Suggest cfg(false) instead of cfg(FALSE)

Author: jamie-osec

compiler/rustc_lint/messages.ftl

@@ -837,6 +837,7 @@ lint_unexpected_cfg_doc_rustc = see <https://doc.rust-lang.org/nightly/rustc/che
 
 lint_unexpected_cfg_from_external_macro_origin = using a cfg inside a {$macro_kind} will use the cfgs from the destination crate and not the ones from the defining crate
 lint_unexpected_cfg_from_external_macro_refer = try referring to `{$macro_name}` crate for guidance on how handle this unexpected cfg
+lint_unexpected_cfg_boolean = consider using a boolean literal
 lint_unexpected_cfg_name = unexpected `cfg` condition name: `{$name}`
 lint_unexpected_cfg_name_expected_names = expected names are: {$possibilities}{$and_more ->
         [0] {""}

compiler/rustc_lint/src/early/diagnostics/check_cfg.rs

@@ -148,6 +148,9 @@ pub(super) fn unexpected_cfg_name(
             between_name_and_value: name_span.between(value_span),
             after_value: value_span.shrink_to_hi(),
         }
+    // Suggest a boolean literal instead
+    } else if value.is_none() && name == sym::FALSE {
+        lints::unexpected_cfg_name::CodeSuggestion::BooleanFalse { span: name_span }
     // Suggest the most probable if we found one
     } else if let Some(best_match) = find_best_match_for_name(&possibilities, name, None) {
         is_feature_cfg |= best_match == sym::feature;

compiler/rustc_lint/src/lints.rs

@@ -2401,6 +2401,16 @@ pub(crate) mod unexpected_cfg_name {
             #[subdiagnostic]
             expected_names: Option<ExpectedNames>,
         },
+        #[suggestion(
+            lint_unexpected_cfg_boolean,
+            applicability = "machine-applicable",
+            style = "verbose",
+            code = "false"
+        )]
+        BooleanFalse {
+            #[primary_span]
+            span: Span,
+        },
     }
 
     #[derive(Subdiagnostic)]

compiler/rustc_span/src/symbol.rs

@@ -235,6 +235,7 @@ symbols! {
         Equal,
         Err,
         Error,
+        FALSE,
         File,
         FileType,
         FmtArgumentsNew,

tests/ui/check-cfg/false.fixed

@@ -0,0 +1,12 @@
+//! Check that `cfg(false)` is suggested instead of cfg(FALSE)
+//
+//@ check-pass
+//@ no-auto-check-cfg
+//@ compile-flags: --check-cfg=cfg()
+//@ run-rustfix
+
+#[cfg(false)]
+//~^ WARNING unexpected `cfg` condition name: `FALSE`
+pub fn g() {}
+
+pub fn main() {}

tests/ui/check-cfg/false.rs

@@ -0,0 +1,12 @@
+//! Check that `cfg(false)` is suggested instead of cfg(FALSE)
+//
+//@ check-pass
+//@ no-auto-check-cfg
+//@ compile-flags: --check-cfg=cfg()
+//@ run-rustfix
+
+#[cfg(FALSE)]
+//~^ WARNING unexpected `cfg` condition name: `FALSE`
+pub fn g() {}
+
+pub fn main() {}

tests/ui/check-cfg/false.stderr

@@ -0,0 +1,17 @@
+warning: unexpected `cfg` condition name: `FALSE`
+  --> $DIR/false.rs:8:7
+   |
+LL | #[cfg(FALSE)]
+   |       ^^^^^
+   |
+   = help: to expect this configuration use `--check-cfg=cfg(FALSE)`
+   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
+   = note: `#[warn(unexpected_cfgs)]` on by default
+help: consider using a boolean literal
+   |
+LL - #[cfg(FALSE)]
+LL + #[cfg(false)]
+   |
+
+warning: 1 warning emitted
+