~, ~[] and ~str not containing @ shouldn't have gc headers

[thestinger]

There's a 4-word header added to every owned pointer right now:

struct rust_opaque_box {
    ref_cnt_t ref_count;
    type_desc *td;
    rust_opaque_box *prev;
    rust_opaque_box *next;
};

[catamorphism]

Not critical for 0.7. Nominating for milestone 5 (production-ready).

[graydon]

Yeah, performance sadness is a production ready thing. Accepted.

[emberian]

This is recently in progress by @thestinger

[thestinger]

I landed #7495 and #7521 with the initial work on this. There's no longer anything reading/writing to the exchange allocation headers (on ~T and ~[]) and they are no longer initialized.

I really don't know enough about trans to finish this off by myself. I have work started here: https://github.com/thestinger/rust/commits/vec but I'm not having much luck tracking down and fixing the remaining issues.

[graydon]

Track me down more often once we've got 0.7 out, I know a lot of the awkward bits on this

[thestinger]

This is now done for ~T. I'm going to open a new bug about ~[]/~str (already have #7496 for ~fn).