Auto merge of #119473 - Urgau:check-cfg-explicit-none, r=petrochenkov

Add explicit `none()` value variant in check-cfg

This PR adds an explicit none value variant in check-cfg values: `values(none())`.

Currently the only way to define the none variant is with an empty `values()` which means that if someone has a cfg that takes none and strings they need to use two invocations: `--check-cfg=cfg(foo) --check-cfg=cfg(foo, values("bar"))`.
Which would now be `--check-cfg=cfg(foo, values(none(),"bar"))`, this is simpler and easier to understand.

`--check-cfg=cfg(foo)`, `--check-cfg=cfg(foo, values())` and `--check-cfg=cfg(foo, values(none()))` would be equivalent.

*Another motivation for doing this is to make empty `values()` actually means no-values, but this is orthogonal to this PR and adding `none()` is sufficient in it-self.*

`@rustbot` label +F-check-cfg
r? `@petrochenkov`

1 files changed, 8 insertions, 1 deletions

diff --git a/compiler/rustc_interface/src/interface.rs b/compiler/rustc_interface/src/interface.rs
index 03335996c03..32fba6ade88 100644
--- a/compiler/rustc_interface/src/interface.rs
+++ b/compiler/rustc_interface/src/interface.rs
@@ -202,8 +202,15 @@ 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 {