Guide: Puts the toc again in the doc

Closes #17505
Fix atomic::fence documentation typos
Fix handling of unboxed closure type param substitutions

- When selecting an implicit trait impl for an unboxed closure, plumb
  through and use the substitutions from impl selection instead of
  using those from the current param environment in trans, which may
  be incorrect.
- When generating a function declaration for an unboxed closure, plumb
  through the substitutions from the param environment of the closure
  as above.  Also normalize the type to avoid generating duplicate
  declarations due to regions being inconsistently replaced with
  ReStatic elsewhere.
- Do not place the closure type in the self param space when
  translating the unboxed closure callee, etc.  It is not actually
  used, and doing so conflicts with the self substitution from
  default trait methods.

Closes #18661
Closes #18685
Add regression test for #18661
Add regression test for #18685
Fix panic due to overly long borrow of RefCell

Closes #18711
Add regression test for #18711
Remove incorrect assert in trans

As an optimization, once unboxed closures receive their environment by
value if it fits within the size of an `int`.  An assert in this code
path assumed that this would only occur if the environment had no more
than a single free variable in it, but multiple smaller free variables
can easily be packed into the space of an `int`, particularly if any
of them are 0-sized.  The assert can simply be removed.

Closes #18652
Add regression test for #18652
Derive Clone for TypeId.
Remove unnecessary `.to_string()`s from `Vec.swap_remove`'s doc example.
Prepend should be append in the 30 minute intro

The examples are about adding to the end of the array, not the
beginning.
rustc: Process #[cfg]/#[cfg_attr] on crates

This commit implements processing these two attributes at the crate level as
well as at the item level. When #[cfg] is applied at the crate level, then the
entire crate will be omitted if the cfg doesn't match. The #[cfg_attr] attribute
is processed as usual in that the attribute is included or not depending on
whether the cfg matches.

This was spurred on by motivations of #18585 where #[cfg_attr] annotations will
be applied at the crate-level.

cc #18585
auto merge of #18688 : bkoropoff/rust/unboxed-closure-subst-fixes, r=nikomatsakis

This resolves some issues that remained after adding support for monomorphizing unboxed closures in trans.

There were a few places where a set of substitutions for an unboxed closure type were dropped on the floor and later recalculated from scratch based on the def ID, but this failed spectacularly when the closure originated from a different param environment.  The substitutions are now plumbed through end-to-end.  Closes #18661

There was also a conflict in the meaning of the self param space within the body of the unboxed closure.  Trans attempted to insert the unboxed closure type as the self type, but this could conflict with the self type from the param environment when an unboxed closure was used within a default method on a trait.  Since the body of an unboxed closure cannot refer to its own self type or value, there's no need for it to actually use the self space.  The downstream consumers of the substitutions in trans do not seem to need it either since they look up the type of the closure some other way, so I just stopped setting it.  Closes #18685.

r? @pcwalton @nikomatsakis
Make TyTrait embed a `TraitRef`, so that when we extend TraitRef, it naturally carries over to object types.

I wanted to embed an `Rc<TraitRef>`, but I was foiled by the current
static rules, which prohibit non-Sync values from being stored in
static locations. This means that the constants for `ty_int` and so
forth cannot be initialized.
Update parser with `for` syntax
Update tests where we don't report errors twice anymore
Purge the old `once_fns`, which are not coming back
Fix tidy error
Implement len() for EnumSet.