Macros: Unexpected token when metavariable is "tt", works with "expr"

[jonas-schievink]

http://is.gd/GKzXeD

struct S(u8);

macro_rules! mac {
    ( $val:tt ) => ( S($val) );
    //( $val:expr ) => ( S($val) );
}

fn main() {
    let s: S = mac!(4);
    println!("{}", s.0);
}

<anon>:4:24: 4:28 error: unexpected token: `4`
<anon>:4     ( $val:tt ) => ( S($val) );
                                ^~~~

The token 4 should be perfectly valid there. Replacing tt with expr in the macro "fixes" this, but since expr is much stricter, this limits what can be expressed.

Is this intended behaviour?

Related: #24145

[jonas-schievink]

Indirecting via another macro that interprets the tt as an expr works around the issue as well:

struct S(u8);

macro_rules! mkexpr {
    ( $e:expr ) => ( $e );
}

macro_rules! mac {
    //( $val:tt ) => ( S($val) );   // Doesn't work
    ( $val:tt ) => ( S(mkexpr!($val)) );
}

fn main() {
    let s: S = mac!(4);
    println!("{}", s.0);
}

[huonw]

Dupe of #5846. Thanks for filing!