These headers and flags were historically needed, but are now unnecessary due
to various changes in how coverage information is stored in MIR.

In user-facing Rust, `dyn` always has at least one predicate following
it. Unfortunately, because we filter out marker traits from receivers at
callsites and `dyn Sync` is, for example, legal, this results in us
having `dyn` types with no predicates on occasion in our alias set
encoding. This patch handles cases where there are no predicates in a
`dyn` type which are relevant to its alias set.

Fixes rust-lang#122998

Previously, we only rewrote `&self` and `&mut self` receivers. By
instantiating the method from the trait definition, we can make this
work work with arbitrary legal receivers instead.

I will be on vacation for the next three weeks. I will re-add myself
when I return.

It has a single call site, and afterwards all the calls to
`parse_expr_tuple_field_access` are in a single method, which is nice.

It's clearer at the call site.

Pass in the span for the field rather than using `prev_token`.
Also rename it `mk_expr_tuple_field_access`, because it doesn't do any
actual parsing, it just creates an expression with what it's given.

Not much of a clarity win by itself, but unlocks additional subsequent
simplifications.

For the `MiddleDot` case, current behaviour:
- For a case like `1.2`, `sym1` is `1` and `sym2` is `2`, and `self.token`
  holds `1.2`.
- It creates a new ident token from `sym1` that it puts into `self.token`.
- Then it does `bump_with` with a new dot token, which moves the `sym1`
  token into `prev_token`.
- Then it does `bump_with` with a new ident token from `sym2`, which moves the
  `dot` token into `prev_token` and discards the `sym1` token.
- Then it does `bump`, which puts whatever is next into `self.token`,
  moves the `sym2` token into `prev_token`, and discards the `dot` token
  altogether.

New behaviour:
- Skips creating and inserting the `sym1` and dot tokens, because they are
  unnecessary.
- This also demonstrates that the comment about `Spacing::Alone` is
  wrong -- that value is never used. That comment was added in rust-lang#77250,
  and AFAICT it has always been incorrect.

The commit also expands comments. I found this code hard to read
previously, the examples in comments make it easier.

…_expr, r=est31

Tweak `parse_dot_suffix_expr`

I find this function hard to understand, so I rewrote it.

r? ```@est31```

…r=onur-ozkan

Add more comments to the bootstrap code that handles `tests/coverage`

At the bootstrap level, coverage tests are a bit more complicated than other test suites, because we want to run the same set of test files in multiple different modes, in a way that's convenient and flexible when invoked manually.

This PR adds a few more comments to explain what's going on.

…pratt

Clarify transmute example

The example claims using an iterator will copy the entire vector, but this is not true in practice thanks to internal specializations in the stdlib (see https://godbolt.org/z/cnxo3MYs5 for confirmation that this doesn't reallocate nor iterate over the vec's elements). Since neither the copy nor the optimization is guaranteed I opted for saying that they _may_ happen.

…ulacrum

Clean up unnecessary headers/flags in coverage mir-opt tests

During rust-lang#122542, I noticed that some of the headers and flags I had copied over from `tests/mir-opt/instrument_coverage.rs`  were unnecessary. And while working to remove those, I noticed even more that could be removed or replaced.

CFI: Handle dyn with no principal

In user-facing Rust, `dyn` always has at least one predicate following it. Unfortunately, because we filter out marker traits from receivers at callsites and `dyn Sync` is, for example, legal, this results in us having `dyn` types with no predicates on occasion in our alias set encoding. This patch handles cases where there are no predicates in a `dyn` type which are relevant to its alias set.

Fixes rust-lang#122998

r? workingjubilee

…compiler-errors

CFI: Support complex receivers

Right now, we only support rewriting `&self` and `&mut self` into `&dyn MyTrait` and `&mut dyn MyTrait`. This expands it to handle the full gamut of receivers by calculating the receiver based on *substitution* rather than based on a rewrite. This means that, for example, `Arc<Self>` will become `Arc<dyn MyTrait>` appropriately with this change.

This approach also allows us to support associated type constraints as well, so we will correctly rewrite `&self` into `&dyn MyTrait<T=i32>`, for example.

r? ```@workingjubilee```

… r=nnethercote

Temporarily remove nnethercote from the review rotation.

I will be on vacation for the next three weeks. I will re-add myself when I return.

r? `@nnethercote`