Suggest cfg(false) instead of cfg(FALSE)

Author: jamie-osec

compiler/rustc_lint/messages.ftl

@@ -829,6 +829,7 @@ lint_unexpected_cfg_add_build_rs_println = or consider adding `{$build_rs_printl
 lint_unexpected_cfg_add_cargo_feature = consider using a Cargo feature instead
 lint_unexpected_cfg_add_cargo_toml_lint_cfg = or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:{$cargo_toml_lint_cfg}
 lint_unexpected_cfg_add_cmdline_arg = to expect this configuration use `{$cmdline_arg}`
+lint_unexpected_cfg_boolean = write it in lowercase (notice the capitalization)
 lint_unexpected_cfg_cargo_update = the {$macro_kind} `{$macro_name}` may come from an old version of the `{$crate_name}` crate, try updating your dependency with `cargo update -p {$crate_name}`
 
 lint_unexpected_cfg_define_features = consider defining some features in `Cargo.toml`

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

@@ -148,6 +148,13 @@ pub(super) fn unexpected_cfg_name(
             between_name_and_value: name_span.between(value_span),
             after_value: value_span.shrink_to_hi(),
         }
+    // Suggest a literal `false` instead
+    // Detect miscapitalized `False`/`FALSE` etc, ensuring that this isn't `r#false`
+    } else if value.is_none()
+        && name.as_str().eq_ignore_ascii_case("false")
+        && name != rustc_span::kw::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)]

tests/ui/check-cfg/false.rs

@@ -0,0 +1,28 @@
+//! Check that `cfg(false)` is suggested instead of cfg(FALSE)
+//
+//@ check-pass
+//@ no-auto-check-cfg
+//@ compile-flags: --check-cfg=cfg()
+
+#[cfg(FALSE)]
+//~^ WARNING unexpected `cfg` condition name: `FALSE`
+//~| HELP: to expect this configuration use
+//~| HELP: write it in lowercase (notice the capitalization)
+pub fn a() {}
+
+#[cfg(False)]
+//~^ WARNING unexpected `cfg` condition name: `False`
+//~| HELP: to expect this configuration use
+//~| HELP: write it in lowercase (notice the capitalization)
+pub fn b() {}
+
+#[cfg(r#false)]
+//~^ WARNING unexpected `cfg` condition name: `r#false`
+//~| HELP: to expect this configuration use
+// No capitalization help for r#false
+pub fn c() {}
+
+#[cfg(false)]
+pub fn d() {}
+
+pub fn main() {}

tests/ui/check-cfg/false.stderr

@@ -0,0 +1,40 @@
+warning: unexpected `cfg` condition name: `FALSE`
+  --> $DIR/false.rs:7:7
+   |
+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: write it in lowercase (notice the capitalization)
+   |
+LL - #[cfg(FALSE)]
+LL + #[cfg(false)]
+   |
+
+warning: unexpected `cfg` condition name: `False`
+  --> $DIR/false.rs:13:7
+   |
+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
+help: write it in lowercase (notice the capitalization) (notice the capitalization)
+   |
+LL - #[cfg(False)]
+LL + #[cfg(false)]
+   |
+
+warning: unexpected `cfg` condition name: `r#false`
+  --> $DIR/false.rs:19:7
+   |
+LL | #[cfg(r#false)]
+   |       ^^^^^^^
+   |
+   = help: to expect this configuration use `--check-cfg=cfg(r#false)`
+   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
+
+warning: 3 warnings emitted
+