Error on constant pointer dereference

[huonw]

After #6990 lands the following fails with an error message. (It currently ICEs.)

static x: &'static uint = &'static 1;
static y: uint = *x;
fn main(){
    match 1 {
        y => {}
        _ => {}
    }
}

const.rs:2:17: 2:19 error: Unsupported constant expr
const.rs:2 static y: uint = *x;
                            ^~

I'm not sure whether this is correctly not-constant, or whether it should compile fine. In the former case, is this something that should be handled in check_const.rs rather than const_eval.rs (where it is handled currently (0d0c004))?

That testcase requires the match with the y.

(This is filed because I wasn't sure about whether this is a valid testcase for #6990 or whether it shouldn't be triggering this code path at all.)

[emberian]

Especially with static mut I don't think it makes sense to support dereferencing in constant expressions.

@graydon?

[huonw]

Triage: still a bug; dereferencing in constant expressions does seem a little peculiar.

[pnkfelix]

Defining the semantics of constant expressions continues to be something we really should tackle.

See also #5551.

[treeman]

Triage bump, carry on.

[mahkoh]

No longer valid.

[alexcrichton]

Flagging as needstest