Add way to express no-values with check-cfg

2 files changed, 14 insertions, 1 deletions

diff --git a/compiler/rustc_interface/src/interface.rs b/compiler/rustc_interface/src/interface.rs
index 32fba6ade88..5ca88090996 100644
--- a/compiler/rustc_interface/src/interface.rs
+++ b/compiler/rustc_interface/src/interface.rs
@@ -218,7 +218,9 @@ pub(crate) fn parse_check_cfg(dcx: &DiagCtxt, specs: Vec<String>) -> CheckCfg {
             }
         }
 
-        if values.is_empty() && !values_any_specified && !any_specified {
+        if !values_specified && !any_specified {
+            // `cfg(name)` is equivalent to `cfg(name, values(none()))` so add
+            // an implicit `none()`
             values.insert(None);
         } else if !values.is_empty() && values_any_specified {
             error!(
diff --git a/compiler/rustc_lint/src/context/diagnostics.rs b/compiler/rustc_lint/src/context/diagnostics.rs
index 9e6a6f70eac..312874db3f5 100644
--- a/compiler/rustc_lint/src/context/diagnostics.rs
+++ b/compiler/rustc_lint/src/context/diagnostics.rs
@@ -354,6 +354,15 @@ pub(super) fn builtin(
                         Applicability::MaybeIncorrect,
                     );
                 }
+            } else {
+                db.note(format!("no expected values for `{name}`"));
+
+                let sp = if let Some((_value, value_span)) = value {
+                    name_span.to(value_span)
+                } else {
+                    name_span
+                };
+                db.span_suggestion(sp, "remove the condition", "", Applicability::MaybeIncorrect);
             }
 
             // We don't want to suggest adding values to well known names
@@ -373,6 +382,8 @@ pub(super) fn builtin(
                 if name == sym::feature {
                     if let Some((value, _value_span)) = value {
                         db.help(format!("consider adding `{value}` as a feature in `Cargo.toml`"));
+                    } else {
+                        db.help("consider defining some features in `Cargo.toml`");
                     }
                 } else if !is_cfg_a_well_know_name {
                     db.help(format!("consider using a Cargo feature instead or adding `println!(\"cargo:rustc-check-cfg={inst}\");` to the top of a `build.rs`"));