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bors a8a0df1b51 Auto merge of #119930 - Urgau:check-cfg-empty-values-means-empty, r=petrochenkov 
Add way to express that no values are expected with check-cfg

This PR adds way to express no-values (no values expected) with `--check-cfg` by making empty `values()` no longer mean `values(none())` (internal: `&[None]`) and now be an empty list (internal: `&[]`).

### Context

Currently `--check-cfg` has a way to express that _any value is expected_ with `values(any())`, but has no way to do the inverse and say that _no value is expected_.

This would be particularly useful for build systems that control a config name and it's values as they could always declare a config name as expected and if in the current state they have values pass them and if not pass an empty list.

To give a more concrete example, Cargo `--check-cfg` currently needs to generate:
 - `--check-cfg=cfg(feature, values(...))` for the case with declared features
 - and `--check-cfg=cfg()` for the case without any features declared

This means that when there are no features declared, users will get an `unexpected config name` but from the point of view of Cargo the config name `feature` is expected, it's just that for now there aren't any values for it.

See [Cargo `check_cfg_args` function](92395d9010/src/cargo/core/compiler/mod.rs (L1263-L1281)) for more details.

### De-specializing *empty* `values()`

To solve this issue I propose that we "de-specialize" empty `values()` to no longer mean `values(none())` but to actually mean empty set/list. This is one of the last source of confusion for my-self and others with the `--check-cfg` syntax.

> The confusing part here is that an empty `values()` currently means the same as `values(none())`, i.e. an expected list of values with the _none_ variant (as in `#[cfg(name)]` where the value is none) instead of meaning an empty set.

Before the new `cfg()` syntax, defining the _none_ variant was only possible under certain circumstances, so in https://github.com/rust-lang/rust/pull/111068 I decided to make `values()` to mean the _none_ variant, but it is no longer necessary since https://github.com/rust-lang/rust/pull/119473 which introduced the `none()` syntax.

A simplified representation of the proposed "de-specialization" would be:

| Syntax                                  | List/set of expected values |
|-----------------------------------------|-----------------------------|
| `cfg(name)`/`cfg(name, values(none()))` | `&[None]`                   |
| `cfg(name, values())`                   | `&[]`                       |

Note that I have my-self made the mistake of using an empty `values()` as meaning empty set, see https://github.com/rust-lang/cargo/pull/13011.

`@rustbot` label +F-check-cfg
r? `@petrochenkov`
cc `@epage`
2024-01-17 14:01:05 +00:00
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README.md

rust-analyzer logo

rust-analyzer is a modular compiler frontend for the Rust language. It is a part of a larger rls-2.0 effort to create excellent IDE support for Rust.

Quick Start

https://rust-analyzer.github.io/manual.html#installation

Documentation

If you want to contribute to rust-analyzer or are just curious about how things work under the hood, check the ./docs/dev folder.

If you want to use rust-analyzer's language server with your editor of choice, check the manual folder. It also contains some tips & tricks to help you be more productive when using rust-analyzer.

Security and Privacy

See the corresponding sections of the manual.

Communication

For usage and troubleshooting requests, please use "IDEs and Editors" category of the Rust forum:

https://users.rust-lang.org/c/ide/14

For questions about development and implementation, join rust-analyzer working group on Zulip:

https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Frust-analyzer

Quick Links

License

rust-analyzer is primarily distributed under the terms of both the MIT license and the Apache License (Version 2.0).

See LICENSE-APACHE and LICENSE-MIT for details.