1. Added big comment block explaining the test framework.

2. Added tests exericising Rc and Arc. This was inspired by a comment
   from eefriedman on PR #28861.

3. Made the cycle-detection not issue false-positives on acyclic dags.

   Doing this efficiently required revising the framework; instead of
   visiting all children (i.e. doing a traversal), now each test is
   responsible for supplying the path that will act as a witness to
   the cycle.

   Luckily for me, all of the pre-existing tests worked with a trivial
   path built from "always tke your first left", but new tests I added
   did require other input paths (i.e., "first turn right, then left".

   (The path representation is a bit-string and its branches are
    n-ary, not word phrases and binary branches as you might think
    from the outline above.)

I had to fix a few things. Notable changes:

1. I removed the MIR support for constants, instead falling back to the existing `ConstVal`. I still think we ought to reform how we handle constants, but it's not clear to me that the approach I was taking is correct, and anyway I think we ought to do it separately.
2. I adjusted how we handle bindings in matches: we now *declare* all the bindings up front, rather than doing it as we encounter them. This is not only simpler, since we don't have to check if a binding has already been declared, it avoids ICEs if any of the arms turn out to be unreachable.
3. I do MIR construction *after* `check_match`, because it detects various broken cases. I'd like for `check_match` to be subsumed by MIR construction, but we can do that as a separate PR (if indeed it makes sense).

I did a crater run and found no regressions in the wild: https://gist.github.com/nikomatsakis/0038f90e10c8ad00f2f8

if they represent the same constant; otherwise the match algorithm
goes into infinite recursion when a pattern contains `NaN`

cause panics

and track which arms are reached (though in fact we don't make use of
this right now -- we might later if we absorb the checking of patterns
into MIR, as I would like)

Add the Tock paper.

trees in MIR

various annoying edge cases

The output of rustdoc includes resources licensed under the SIL Open
Font License, the MIT license, and the Apache License 2.0. All of these
licenses permit redistribution provided that the license text is also
redistributed. Previously this was not the case, making rustdoc output
unsuitable for distribution by default. This resolves that problem by
including the license texts in rustdoc output.

See also: https://users.rust-lang.org/t/licensing-rustdoc-output/2880

`--explain` support for E0163 and E0164.

Part of rust-lang/rust#24407

The reason this was not failing is fascinating.  The variable $(rustc)
is empty, so the make recipe was expanded as " -o foo foo.rs".  make
interpreted this as an instruction to run the command "o foo foo.rs"
and ignore any failure that occurred, because it uses a leading '-' on
a command to signal that behavior.

"Rust Inside Other Languages" in the Rust book includes a library example. The reference specifies printing "done!" when the code finishes running, and the language examples (Ruby, Python, JS) all do this in their code. 

However, the Rust library example code *also* does this, so that the examples as written would output "done!" twice.

This commit removes the "done!" from the Rust example code to tidy up the output to match expectations.

The "Rust Inside Other Languages" page includes a library example. The
reference specifies printing "done!" when the code finishes running, and
the language examples (Ruby, Python, JS) all do this in their code.
However, the Rust library example code *also* does this, so that the
examples as written would output "done!" twice.

This removes the "done!" from the Rust example code to clarify the docs.

By RFC1214:
>    Before calling a fn, we check that its argument and return types are WF.
    
The previous code only checked the trait-ref, which was not enough
in several cases.
    
As this is a soundness fix, it is a [breaking-change]. Some new annotations are needed, which I think are because of #18653 and the imperfection of `projection_must_outlive` (that can probably be worked around by moving the wf obligation later).
    
Fixes #28609

r? @nikomatsakis 

The reason this was not failing is fascinating.  The variable $(rustc)
is empty, so the make recipe was expanded as " -o foo foo.rs".  make
interpreted this as an instruction to run the command "o foo foo.rs"
and ignore any failure that occurred, because it uses a leading '-' on
a command to signal that behavior.

r? @steveklabnik 

libcore.rlib reduced from 19121 kiB to 15934 kiB - 20% win.

The librustc encoded AST is 9013500 bytes long - for the record, librustc consists of about 2254126 characters. Might be worth looking at.

r? @EddyB

Addds `--explain` support for E0163 and E0164.

also, ensure that callers are checked.

This adds a paragraph on how to generate documentation without sloooow `make doc`. I'm not a native English speaker, so there might be some language related bugs (I wish English was as hard to get wrong, as Rust)

This also includes whitespace cleanup of contributing.md in a separate commit. Whiltespace is not significant in github flavored markdown, and my Emacs just cleans ws automatically :)

r? @steveklabnik

part of #28810

it doesn't seem to be necessary and is potentially harmful.

r? @nikomatsakis 

- Successful merges: #28736, #28805, #28812, #28814, #28819
- Failed merges: 

It's not really clear here, since the example is rendered, where it
starts and ends. So let's use <hr>s to split it up.

The list was grabled.

r? @steveklabnik

Our docs were very basic for the various versions of from_utf8, so
this commit beefs them up.

It also improves docs for the &str variant's error, Utf8Error.

Fix #28402.

Our docs were very basic for the various versions of from_utf8, so
this commit beefs them up.

It also improves docs for the &str variant's error, Utf8Error.

It's not really clear here, since the example is rendered, where it
starts and ends. So let's use <hr>s to split it up.

fixes #26920