clippy_lint: Test for BoxedLocal false-positive in C-FFI and fix C-FFI Abi comparison.

Author: JPTIZ

clippy_lints/src/escape.rs

@@ -68,7 +68,7 @@ impl<'tcx> LateLintPass<'tcx> for BoxedLocal {
         hir_id: HirId,
     ) {
         if let Some(header) = fn_kind.header() {
-            if header.abi == Abi::Cdecl {
+            if header.abi == Abi::C {
                 return;
             }
         }

tests/ui/escape_analysis.rs

@@ -174,3 +174,8 @@ mod issue_3739 {
         };
     }
 }
+
+/// Issue #5542
+///
+/// This shouldn't warn for `boxed_local` as it is a function implemented in C.
+pub extern "C" fn do_now_warn_me(_c_pointer: Box<String>) -> () {}