Add hint about the return code of panic!

I hope the link works on all cases, since the `unreachable` doc is copied to `std::` as well.

incr.comp.: Don't include span information in the ICH of type definitions

This should improve some of the `regex` tests on perf.rlo. Not including spans into the ICH is harmless until we also cache warnings. To really solve the problem, we need to do more refactoring (see #43088).

r? @nikomatsakis

Allow setting the limit on std::io::Take.

Fixes https://github.com/rust-lang/rust/issues/27269.

update crate dependencies

I wanted to update mdbook's version. This ended up updating a bunch of other stuff too.

I am not sure if updating this much stuff is considered a Good Idea or not; happy to figure out how to make it smaller if someone can help me figure out how to use x.py to do it.

I wanted to update mdbook's version. This ended up updating a bunch of other stuff too.

Redox: Use O_NOFOLLOW for lstat()

compilertest (UI test): Support custom normalization.

Closes #42434.

Adds this header for UI tests:

```rust
// normalize-stderr-32bit: "fn() (32 bits)" -> "fn() ($PTR bits)"
```

It will normalize the `stderr` output on 32-bit platforms, by replacing all instances of `fn() (32 bits)` by `fn() ($PTR bits)`.

Extends the UI tests in #42304 and #41968 to 32-bit targets.

r? @nikomatsakis

Update compiler_builtins submodule for probestack fix

Closes #43102

Downgrade ProjectionTy's TraitRef to its substs

Addresses the second part of #42171 by removing the `TraitRef` from
`ProjectionTy`, and directly storing its `Substs`.

Closes #42171.

Addresses the second part of #42171 by removing the `TraitRef` from
`ProjectionTy`, and directly storing its `Substs`.

Closes #42171.

Clean up some code

From #42669.

r? @nikomatsakis

Prepare for `normalize-std???` which will share the same logic.
Added `ignore-32bit` and `ignore-64bit`.

Use similar compression settings as before updating to use flate2

Fixes #42879

(My first PR to rust-lang yay)

This changes the compression settings back to how they were before the change to use the flate2 crate rather than the in-tree flate library. The specific changes are to use the `Fast` compression level (which should be equivialent to what was used before), and use a raw deflate stream rather than wrapping the stream in a zlib wrapper. The [zlib](https://tools.ietf.org/html/rfc1950) wrapper adds an extra 2 bytes of header data, and 4 bytes for a checksum at the end. The change to use a faster compression level did give some compile speedups in the past (see #37298). Having to calculate a checksum also added a small overhead, which didn't exist before the change to flate2.

r? @alexcrichton

Only match a fragment specifier the if it starts with certain tokens.

When trying to match a fragment specifier, we first predict whether the current token can be matched at all. If it cannot be matched, don't bother to push the Earley item to `bb_eis`. This can fix a lot of issues which otherwise requires full backtracking (#42838).

In this PR the prediction treatment is not done for `:item`, `:stmt` and `:tt`, but it could be expanded in the future.

Fixes #24189.
Fixes #26444.
Fixes #27832.
Fixes #34030.
Fixes #35650.
Fixes #39964.
Fixes the 4th comment in #40569.
Fixes the issue blocking #40984.

incr.comp.: Deduplicate some DepNodes and introduce anonymous DepNodes

This is a parallel PR to the pending https://github.com/rust-lang/rust/pull/42769. It implements most of what is possible in terms of DepNode re-opening without having anonymous DepNodes yet (https://github.com/rust-lang/rust/issues/42298).

r? @nikomatsakis

Test src/doc once more

This was accidentally broken in https://github.com/rust-lang/rust/pull/42437 since we filtered too early to recurse into sub-directories.

In theory, @bors p=10

r? @alexcrichton

Fixes #42879

Fix feature gate for `#[link_args(..)]` attribute

Fix feature gate for `#[link_args(..)]` attribute so that it will fire regardless of context of attribute.

See also #29596 and #43106

regardless of context of attribute.

Extend the gating test to include the attribute in "weird" places.

Store all generic arguments for method calls in AST/HIR

The first part of https://github.com/rust-lang/rust/pull/42492.
Landed separately to start the process of merging libsyntax changes breaking rustfmt, which is not easy these days.