Add way to express that no values are expected with check-cfg

[Urgau]

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 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 #111068 I decided to make values() to mean the none variant, but it is no longer necessary since #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 rust-lang/cargo#13011.

@rustbot label +F-check-cfg
r? @petrochenkov
cc @epage

[petrochenkov]

@bors r+

[bors]

📌 Commit 41b69aa has been approved by petrochenkov

It is now in the queue for this repository.

[bors]

⌛ Testing commit 41b69aa with merge c58a5da...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing c58a5da to master...

[rust-timer]

Finished benchmarking commit (c58a5da): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.2%	[0.7%, 1.6%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.3%	[-4.1%, -1.4%]	3
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	5.7%	[3.1%, 9.3%]	17
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 665.924s -> 664.091s (-0.28%)
Artifact size: 308.39 MiB -> 308.23 MiB (-0.05%)