Allow first-class struct and tuple constants

[aochagavia]

The following code should compile, but does not:

struct MyStruct { field: usize }
struct Nested { nested: MyStruct }
struct Mix2 { nested: ((usize,),) }

const STRUCT: MyStruct = MyStruct { field: 42 };
const TUP: (usize,) = (43,);
const NESTED_S: Nested = Nested { nested: MyStruct { field: 5 } };
const NESTED_T: ((usize,),) = ((4,),);
const MIX_1: ((Nested,),) = ((Nested { nested: MyStruct { field: 3 } },),);
const MIX_2: Mix2 = Mix2 { nested: ((2,),) };
const INSTANT_1: usize = (MyStruct { field: 1 }).field;
const INSTANT_2: usize = (0,).0;

fn main() {
    let a = [0; STRUCT.field];
    let b = [0; TUP.0];
    let c = [0; NESTED_S.nested.field];
    let d = [0; (NESTED_T.0).0];
    let e = [0; (MIX_1.0).0.nested.field];
    let f = [0; (MIX_2.nested.0).0];
    let g = [0; INSTANT_1];
    let h = [0; INSTANT_2];
}

[aochagavia]

@eddyb Someone told me on IRC that you know how check_const.rs works. I am not really sure about the ConstQualif that structs and tuples should have. Could you give me a hint?

[eddyb]

I think all that code is fine, have you checked rustc::middle::const_eval?

[aochagavia]

It doesn't compile on the latest nightly (only a and b work, because of a hack I introduced a couple of months ago).

This are the errors:

$ rustc test.rs
test.rs:17:17: 17:38 error: expected constant integer for repeat count, found no
n-constant expression [E0307]
test.rs:17     let c = [0; NESTED_S.nested.field];
                           ^~~~~~~~~~~~~~~~~~~~~
test.rs:18:17: 18:31 error: expected constant integer for repeat count, found no
n-constant expression [E0307]
test.rs:18     let d = [0; (NESTED_T.0).0];
                           ^~~~~~~~~~~~~~
test.rs:19:17: 19:41 error: expected constant integer for repeat count, found no
n-constant expression [E0307]
test.rs:19     let e = [0; (MIX_1.0).0.nested.field];
                           ^~~~~~~~~~~~~~~~~~~~~~~~
test.rs:20:17: 20:35 error: expected constant integer for repeat count, found no
n-constant expression [E0307]
test.rs:20     let f = [0; (MIX_2.nested.0).0];
                           ^~~~~~~~~~~~~~~~~~
test.rs:21:17: 21:26 error: expected constant integer for repeat count, found va
riable [E0307]
test.rs:21     let g = [0; INSTANT_1];
                           ^~~~~~~~~
test.rs:22:17: 22:26 error: expected constant integer for repeat count, found va
riable [E0307]
test.rs:22     let h = [0; INSTANT_2];
                           ^~~~~~~~~
error: aborting due to 6 previous errors

[eddyb]

Yes, that is const_eval - it just doesn't know how to do those accesses - check_consts hasn't had a chance to run at this point, you're still in typeck.

[aochagavia]

Oops... I actually meant const_eval when I said check_consts. Now I realize I don't need ConstQualif at all... Thanks!