Rustup

changelog: Deprecate `unknown_clippy_lints` (integrated in `unknown_lints`)

r? `@ghost`

Improve safety of `LateContext::qpath_res`

This is my first rustc code change, inspired by hacking on clippy!

The first change is to clear cached `TypeckResults` from `LateContext` when visiting a nested item. I took a hint from [here](https://github.com/rust-lang/rust/blob/5e91c4ecc09312d8b63d250a432b0f3ef83f1df7/compiler/rustc_privacy/src/lib.rs#L1300).

Clippy has a `qpath_res` util function to avoid a possible ICE in `LateContext::qpath_res`. But the docs of `LateContext::qpath_res` promise no ICE. So this updates the `LateContext` method to keep its promises, and removes the util function.

Related: rust-lang/rust-clippy#4545

CC ````````````@EddyB```````````` since you've done related work
CC ````````````@flip1995```````````` FYI

Make more traits of the From/Into family diagnostic items

Following traits are now diagnostic items:
- `From` (unchanged)
- `Into`
- `TryFrom`
- `TryInto`

This also adds symbols for those items:
- `into_trait`
- `try_from_trait`
- `try_into_trait`

Related: https://github.com/rust-lang/rust-clippy/pull/6620#discussion_r562482587

Refractor a few more types to `rustc_type_ir`

In the continuation of #79169, ~~blocked on that PR~~.

This PR:
 - moves `IntVarValue`, `FloatVarValue`, `InferTy` (and friends) and `Variance`
 - creates the `IntTy`, `UintTy` and `FloatTy` enums in `rustc_type_ir`, based on their `ast` and `chalk_ir` equilavents, and uses them for types in the rest of the compiler.

~~I will split up that commit to make this easier to review and to have a better commit history.~~
EDIT: done, I split the PR in commits of 200-ish lines each

r? `````@nikomatsakis````` cc `````@jackh726`````

Fix website syntax highlighting

changelog: none

Fix syntax highlighting on website when the docs contain ` ```rust,ignore`

add "cargo dev crater" to run clippy on a fixed set of crates and diff the lint warnings

`cargo dev crater` now does the following:
build clippy in debug mode
for a fixed set of crates:
 download and extract the crate
 run compiled clippy on the crate
 dump the warnings into a file that is inside the repo

We can then do a "git diff" and see what effects our clippy changes had on a tiny fraction of the rust ecosystem and can see when an change unexpectedly added or silenced a lot of warnings.

Checking all the crates took less than 5 minutes on my system.

Should help with https://github.com/rust-lang/rust-clippy/issues/6429

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*Please write a short comment explaining your change (or "none" for internal only changes)*
changelog: extend cargo dev to run clippy against a fixed set of crates and compare warnings

New lint: exhaustive_enums, exhaustive_structs

Fixes #6616

changelog: Added restriction lint: `exhaustive_enums`, `exhaustive_structs`

Documentation for adding configuration to a lint and common abbreviations

This PR adds some commonly used abbreviations to the `basis.md` file and a guide on how to implement a configuration value for a lint.

* [Rendered `/doc/basics.md` (Abbreviation list)](https://github.com/xFrednet/rust-clippy/blob/0000-configuration-documentation/doc/basics.md#common-abbreviations)
* [Rendered `/doc/adding_lints.md` (Configuration value guide)](https://github.com/xFrednet/rust-clippy/blob/0000-configuration-documentation/doc/adding_lints.md#adding-configuration-to-a-lint)

I'm not sure if the guide is written in the best way. Style suggestions are appreciated. 🙃

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Again a big **thank you** for everyone who helped to collect the abbreviation list over on [zulip]. I had a lot of fun, and it was also very informative. Keep up the good work 🙃

[zulip]: https://rust-lang.zulipchat.com/#narrow/stream/257328-clippy/topic/Common.20abbreviations.20in.20basics.2Emd/near/223548065
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changelog: none

* Fixed some spelling

make serde a feature-dep
save clippy version in the crater log

Also sort lint results alphabetically.

cargo dev crater:  share target dir between clippy runs, enable pedantic and cargo lints, ignore tokei for now.