coverage: Check that the function signature span precedes the body

This will normally be true, but in cases where it's not true we're better off
not making any assumptions about the signature.

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -324,14 +324,16 @@ fn extract_hir_info<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> ExtractedHir
     let body_span = get_body_span(tcx, hir_body, def_id);
 
     // The actual signature span is only used if it has the same context and
-    // filename as the body.
+    // filename as the body, and precedes the body.
     let maybe_fn_sig_span = hir_node.fn_sig().map(|fn_sig| fn_sig.span);
     let fn_sig_span = maybe_fn_sig_span
         .filter(|&fn_sig_span| {
             let source_map = tcx.sess.source_map();
             let file_idx = |span: Span| source_map.lookup_source_file_idx(span.lo());
 
-            fn_sig_span.eq_ctxt(body_span) && file_idx(fn_sig_span) == file_idx(body_span)
+            fn_sig_span.eq_ctxt(body_span)
+                && fn_sig_span.hi() <= body_span.lo()
+                && file_idx(fn_sig_span) == file_idx(body_span)
         })
         // If so, extend it to the start of the body span.
         .map(|fn_sig_span| fn_sig_span.with_hi(body_span.lo()))