fix most compiler/ doctests

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -259,7 +259,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
     /// Report an error because the universal region `fr` was required to outlive
     /// `outlived_fr` but it is not known to do so. For example:
     ///
-    /// ```
+    /// ```compile_fail,E0312
     /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x }
     /// ```
     ///