Support macro expansion inside attribute

[edwin0cheng]

e.g :

#[doc = concat!("Hello", " world")]
struct Bar;

It is unstable feature and it is used in rustc master already.

[jonas-schievink]

cc #7779

[jonas-schievink]

I guess this is related to #7049

[mvforell]

This would be very useful for generating doc attributs in macros, e.g. like described at https://stackoverflow.com/a/43353854:

macro_rules! impl_foo {
    ($name:ident, $sname:expr) => {
        #[doc = "Returns a new `"]
        #[doc = $sname]
        #[doc = "`."]
        pub fn myfoo() -> $name {
            42
        }
    };

    ($name:tt) => {
        impl_foo!($name, stringify!($name));
    };
}

impl_foo!(u32);

Currently, the above doc gets rendered as

Returns a new ``.

instead of

Returns a new u32.