Add explicit none() value variant in check-cfg

compiler/rustc_interface/src/interface.rs

@@ -199,8 +199,15 @@ pub(crate) fn parse_check_cfg(dcx: &DiagCtxt, specs: Vec<String>) -> CheckCfg {
                         if !args.is_empty() {
                             error!("`any()` must be empty");
                         }
+                    } else if arg.has_name(sym::none)
+                        && let Some(args) = arg.meta_item_list()
+                    {
+                        values.insert(None);
+                        if !args.is_empty() {
+                            error!("`none()` must be empty");
+                        }
                     } else {
-                        error!("`values()` arguments must be string literals or `any()`");
+                        error!("`values()` arguments must be string literals, `none()` or `any()`");
                     }
                 }
             } else {

src/doc/unstable-book/src/compiler-flags/check-cfg.md

@@ -36,12 +36,16 @@ present in the list of expected values. If `"value"` is not in it, then `rustc`
 *The command line `--cfg` arguments are currently *NOT* checked but may very well be checked in
 the future.*
 
+To check for the _none_ value (ie `#[cfg(foo)]`) one can use the `none()` predicate inside
+`values()`: `values(none())`. It can be followed or precessed by any number of `"value"`.
+
 To enable checking of values, but to provide an *none*/empty set of expected values
 (ie. expect `#[cfg(name)]`), use these forms:
 
 ```bash
 rustc --check-cfg 'cfg(name)'
 rustc --check-cfg 'cfg(name, values())'
+rustc --check-cfg 'cfg(name, values(none()))'
 ```
 
 To enable checking of name but not values (i.e. unknown expected values), use this form:
@@ -112,7 +116,7 @@ This table describe the equivalence of a `--cfg` argument to a `--check-cfg` arg
 | `--cfg`                       | `--check-cfg`                                              |
 |-----------------------------|----------------------------------------------------------|
 | *nothing*                     | *nothing* or `--check-cfg=cfg()` (to enable the checking)  |
-| `--cfg foo`                   | `--check-cfg=cfg(foo) or --check-cfg=cfg(foo, values())`   |
+| `--cfg foo`                   | `--check-cfg=cfg(foo), --check-cfg=cfg(foo, values())` or `--check-cfg=cfg(foo, values(none()))`   |
 | `--cfg foo=""`                | `--check-cfg=cfg(foo, values(""))`                         |
 | `--cfg foo="bar"`             | `--check-cfg=cfg(foo, values("bar"))`                      |
 | `--cfg foo="1" --cfg foo="2"` | `--check-cfg=cfg(foo, values("1", "2"))`                   |

tests/ui/check-cfg/invalid-arguments.cfg_none.stderr

@@ -0,0 +1,2 @@
+error: invalid `--check-cfg` argument: `cfg(none())` (`cfg()` arguments must be simple identifiers, `any()` or `values(...)`)
+

tests/ui/check-cfg/invalid-arguments.ident_in_values_1.stderr

@@ -1,2 +1,2 @@
-error: invalid `--check-cfg` argument: `cfg(foo,values(bar))` (`values()` arguments must be string literals or `any()`)
+error: invalid `--check-cfg` argument: `cfg(foo,values(bar))` (`values()` arguments must be string literals, `none()` or `any()`)
 

tests/ui/check-cfg/invalid-arguments.ident_in_values_2.stderr

@@ -1,2 +1,2 @@
-error: invalid `--check-cfg` argument: `cfg(foo,values("bar",bar,"bar"))` (`values()` arguments must be string literals or `any()`)
+error: invalid `--check-cfg` argument: `cfg(foo,values("bar",bar,"bar"))` (`values()` arguments must be string literals, `none()` or `any()`)
 

tests/ui/check-cfg/invalid-arguments.none_not_empty.stderr

@@ -0,0 +1,2 @@
+error: invalid `--check-cfg` argument: `cfg(foo,values(none("test")))` (`none()` must be empty)
+

tests/ui/check-cfg/invalid-arguments.rs

@@ -7,6 +7,7 @@
 // revisions: values_any_missing_values values_any_before_ident ident_in_values_1
 // revisions: ident_in_values_2 unknown_meta_item_1 unknown_meta_item_2 unknown_meta_item_3
 // revisions: mixed_values_any mixed_any any_values giberich unterminated
+// revisions: none_not_empty cfg_none
 //
 // compile-flags: -Z unstable-options
 // [anything_else]compile-flags: --check-cfg=anything_else(...)
@@ -24,9 +25,11 @@
 // [unknown_meta_item_1]compile-flags: --check-cfg=abc()
 // [unknown_meta_item_2]compile-flags: --check-cfg=cfg(foo,test())
 // [unknown_meta_item_3]compile-flags: --check-cfg=cfg(foo,values(test()))
+// [none_not_empty]compile-flags: --check-cfg=cfg(foo,values(none("test")))
 // [mixed_values_any]compile-flags: --check-cfg=cfg(foo,values("bar",any()))
 // [mixed_any]compile-flags: --check-cfg=cfg(any(),values(any()))
 // [any_values]compile-flags: --check-cfg=cfg(any(),values())
+// [cfg_none]compile-flags: --check-cfg=cfg(none())
 // [giberich]compile-flags: --check-cfg=cfg(...)
 // [unterminated]compile-flags: --check-cfg=cfg(
 

tests/ui/check-cfg/invalid-arguments.unknown_meta_item_3.stderr

@@ -1,2 +1,2 @@
-error: invalid `--check-cfg` argument: `cfg(foo,values(test()))` (`values()` arguments must be string literals or `any()`)
+error: invalid `--check-cfg` argument: `cfg(foo,values(test()))` (`values()` arguments must be string literals, `none()` or `any()`)
 

tests/ui/check-cfg/no-expected-values.rs

@@ -6,7 +6,7 @@
 // compile-flags: --check-cfg=cfg(values,simple,mixed,empty)
 // [simple]compile-flags: --check-cfg=cfg(test) --check-cfg=cfg(feature)
 // [mixed]compile-flags: --check-cfg=cfg(test,feature)
-// [empty]compile-flags: --check-cfg=cfg(test,feature,values())
+// [empty]compile-flags: --check-cfg=cfg(test,feature,values(none()))
 
 #[cfg(feature = "foo")]
 //~^ WARNING unexpected `cfg` condition value

tests/ui/check-cfg/values-none.explicit.stderr

@@ -0,0 +1,27 @@
+warning: unexpected `cfg` condition value: `too`
+  --> $DIR/values-none.rs:11:7
+   |
+LL | #[cfg(foo = "too")]
+   |       ^^^--------
+   |          |
+   |          help: remove the value
+   |
+   = note: no expected value for `foo`
+   = help: to expect this configuration use `--check-cfg=cfg(foo, values("too"))`
+   = 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: `bar`
+  --> $DIR/values-none.rs:16:7
+   |
+LL | #[cfg(foo = "bar")]
+   |       ^^^--------
+   |          |
+   |          help: remove the value
+   |
+   = note: no expected value for `foo`
+   = help: to expect this configuration use `--check-cfg=cfg(foo, values("bar"))`
+   = 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
+

tests/ui/check-cfg/values-none.implicit.stderr

@@ -0,0 +1,27 @@
+warning: unexpected `cfg` condition value: `too`
+  --> $DIR/values-none.rs:11:7
+   |
+LL | #[cfg(foo = "too")]
+   |       ^^^--------
+   |          |
+   |          help: remove the value
+   |
+   = note: no expected value for `foo`
+   = help: to expect this configuration use `--check-cfg=cfg(foo, values("too"))`
+   = 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: `bar`
+  --> $DIR/values-none.rs:16:7
+   |
+LL | #[cfg(foo = "bar")]
+   |       ^^^--------
+   |          |
+   |          help: remove the value
+   |
+   = note: no expected value for `foo`
+   = help: to expect this configuration use `--check-cfg=cfg(foo, values("bar"))`
+   = 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
+

tests/ui/check-cfg/values-none.rs

@@ -0,0 +1,23 @@
+// check-pass
+//
+// revisions: explicit implicit
+// compile-flags: -Zunstable-options
+// [explicit]compile-flags: --check-cfg=cfg(foo,values(none()))
+// [implicit]compile-flags: --check-cfg=cfg(foo)
+// [simple]  compile-flags: --check-cfg=cfg(foo,values(none(),"too"))
+// [concat_1]compile-flags: --check-cfg=cfg(foo) --check-cfg=cfg(foo,values("too"))
+// [concat_2]compile-flags: --check-cfg=cfg(foo,values("too")) --check-cfg=cfg(foo)
+
+#[cfg(foo = "too")]
+//[explicit]~^ WARNING unexpected `cfg` condition value
+//[implicit]~^^ WARNING unexpected `cfg` condition value
+fn foo_too() {}
+
+#[cfg(foo = "bar")]
+//~^ WARNING unexpected `cfg` condition value
+fn foo_bar() {}
+
+#[cfg(foo)]
+fn foo() {}
+
+fn main() {}