This is a [breaking-change] for plugin authors.
You must now create a dep-graph earlier.

Previously, it would attempt to resolve some external crates that weren't
necessary for dep-info output.

Fixes #33231.

Note that this whole PR is a [breaking-change] for clients of the Compiler API.

This commit still does a lot of building in pretty because we always run after parsing.

Major changes:
- Remove old snippet rendering code and use the new stuff.
- Introduce `span_label` method to add a label
- Remove EndSpan mode and replace with a fn to get the last
  character of a span.
- Stop using `Option<MultiSpan>` and just use an empty `MultiSpan`
- and probably a bunch of other stuff :)

In fact, we make JSOn the default and add an option for save-analysis-csv for the legacy behaviour.

We also rename some bits and pieces `dxr` -> `save-analysis`

for now, this houses `svh` and the code to check `assert_dep_graph` is
sane

/# This is a combination of 16 commits.
/# The first commit's message is:
allow RUST_BACKTRACE=disabled to act as if unset

When RUST_BACKTRACE is set to "disabled" then this acts as if the env.
var is unset.

/# This is the 2nd commit message:

case insensitive "DiSaBLeD" RUST_BACKTRACE value

previously it expected a lowercase "disabled" to treat the env. var as
unset

/# This is the 3rd commit message:

RUST_BACKTRACE=0 acts as if unset

previously RUST_BACKTRACE=disabled was doing the same thing

/# This is the 4th commit message:

RUST_BACKTRACE=0|n|no|off acts as if unset

previously only RUST_BACKTRACE=0 acted as if RUST_BACKTRACE was unset
Now added more options (case-insensitive): 'n','no' and 'off'
eg. RUST_BACKTRACE=oFF

/# This is the 5th commit message:

DRY on the value of 2

DRY=don't repeat yourself
Because having to remember to keep the two places of '2' in sync is not
ideal, even though this is a simple enough case.

/# This is the 6th commit message:

Revert "DRY on the value of 2"

This reverts commit 95a0479d5cf72a2b2d9d21ec0bed2823ed213fef.

Nevermind this DRY on 2, because we already have a RY on 1,
besides the code is less readable this way...

/# This is the 7th commit message:

attempt to document unsetting RUST_BACKTRACE

/# This is the 8th commit message:

curb allocations when checking for RUST_BACKTRACE

this means we don't check for case-insensitivity anymore

/# This is the 9th commit message:

as decided, RUST_BACKTRACE=0 turns off backtrace

/# This is the 10th commit message:

RUST_TEST_NOCAPTURE=0 acts as if unset

(that is, capture is on)

Any other value acts as if nocapture is enabled (that is, capture is off)

/# This is the 11th commit message:

update other RUST_TEST_NOCAPTURE occurrences

apparently only one place needs updating

/# This is the 12th commit message:

update RUST_BACKTRACE in man page

/# This is the 13th commit message:

handle an occurrence of RUST_BACKTRACE

/# This is the 14th commit message:

ensure consistency with new rules for backtrace

/# This is the 15th commit message:

a more concise comment for RUST_TEST_NOCAPTURE

/# This is the 16th commit message:

update RUST_TEST_NOCAPTURE in man page

Gated cfg attributes are not available on the stable and beta release
channels, therefore they should not be presented to users of those
channels in order to avoid confusion.

The configuration returned by `config::build_configuration` needs to
be modified with `target_features::add_configuration` in order to also
contain the target features. This is already done for the
configuration used when compiling and when creating the documentation,
but was missing in the `cfg` printing code.

This commit adds support for *truly* unstable options in the compiler, as well
as adding warnings for the start of the deprecation path of
unstable-but-not-really options. Specifically, the following behavior is now in
place for handling unstable options:

* As before, an unconditional error is emitted if an unstable option is passed
  and the `-Z unstable-options` flag is not present. Note that passing another
  `-Z` flag does not require passing `-Z unstable-options` as well.
* New flags added to the compiler will be in the `Unstable` category as opposed
  to the `UnstableButNotReally` category which means they will unconditionally
  emit an error when used on stable.
* All current flags are in a category where they will emit warnings when used
  that the option will soon be a hard error.

Also as before, it is intended that `-Z` is akin to `#![feature]` in a crate
where it is required to unlock unstable functionality. A nightly compiler which
is used without any `-Z` flags should only be exercising stable behavior.

[breaking-change] for syntax extensions