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`

tests/ui/check-cfg/cargo-feature.none.stderr

@@ -1,34 +1,36 @@
-warning: unexpected `cfg` condition name: `feature`
-  --> $DIR/cargo-feature.rs:13:7
+warning: unexpected `cfg` condition value: `serde`
+  --> $DIR/cargo-feature.rs:14:7
    |
 LL | #[cfg(feature = "serde")]
-   |       ^^^^^^^^^^^^^^^^^
+   |       ^^^^^^^^^^^^^^^^^ help: remove the condition
    |
-   = help: consider defining some features in `Cargo.toml`
+   = note: no expected values for `feature`
+   = help: consider adding `serde` as a feature in `Cargo.toml`
    = note: see <https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#check-cfg> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
 
-warning: unexpected `cfg` condition name: `feature`
+warning: unexpected `cfg` condition value: (none)
   --> $DIR/cargo-feature.rs:18:7
    |
 LL | #[cfg(feature)]
-   |       ^^^^^^^
+   |       ^^^^^^^ help: remove the condition
    |
+   = note: no expected values for `feature`
    = help: consider defining some features in `Cargo.toml`
    = note: see <https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#check-cfg> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition name: `tokio_unstable`
-  --> $DIR/cargo-feature.rs:23:7
+  --> $DIR/cargo-feature.rs:22:7
    |
 LL | #[cfg(tokio_unstable)]
    |       ^^^^^^^^^^^^^^
    |
-   = help: expected names are: `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
+   = help: expected names are: `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
    = help: consider using a Cargo feature instead or adding `println!("cargo:rustc-check-cfg=cfg(tokio_unstable)");` to the top of a `build.rs`
    = note: see <https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#check-cfg> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition name: `CONFIG_NVME`
-  --> $DIR/cargo-feature.rs:27:7
+  --> $DIR/cargo-feature.rs:26:7
    |
 LL | #[cfg(CONFIG_NVME = "m")]
    |       ^^^^^^^^^^^^^^^^^

tests/ui/check-cfg/cargo-feature.rs

@@ -5,19 +5,18 @@
 // check-pass
 // revisions: some none
 // rustc-env:CARGO=/usr/bin/cargo
-// compile-flags: --check-cfg=cfg() -Z unstable-options
+// compile-flags: -Z unstable-options
+// [none]compile-flags: --check-cfg=cfg(feature,values())
 // [some]compile-flags: --check-cfg=cfg(feature,values("bitcode"))
 // [some]compile-flags: --check-cfg=cfg(CONFIG_NVME,values("y"))
 // [none]error-pattern:Cargo.toml
 
 #[cfg(feature = "serde")]
-//[none]~^ WARNING unexpected `cfg` condition name
-//[some]~^^ WARNING unexpected `cfg` condition value
+//~^ WARNING unexpected `cfg` condition value
 fn ser() {}
 
 #[cfg(feature)]
-//[none]~^ WARNING unexpected `cfg` condition name
-//[some]~^^ WARNING unexpected `cfg` condition value
+//~^ WARNING unexpected `cfg` condition value
 fn feat() {}
 
 #[cfg(tokio_unstable)]

tests/ui/check-cfg/cargo-feature.some.stderr

@@ -1,5 +1,5 @@
 warning: unexpected `cfg` condition value: `serde`
-  --> $DIR/cargo-feature.rs:13:7
+  --> $DIR/cargo-feature.rs:14:7
    |
 LL | #[cfg(feature = "serde")]
    |       ^^^^^^^^^^^^^^^^^
@@ -16,10 +16,11 @@ LL | #[cfg(feature)]
    |       ^^^^^^^- help: specify a config value: `= "bitcode"`
    |
    = note: expected values for `feature` are: `bitcode`
+   = help: consider defining some features in `Cargo.toml`
    = note: see <https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#check-cfg> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition name: `tokio_unstable`
-  --> $DIR/cargo-feature.rs:23:7
+  --> $DIR/cargo-feature.rs:22:7
    |
 LL | #[cfg(tokio_unstable)]
    |       ^^^^^^^^^^^^^^
@@ -29,7 +30,7 @@ LL | #[cfg(tokio_unstable)]
    = note: see <https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#check-cfg> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `m`
-  --> $DIR/cargo-feature.rs:27:7
+  --> $DIR/cargo-feature.rs:26:7
    |
 LL | #[cfg(CONFIG_NVME = "m")]
    |       ^^^^^^^^^^^^^^---

tests/ui/check-cfg/concat-values.rs

@@ -1,6 +1,7 @@
 // check-pass
 // compile-flags: -Z unstable-options
 // compile-flags: --check-cfg=cfg(my_cfg,values("foo")) --check-cfg=cfg(my_cfg,values("bar"))
+// compile-flags: --check-cfg=cfg(my_cfg,values())
 
 #[cfg(my_cfg)]
 //~^ WARNING unexpected `cfg` condition value
@@ -10,4 +11,7 @@ fn my_cfg() {}
 //~^ WARNING unexpected `cfg` condition value
 fn my_cfg() {}
 
+#[cfg(any(my_cfg = "foo", my_cfg = "bar"))]
+fn foo_and_bar() {}
+
 fn main() {}

tests/ui/check-cfg/concat-values.stderr

@@ -1,5 +1,5 @@
 warning: unexpected `cfg` condition value: (none)
-  --> $DIR/concat-values.rs:5:7
+  --> $DIR/concat-values.rs:6:7
    |
 LL | #[cfg(my_cfg)]
    |       ^^^^^^
@@ -10,7 +10,7 @@ LL | #[cfg(my_cfg)]
    = note: `#[warn(unexpected_cfgs)]` on by default
 
 warning: unexpected `cfg` condition value: `unk`
-  --> $DIR/concat-values.rs:9:7
+  --> $DIR/concat-values.rs:10:7
    |
 LL | #[cfg(my_cfg = "unk")]
    |       ^^^^^^^^^^^^^^

tests/ui/check-cfg/empty-values.rs

@@ -0,0 +1,14 @@
+// Check that we detect unexpected value when none are allowed
+//
+// check-pass
+// compile-flags: --check-cfg=cfg(foo,values()) -Zunstable-options
+
+#[cfg(foo = "foo")]
+//~^ WARNING unexpected `cfg` condition value
+fn do_foo() {}
+
+#[cfg(foo)]
+//~^ WARNING unexpected `cfg` condition value
+fn do_foo() {}
+
+fn main() {}

tests/ui/check-cfg/empty-values.stderr

@@ -0,0 +1,23 @@
+warning: unexpected `cfg` condition value: `foo`
+  --> $DIR/empty-values.rs:6:7
+   |
+LL | #[cfg(foo = "foo")]
+   |       ^^^^^^^^^^^ help: remove the condition
+   |
+   = note: no expected values for `foo`
+   = help: to expect this configuration use `--check-cfg=cfg(foo, values("foo"))`
+   = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
+   = note: `#[warn(unexpected_cfgs)]` on by default
+
+warning: unexpected `cfg` condition value: (none)
+  --> $DIR/empty-values.rs:10:7
+   |
+LL | #[cfg(foo)]
+   |       ^^^ help: remove the condition
+   |
+   = note: no expected values for `foo`
+   = help: to expect this configuration use `--check-cfg=cfg(foo)`
+   = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
+
+warning: 2 warnings emitted
+