doc: Clarify supported platforms and tiers

[alexcrichton]

This commit expands the "supported platforms" section of the book to include
documentation on the tiers that Rust currently has as well as organizing all
supported platforms into these various tiers. Infrastructure improvements over
the next few months are likely to change the location of may of these platforms
over, but for now this should faithfully represent what we've got today!

[rust-highfive]

r? @steveklabnik

(rust_highfive has picked a reviewer for you, use r? to override)

[alexcrichton]

r? @brson

[brson]

This sentence contains 'support' twice, awkward. "The Rust compiler runs on, and compiles to, a great number of platforms, though not all platforms are equally supported. Rust's support levels are organized into three tiers, each with a different set of guarantees." Not sure, tired.

[brson]

"artifacts are provided"

[brson]

I consider this redundant with the previous bullet.

[brson]

Again, I consider this bullet redundant. 'artifacts' = 'installers'

[brson]

I do think it would be ideal to punt the fine details about specific low-tier platforms somewhere else, but this location is ok for now.

r? @steveklabnik

[alexcrichton]

Updated!

[nagisa]

On that particular configuration buildbots use. There’re some tests that do not pass on certain configurations.

[alexcrichton]

While a good point, I'm not sure if there's a great way to put this succinctly. We test a good deal of configurations but it's basically impossible for us to test every configuration so I'm not sure we'd gain much in breaking down exactly what configurations of what tests are run here.

[mdinger]

s/Specifially/Specifically/

[mdinger]

I didn't think the list read correctly based on the preceding statement. Satisfy sounds like it should be followed by a series of check marks or something and I couldn't work it in with that. I tried to set it up so each statement starts with a qualified verb (must do/may build/can compile/does not fly) (I don't know what they're actually called so I'm sorta guessing here).

The 3rd bullet may not be exactly what was meant originally though. I'm not really sure.

---

Specifically, these platforms:
* must be automatically built
* may or may not run the compiler test-suite
* successfully compile the `rust-lang/rust` master branch. This requires either:
  - just compiling the std library or
  - running the full bootstrap
* provide official release artifacts

[larsbergstrom]

@alexcrichton One thing that the Gecko tiers page (https://developer.mozilla.org/en/docs/Supported_build_configurations) has is contact names for the community members who are the official maintainer, where such a thing exists. Perhaps that makes sense here, too?

[lambda]

A questions on this; this appears to indicate the platforms that rustc supports running on, but that can be different than the platforms it supports targeting. In particular, I believe that limited support for targeting Windows XP has landed. Would it be at all useful to make the distinction between level of support for running on vs. targeting a platform in this description, or maybe adding a "Tier 4" for such platforms that can be targeted by a limited subset of the language and stdlib but not necessarily run the compiler or support all of std? Or is XP supposed to be covered by the i686-pc-windows-msvc target? In that case, you may want to update the comment about that target.

[SimonSapin]

What do the columns mean? I’m guessing that rustc and cargo columns are for "runs on platform X", while std is "can cross-compile targetting platform X", but this should be documented.

[alexcrichton]

@larsbergstrom

Sounds like a good idea to me! I'd want to clear it with everyone before adding them to this page, but I suspect those maintainers can be added here over time.

---

@lambda

Ah good point! XP support falls into the tier 3 category I believe where it's not guaranteed to build or work but support is in theory there, I'll add a note.

---

@SimonSapin

It means that the corresponding column is expected to build for that platform. In general there are many ways to cross to a platform (e.g. from any host), but I'll try to clarify.

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit 93e419c has been approved by steveklabnik

[bors]

⌛️ Testing commit 93e419c with merge 4aec7c7...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt