rustdoc: Only record the same impl once

Due to inlining it is possible to visit the same module multiple times during `<Cache as DocFolder>::fold_crate`, so we keep track of the modules we've already visited.

fixes #33054

r? @alexcrichton

Remove IPV6_V6ONLY functionality

These settings can only be adjusted before bind time, which doesn't make
sense in the current set of functionality. These methods are stable, but
haven't hit a stable release yet.

Closes #33052

[breaking-change]

r? @alexcrichton

Will also need a backport to the beta.

librustc: remove outdated workaround

Fixed upstream: https://github.com/llvm-mirror/llvm/commit/ca07e256f62f

@alexcrichton following up from https://github.com/rust-lang/rust/pull/31709#discussion_r59125612

cc @ranma42

rustbuild: Run all markdown documentation tests

This commit adds support to rustbuild to run all documentation tests, basically
running `rustdoc --test` over all our documentation.

Sanity check Python on OSX for LLDB tests

Two primary changes:

* Don't get past the configure stage if `python` isn't coming from `/usr/bin`
* Call `debugger.Terminate()` to prevent segfaults on newer versions of LLDB.

Closes #32994

port compiletest to use JSON output

This uncovered a lot of bugs in compiletest and also some shortcomings
of our existing JSON output. We had to add information to the JSON
output, such as suggested text and macro backtraces. We also had to fix
various bugs in the existing tests.

Joint work with @jonathandturner.

r? @alexcrichton

Implement `append` for b-trees.

I have finally found time to revive #26227, this time only with an `append` implementation.

The algorithm implemented here is linear in the size of the two b-trees. It firsts creates
a `MergeIter` from the two b-trees and then builds a new b-tree by pushing
key-value pairs from the `MergeIter` into nodes at the right heights.

Three functions for stealing have been added to the implementation of `Handle` as
well as a getter for the height of a `NodeRef`.

The docs have been updated with performance information about `BTreeMap::append` and
the remark about B has been removed now that it is the same for all instances of `BTreeMap`.

cc @gereeter @Gankro @apasel422

configure: Support --disable-option-checking

I'm trying to package Rust for Fedora (this is nothing official (yet)).

The standard RPM packaging process involves running `./configure` with a whole lot of options that are commonly recognized by autotools configure scripts, but not by Rust's one. Since it does not make much sense to support all of this options, I think it would be great to support at least `--disable-option-checking`, so Rust's configure script would not fail.

[The old attempt](https://github.com/fabiand/rust-spec/blob/master/rustc.spec) to package Rust used a sed script (at line 72), but this is not the recommended way to do that.

Due to inlining it is possible to visit the same module multiple times
during `<Cache as DocFolder>::fold_crate`, so we keep track of the
modules we've already visited.

Move def id collection and extern crate handling to before AST->HIR lowering

r? @jseyfried, @EddyB, or @nikomatsakis

The algorithm implemented here is linear in the size of the two b-trees. It
firsts creates a `MergeIter` from the two b-trees and then builds a new b-tree
by pushing key-value pairs from the `MergeIter` into nodes at the right heights.

Three functions for stealing have been added to the implementation of `Handle` as
well as a getter for the height of a `NodeRef`.

The docs have been updated with performance information about `BTreeMap::append` and
the remark about B has been removed now that it is the same for all instances of `BTreeMap`.

Split core::iter module implementation into parts

Split core::iter module implementation into parts

split iter.rs into a directory of (implementation private) modules.

+ mod (adaptor structs whose private fields need to be available both for them and Iterator
  + iterator (Iterator trait)
  + traits (FromIterator, etc; all traits but Iterator itself)
  + range (range related)
  + sources (Repeat, Once, Empty)

rustdoc: Fix the strip-hidden `ImplStripper`

Instead of stripping impls which reference *stripped* items, we keep impls which reference *retained* items. We do this because when we strip an item we immediately return, and do not recurse into it - leaving the contained items non-stripped from the point of view of the `ImplStripper`.

fixes #33069

r? @alexcrichton

This test was relying on buggy behavior.

Also add a comment or two to pacify the merciless self-critic, who hates
a closure without a comment.

the current tidy panics give you no idea why it failed

This uncovered a lot of bugs in compiletest and also some shortcomings
of our existing JSON output. We had to add information to the JSON
output, such as suggested text and macro backtraces. We also had to fix
various bugs in the existing tests.

Joint work with jntrnr.

These settings can only be adjusted before bind time, which doesn't make
sense in the current set of functionality. These methods are stable, but
haven't hit a stable release yet.

Closes #33052

[breaking-change]

MIR: Do not require END_BLOCK to always exist

Basically, all this does, is removing restriction for END_BLOCK to exist past the first invocation of RemoveDeadBlocks pass. This way for functions whose CFG does not reach the `END_BLOCK` end up not containing the block.

As far as the implementation goes, I’m not entirely satisfied with the `BasicBlock::end_block`. I had hoped to make `new` a `const fn` and then just have a `const END_BLOCK` private to mir::build, but it turns out that constant functions don’t yet support conditionals nor a way to assert.

rustbuild: Package librustc & co for cross-hosts

Currently the `rust-std` package produced by rustbuild only contains the
standard library plus libtest, but the makefiles actually produce a `rust-std`
package with all known target libraries (including libsyntax, librustc, etc).
Tweak the behavior so the dependencies of the `dist-docs` step in rustbuild
depend on the compiler libraries as well (so that they're all packaged).

Closes #32984

Fixed upstream: https://github.com/llvm-mirror/llvm/commit/ca07e256f62f

doc: Update our tier support

This modifies our listing of tiered platforms a few ways:

* All lists are alphabetized based on target now
* Lots of targets are moved up to "Tier 2" as we're gating on all these builds
  and official releases are provided (and installable via rustup).
* A few targets now list having a compiler + cargo now as well.

No more platforms have been moved up to Tier 1 at this time, however. The only
real candidate is ``x86_64-unknown-linux-musl`, but that's not *quite* to a tier
1 level of quality just yet so let's hold off for another release or so to iron
it out a bit.

Compute `target_feature` from LLVM

This is a work-in-progress fix for #31662.

The logic that computes the target features from the command line has been replaced with queries to the `TargetMachine`.

Compute LLVM-agnostic type layouts in rustc.

Layout for monomorphic types, and some polymorphic ones (e.g. `&T` where `T: Sized`),
can now be computed by rustc without involving LLVM in the actual process.

This gives rustc the ability to evaluate `size_of` or `align_of`, as well as obtain field offsets.
MIR-based CTFE will eventually make use of these layouts, as will MIR trans, shortly.

Layout computation also comes with a `[breaking-change]`, or two:
* `"data-layout"` is now mandatory in custom target specifications, reverting the decision from #27076.
This string is needed because it describes endianness, pointer size and alignments for various types.
We have the first two and we could allow tweaking alignments in target specifications.
Or we could also extract the data layout from LLVM and feed it back into rustc.
However, that can vary with the LLVM version, which is fragile and undermines stability.
For built-in targets, I've added a check that the hardcoded data-layout matches LLVM defaults.
* `transmute` calls are checked in a stricter fashion, which fixes #32377

To expand on `transmute`, there are only 2 allowed patterns: between types with statically known sizes and between pointers with the same potentially-unsized "tail" (which determines the type of unsized metadata they use, if any).
If you're affected, my suggestions are:
* try to use casts (and raw pointer deref) instead of transmutes
* *really* try to avoid `transmute` where possible
* if you have a structure, try working on individual fields and unpack/repack the structure instead of transmuting it whole, e.g. `transmute::<RefCell<Box<T>>, RefCell<*mut T>>(x)` doesn't work, but `RefCell::new(Box::into_raw(x.into_inner()))` does (and `Box::into_raw` is just a `transmute`)

Add `contains` to `VecDeque` and `LinkedList` (+ tests)

This implements [RFC 1552](https://github.com/rust-lang/rfcs/blob/master/text/1552-contains-method-for-various-collections.md). Tracking issue: #32630

Sorry for the late response. This is my first contribution, so please tell me if anything isn't optimal!

mk: Bootstrap from stable instead of snapshots

This commit removes all infrastructure from the repository for our so-called
snapshots to instead bootstrap the compiler from stable releases. Bootstrapping
from a previously stable release is a long-desired feature of distros because
they're not fans of downloading binary stage0 blobs from us. Additionally, this
makes our own CI easier as we can decommission all of the snapshot builders and
start having a regular cadence to when we update the stage0 compiler.

A new `src/etc/get-stage0.py` script was added which shares some code with
`src/bootstrap/bootstrap.py` to read a new file, `src/stage0.txt`, which lists
the current stage0 compiler as well as cargo that we bootstrap from. This script
will download the relevant `rustc` package an unpack it into `$target/stage0` as
we do today.

One problem of bootstrapping from stable releases is that we're not able to
compile unstable code (e.g. all the `#![feature]` directives in libcore/libstd).
To overcome this we employ two strategies:

* The bootstrap key of the previous compiler is hardcoded into `src/stage0.txt`
  (enabled as a result of #32731) and exported by the build system. This enables
  nightly features in the compiler we download.
* The standard library and compiler are pinned to a specific stage0, which
  doesn't change, so we're guaranteed that we'll continue compiling as we start
  from a known fixed source.

The process for making a release will also need to be tweaked now to continue to
cadence of bootstrapping from the previous release. This process looks like:

1. Merge `beta` to `stable`
2. Produce a new stable compiler.
3. Change `master` to bootstrap from this new stable compiler.
4. Merge `master` to `beta`
5. Produce a new beta compiler
6. Change `master` to bootstrap from this new beta compiler.

Step 3 above should involve very few changes as `master` was previously
bootstrapping from `beta` which is the same as `stable` at that point in time.
Step 6, however, is where we benefit from removing lots of `#[cfg(stage0)]` and
get to use new features. This also shouldn't slow the release too much as steps
1-5 requires little work other than waiting and step 6 just needs to happen at
some point during a release cycle, it's not time sensitive.

Closes #29555
Closes #29557

Assert that the feature strings are NUL terminated, so that they will
be well-formed as C strings.

This is a safety check to ease the maintaninace and update of the
feature lists.

This simplifies the code a bit and makes the types nicer, too.

rustbuild: Fix --enable-rpath usage

This commit fixes the `--enable-rpath` configure flag in rustbuild to work
despite the compile-time directories being different than the runtime
directories. This unfortunately means that we can't use `-C rpath` out of the
box but hopefully the portability story here isn't too bad as
`src/librustc_back/rpath.rs` isn't *too* complicated.

Closes #32886

Improve computation of offset in `EscapeUnicode`

Unify the computation of `offset` and use `leading_zeros` instead of manually scanning the bits.
This PR removes some duplicated code and makes it a little simpler .
The computation of `offset` is also faster, but it is unlikely to have an impact on actual code.

(split from #31049)