Specify trait implementation, resolution, and coherence for 1.0

[pnkfelix]

Niko has a grab-bag of issues over on #5527.

There are a bunch of desiderata for traits in Rust, and we won't get to all of them for 1.0.

But we do need to firm up exactly what the core semantics are (for how trait matching is done, and what the limits are for the backward-compatible core of the language), with a specification against which we can compare the implementation.

[pnkfelix]

As an example of the sort of thing we need to work out, see #8075 (and #3429).

[pnkfelix]

Accepted for 1.0, P-backcompat-lang.

[reem]

Visiting for triage: some progress has been made (particularly with all the new multi dispatch and associated type rules), but more needs to be done to fully specify everything.

[aturon]

Nominating for removal from P-backcompat-lang. The spec/documentation side is probably covered by existing bugs; this issue should perhaps be closed.

[pnkfelix]

Assigning 1.0 milestone (specifically the writeup of the semantics -- I consider it polish).

[pnkfelix]

(I probably should have removed P-backcompat-lang from this ticket when I assigned this to 1.0 and categorized it as a polish issue. It is possible that writing up the semantics will expose bugs that would necessitate a change to the language specification, but it is highly unlikely, and probably does not justify the label.)

[pnkfelix]

closing; if someone can come up with a spec, great, but we're not going to block anything ourselves to do so.