small cleanups in rustc_errors / emitter

This is either moving code around so it gets called less often or using if let instead of match in a few cases.

Add Result::into_err where the Ok variant is the never type

Equivalent of #66045 but for the inverse situation where `T: Into<!>` rather than `E: Into<!>`.

I'm using the same feature gate name. I can't see why one of these methods would be OK to stabilize but not the other.

Tracking issue: #61695

Add documentation for rustdoc-gui tests

I think a bit of documentation doesn't hurt in this case considering how "out of the ordinary" this is.

r? ``@jyn514``

Remove Option::{unwrap_none, expect_none}.

This removes `Option::unwrap_none` and `Option::expect_none` since we're not going to stabilize them, see https://github.com/rust-lang/rust/issues/62633.

Closes #62633

stabilize debug_non_exhaustive

tracking issue: https://github.com/rust-lang/rust/issues/67364

but it is still an open question whether the other `Debug*` struct's should have a similar method. I would guess that would best be put underneath a new feature gate, as this one seems uncontroversial enough to stabilize as is

Use `EvaluatedToOkModuloRegions` whenever we erase regions

Fixes #80691

When we evaluate a trait predicate, we convert an
`EvaluatedToOk` result to `EvaluatedToOkModuloRegions` if we erased any
regions. We cache the result under a region-erased 'freshened'
predicate, so `EvaluatedToOk` may not be correct for other predicates
that have the same cache key.

Fixes #80691

When we evaluate a trait predicate, we convert an
`EvaluatedToOk` result to `EvaluatedToOkModuloRegions` if we erased any
regions. We cache the result under a region-erased 'freshened'
predicate, so `EvaluatedToOk` may not be correct for other predicates
that have the same cache key.

2229 migration: Don't try resolve regions before writeback

In the analysis use `resolve_vars_if_possible` instead of `fully_resolve`,
because we might not have performed regionck yet.

Fixes: #83176

r? `@nikomatsakis`

Run analyses before thir-tree dumps

Fixes #83048

Expose str::SplitInclusive in alloc and therefore in std

This seems to have been omitted from the beginning when this feature was first introduced in 86bf9629.  Most users won't need to name this type which is probably why this wasn't noticed in the meantime.

See #83372 for a different but related bug.

### Notes for reviewers

I think I have got this right but TBH I am not very familiar with the relationship between core and std and so on.  <strike>I also haven't don't any kind of test (not even a build) yet.  I will do a local docs build to see that the type now appears in the std docs.</strike>  I did a local docs build and it has made this type appear as `std::str::SplitInclusive` as expected

The linkification of the return value from `str::split_inclusive` teleports me to the online url for `core::str::SplitInclusive`.  I think this may be a rustdoc anomaly (similar to #79630 maybe) but I am not sure.  Perhaps it means I haven't done the `std` -> `core` referrence correctly.

I made this insta-stable since it seems like simply a bug.  Please LMK if that is not right.  *(edited to add:)* In particular, IDK how this ought to relate to the (?)current release process.

implement `feature(const_generics_defaults)`

Implements const generics defaults `struct Example<const N: usize=3>`, as well as a query for getting the default of a given const-parameter's def id. There are some remaining FIXME's but they were specified as not blocking for merging this PR. This also puts the defaults behind the unstable feature gate `#![feature(const_generics_defaults)]`.

~~This currently creates a field which is always false on `GenericParamDefKind` for future use when
consts are permitted to have defaults. I'm not sure if this is exactly what is best for adding default parameters, but I mimicked the style of type defaults, so hopefully this is ok.~~

r? `@lcnr`

Rollup of 9 pull requests

Successful merges:

 - #83051 (Sidebar trait items order)
 - #83313 (Only enable assert_dep_graph when query-dep-graph is enabled.)
 - #83353 (Add internal io::Error::new_const to avoid allocations.)
 - #83391 (Allow not emitting `uwtable` on Android)
 - #83392 (Change `-W help` to display edition level.)
 - #83393 (Codeblock tooltip position)
 - #83399 (rustdoc: Record crate name instead of using `None`)
 - #83405 (Slight visual improvements to warning boxes in the docs)
 - #83415 (Remove unnecessary `Option` wrapping around `Crate.module`)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Remove unnecessary `Option` wrapping around `Crate.module`

I'm wondering if it was originally there so that we could `take` the
module which enables `after_krate` to take an `&Crate`. However, the two
impls of `after_krate` only use `Crate.name`, so we can pass just the
name instead.

Slight visual improvements to warning boxes in the docs

First I noticed that sometimes the thumbs-down emoji in the docs is hard to see and hard to look at because the yellow emoji color and the color of the box below are so bright. Especially if you look at the screen late at night you can notice it. I thought I should change that so I added a black outline around the emoji. It works using the [`text-shadow`](https://developer.mozilla.org/en-US/docs/Web/CSS/text-shadow) property. It may be a bit hacky but it seems to work well and browser compatibility looks pretty good too: [browser compatibility](https://developer.mozilla.org/en-US/docs/Web/CSS/text-shadow#browser_compatibility).
For consistency the microscope has the black border too.
Alternatively I had `drop-shadow(0px 0px 1px black);` in mind but its [browser compatibility](https://developer.mozilla.org/en-US/docs/Web/CSS/filter-function/drop-shadow()#browser_compatibility) doesn't look as good and the blurry shadow probably doesn't look as good either.

Then, I thought that now that I'm at it I could also try changing the purple color to a color you would rather expect to see for deprecation: red. For the red I've taken the blue and reused it as a foundation and moved it to the red color spectrum.
But then I thought that the purple color could still be reused for something else: for the boxes that tell you about portability (e.g. _only supported on Unix_). These are currently blue.

I think blue doesn't really represent danger like it should. Not being cross-platform represents a danger because if you want to compile for a different platform, your code may not compile anymore. Blue looks too friendly and is in my opinion more suitable for a box containing general information like for instance "This is available since 1.0.0". None of the current three box types (unstable, deprecated and portability) are that.

I think purple is a better fit for it because it's kind of in the middle between "use it" and "don't use it". Deprecated is definitely "don't use it". To illustrate this better, here's a color spectrum:

Blue = friendly,  "use it".
![image](https://user-images.githubusercontent.com/35064754/112139891-9a6b0f80-8bd3-11eb-94e1-dc747a3d4cf9.png)
Red = danger, "don't use it".

And the purple in the middle (the color that the portability box now has) probably represents "use it if you have to", so it's not entirely friendly and not entirely a danger. That is why I think it fits.

However I made one change to that existing purple: I made the outer color a bit brighter because it's outstandingly dark compared to the other outer colors of the other boxes.

This is all subjective but in my opinion it looks nicer. At first you might need to get used to it though. Notice the box colors and the black outlines around the emoji shapes:
![image](https://user-images.githubusercontent.com/35064754/112139327-ebc6cf00-8bd2-11eb-88ac-25219b43a1a0.png)
![image](https://user-images.githubusercontent.com/35064754/112139392-000acc00-8bd3-11eb-90c2-81feec93c521.png)

rustdoc: Record crate name instead of using `None`

Fixes #83365.

r? `@jyn514`

Codeblock tooltip position

The codeblocks tooltips were misplaced. Normally, there is no top margin applied to a tooltip unless the codeblock is the first element of the doc block. The CSS rule was too vague though, applying it to all tooltips where the codeblock was the first child of its parent. Which can be easily seen with lists:

Before:

![Screenshot from 2021-03-22 22-05-16](https://user-images.githubusercontent.com/3050060/112059812-a667ba80-8b5c-11eb-88dd-1c598ceb3766.png)

After:

![Screenshot from 2021-03-22 22-06-31](https://user-images.githubusercontent.com/3050060/112059815-a7005100-8b5c-11eb-9e40-8fc57513e498.png)

r? ``@Nemo157``

Change `-W help` to display edition level.

`-W help` was not honoring the `--edition` flag when displaying the default lint level. It was using the edition for sorting, but not for the final display.

This isn't important right now as there aren't any edition-specific lint levels. Also, the `declare_lint` macro is broken and doesn't even allow setting them right now. However, I figure it wouldn't hurt to fix this before I forget about it, in case edition-specific lints are ever used in the future.

Allow not emitting `uwtable` on Android

`uwtable` is marked as required on Android, so it can't be disabled via `-C force-unwind-tables=no`. However, I found that the reason it's marked as required was to resolve a [backtrace issue in Gecko](https://github.com/rust-lang/rust/issues/49867), and I haven't find any other reasons that make it required ([yet](https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/Unwind.20tables.20are.20strictly.20required.20on.20Windows.20and.20Android)). Therefore, I assume it's safe to turn it off if a (nice) backtrace is not needed, and submit this PR to allow `-C force-unwind-tables=no` when targeting Android.

Note that I haven't tested this change on Android as I don't have an Android environment for testing.

Add internal io::Error::new_const to avoid allocations.

This makes it possible to have a io::Error containing a message with zero allocations, and uses that everywhere to avoid the *three* allocations involved in `io::Error::new(kind, "message")`.

The function signature isn't perfect, because it needs a reference to the `&str`. So for now, this is just a `pub(crate)` function. Later, we'll be able to use `fn new_const<MSG: &'static str>(kind: ErrorKind)` to make that a bit better. (Then we'll also be able to use some ZST trickery if that would result in more efficient code.)

See https://github.com/rust-lang/rust/issues/83352

Only enable assert_dep_graph when query-dep-graph is enabled.

This is a debugging option. The only effect should be on rustc tests.

r? ``@michaelwoerister``

Sidebar trait items order

We were actually sorting `Symbol` and not `String`, creating a completely invalid sort result. I added a test to prevent regressions.

r? ``@jyn514``

Rename `source` to `span` and `span` to `source`

- Rename `clean::Item.source` to `span`
- Rename `clean::Span::span()` to `clean::Span::inner()`
- Rename `rustdoc_json_types::Item.source` to `span`
- rustdoc-json: Rename `Import.span` to `Import.source`

*See also the [discussion on Zulip][z] (this is a bit more than discussed in
that conversation, but all the changes are related).*

r? `@jyn514`

[z]: https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/get.20span.20of.20file.20from.20name/near/229603729

The previous changes mean that we can now remove this `Option`.

I'm wondering if it was originally there so that we could `take` the
module which enables `after_krate` to take an `&Crate`. However, the two
impls of `after_krate` only use `Crate.name`, so we can pass just the
name instead.

Co-authored-by: NJoshua Nelson <joshua@yottadb.com>

A bunch of nits fixed, and a new test for pretty printing the AST.

This currently creates a field which is always false on GenericParamDefKind for future use when
consts are permitted to have defaults

Update const_generics:default locations

Previously just ignored them, now actually do something about them.

Fix using type check instead of value

Add parsing

This adds all the necessary changes to lower const-generics defaults from parsing.

Change P<Expr> to AnonConst

This matches the arguments passed to instantiations of const generics, and makes it specific to
just anonymous constants.

Attempt to fix lowering bugs