This adds parsing for expressions like 'x as Ty[0]' which will
immediately error out, but still give the rest of the parser a valid
parse tree to continue.

This is almost entirely refactoring and message changing, with the
single behavioral change of panicking for unexpected output.

Previously this just errored out on all usages of type ascription,
which isn't helpful.

This is a modified version of estebank's suggestion, with a bit of
extra cleanup now that we don't need the different cases for if we can
turn a span into a string or not.

Implementes suggeseted changes by Centril.

This checks whether the memory location of the cast remains the same
after atttempting to parse a postfix operator after a cast has been
parsed. If the address is not the same, an illegal postfix operator
was parsed.

Previously the code generated a hash of the pointer, which was overly
complex and inefficent. Casting the pointers and comparing them
is simpler and more effcient.

Reference issue rust-lang#67311

Tests combinations of the following pattern features:
- bindings_after_at
- or_patterns
- slice_patterns
- box_patterns

Some of the nested OR paths were being missed

For all of the methods that pick off the first or last element, we can
use subslice patterns to implement them directly, rather than relying on
deeper indexing function calls. At a minimum, this means the generated
code will rely less on inlining for performance, but in some cases it
also optimizes better.

Error::description is deprecated as of version 1.42, as the commit was
not in the release for 1.41.

When encountering an Item in a pat context, point at the item def

```
error[E0308]: mismatched types
  --> $DIR/const-in-struct-pat.rs:8:17
   |
LL | struct foo;
   | ----------- `foo` defined here
...
LL |     let Thing { foo } = t;
   |                 ^^^ expected struct `std::string::String`, found struct `foo`
   |
   = note: `foo` is interpreted as a unit struct, not a new binding
help: you can bind the struct field to a different name
   |
LL |     let Thing { foo: other_foo } = t;
   |                 ^^^^^^^^^^^^^^
```
```
error[E0308]: mismatched types
  --> $DIR/const.rs:14:9
   |
LL | const FOO: Foo = Foo{bar: 5};
   | ----------------------------- constant defined here
...
LL |         FOO => {},
   |         ^^^
   |         |
   |         expected `&Foo`, found struct `Foo`
   |         `FOO` is interpreted as a constant, not a new binding
   |         help: use different name to introduce a new binding: `other_foo`
```

Fix rust-lang#55631, fix rust-lang#48062, cc rust-lang#42876.

Parse & reject postfix operators after casts

This adds an explicit error messages for when parsing `x as Type[0]` or similar expressions. Our add an extra parse case for parsing any postfix operator (dot, indexing, method calls, await) that triggers directly after parsing `as` expressions.

My friend and I worked on this together, but they're still deciding on a github username and thus I'm submitting this for both of us.

It will immediately error out, but will also provide the rest of the parser with a useful parse tree to deal with.

There's one decision we made in how this produces the parse tree. In the situation `&x as T[0]`, one could imagine this parsing as either `&((x as T)[0])` or `((&x) as T)[0]`. We chose the latter for ease of implementation, and as it seemed the most intuitive.

Feedback welcome! This is our first change to the parser section, and it might be completely horrible.

Fixes rust-lang#35813.

Use .next() instead of .nth(0) on iterators.

rustc_expand: Factor out `Annotatable::into_tokens` to a separate method

Minor refactoring salvaged from rust-lang#69594.
r? @Centril

…after-at-tests, r=Centril

test(pattern): add tests for combinations of pattern features

Reference issue rust-lang#67311

Tests combinations of the following pattern features:
- bindings_after_at
- or_patterns
- slice_patterns
- box_patterns

r? @Centril

Use subslice patterns in slice methods

For all of the methods that pick off the first or last element, we can
use subslice patterns to implement them directly, rather than relying on
deeper indexing function calls. At a minimum, this means the generated
code will rely less on inlining for performance, but in some cases it
also optimizes better.

Avoid using `unwrap()` in suggestions

Fixes rust-lang#69725

r? @estebank

…-Simulacrum

Update deprecation version to 1.42 for Error::description

Error::description is deprecated as of version 1.42, as the commit was
not in the release for 1.41.

Fixes rust-lang#69751

…ep, r=cramertj

Don't redundantly repeat field names (clippy::redundant_field_names)