It's unreachable, because AST JSON printing support was removed some
time ago.

`MetaItem::value_str` is good enough. And this makes
`MetaItem::value_str` more like `MetaItem::meta_item_list` and
`name_value_literal`.

It's a highly misleading name, because it's completely different to
`MetaItem::name_value_literal`. Specifically, it doesn't match
`MetaItemKind::NameValue` (e.g. `#[foo = 3]`), it matches
`MetaItemKind::List` (e.g. `#[foo(3)]`).

This makes it clearer what is actually used outside of this crate.

…bute

this directive prevents compiletest from adding any implicit and
automatic --check-cfg arguments

This clarifies something that has puzzled me for some time.

Rewrite the `no-input-file.stderr` test in Rust and support diff

Rewrite the `no-input-file.stderr` test from rust-lang#121876.
Use the `similar` lib to replace the `diff` command.

…estebank

Minor AST cleanups

r? `@estebank`

CI: implement job skipping in Python matrix calculation

This removes the `step` YAML anchor and the corresponding bash script.

Best reviewed commit-by-commit.

r? ``@pietroalbini``

…youxu

Enable `--check-cfg` by default in UI tests

This PR enables-by-default `--check-cfg` in UI tests, now that it has become stable.

To do so this PR does 2 main things:
 - it introduce the `no-auto-check-cfg` directive to `compiletest`, to prevent any `--check-cfg` args (only to be used for `--check-cfg` tests)
 - it updates the _remaining_[^1] UI tests by either:
     - allowing the lint when neither expecting the lint nor giving the check-cfg args make sense
     - give the appropriate check-cfg args
     - or expect the lint, when it useful

[^1]: some preparation work was done in rust-lang#123577 rust-lang#123702

I highly recommend reviewing this PR commit-by-commit.

r? `@jieyouxu`