How does Expr receive literal string?

[TD-Sky]

I'm writing a attribute macro used on struct:

#[mc(key = "this is a literal string")]
struct Foo;

The receiver type:

#[derive(Debug, FromMeta)]
struct Args {
    value: syn::Expr,
}

The value expects a literal string or syn::Path to a value. Parsing it NestedMeta::parse_meta_list and FromMeta::from_list will get error Unknown literal value this is a literal string`. Are there any solutions?

[TedDriggs]

This is likely running into #229, so you should use util::parse_expr::preserve_str_literal to handle it.