
         // If the method is an impl for a trait, don't warn.
         let parent_id = cx.tcx.hir().get_parent_item(hir_id);
         let parent_node = cx.tcx.hir().find(parent_id);