Do not intersect spans coming from different contexts (rust-lang#13942)

The code should not attempt to obtain a snippet by capping the function
signature span with its identifier span without checking that they are
in the same context.

This is the only instance I could identify where placeholders were used
instead of the real snippet when running the CI lintcheck. Moreover, the
placeholders were not even used, as they snippet was obtained
prematurely.

Found in the context of rust-lang#13941

changelog: none

Author: dswij

clippy_lints/src/no_mangle_with_rust_abi.rs

@@ -37,7 +37,9 @@ declare_lint_pass!(NoMangleWithRustAbi => [NO_MANGLE_WITH_RUST_ABI]);
 
 impl<'tcx> LateLintPass<'tcx> for NoMangleWithRustAbi {
     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'tcx>) {
-        if let ItemKind::Fn { sig: fn_sig, .. } = &item.kind {
+        if let ItemKind::Fn { sig: fn_sig, .. } = &item.kind
+            && !item.span.from_expansion()
+        {
             let attrs = cx.tcx.hir().attrs(item.hir_id());
             let mut app = Applicability::MaybeIncorrect;
             let fn_snippet = snippet_with_applicability(cx, fn_sig.span.with_hi(item.ident.span.lo()), "..", &mut app);