Suggest cfg(false) instead of cfg(FALSE)

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 `{$cmdline_arg}`
+lint_unexpected_cfg_boolean = you may have meant to use `{$literal}` (notice the capitalization). Doing so makes this predicate evaluate to `{$literal}` unconditionally
 lint_unexpected_cfg_cargo_update = the {$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

@@ -138,6 +138,16 @@ pub(super) fn unexpected_cfg_name(
     let is_from_external_macro = name_span.in_external_macro(sess.source_map());
     let mut is_feature_cfg = name == sym::feature;
 
+    fn miscapitalized_boolean(name: Symbol) -> Option<bool> {
+        if name.as_str().eq_ignore_ascii_case("false") {
+            Some(false)
+        } else if name.as_str().eq_ignore_ascii_case("true") {
+            Some(true)
+        } else {
+            None
+        }
+    }
+
     let code_sugg = if is_feature_cfg && is_from_cargo {
         lints::unexpected_cfg_name::CodeSuggestion::DefineFeatures
     // Suggest correct `version("..")` predicate syntax
@@ -148,6 +158,21 @@ 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()
+        // If this is a miscapitalized False/FALSE, suggest the boolean literal instead
+        && let Some(boolean) = miscapitalized_boolean(name)
+        // Check this isn't a raw identifier
+        && sess
+            .source_map()
+            .span_to_snippet(name_span)
+            .map_or(true, |snippet| !snippet.contains("r#"))
+    {
+        lints::unexpected_cfg_name::CodeSuggestion::BooleanLiteral {
+            span: name_span,
+            literal: boolean,
+        }
     // 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 +2401,17 @@ pub(crate) mod unexpected_cfg_name {
             #[subdiagnostic]
             expected_names: Option<ExpectedNames>,
         },
+        #[suggestion(
+            lint_unexpected_cfg_boolean,
+            applicability = "machine-applicable",
+            style = "verbose",
+            code = "{literal}"
+        )]
+        BooleanLiteral {
+            #[primary_span]
+            span: Span,
+            literal: bool,
+        },
     }
 
     #[derive(Subdiagnostic)]

tests/ui/check-cfg/false.rs

@@ -0,0 +1,61 @@
+//! 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: you may have meant to use `false` (notice the capitalization).
+pub fn a() {}
+
+#[cfg(False)]
+//~^ WARNING unexpected `cfg` condition name: `False`
+//~| HELP: to expect this configuration use
+//~| HELP: you may have meant to use `false` (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(r#False)]
+//~^ WARNING unexpected `cfg` condition name: `False`
+//~| HELP: to expect this configuration use
+// No capitalization help for r#False
+pub fn d() {}
+
+#[cfg(false)]
+pub fn e() {}
+
+#[cfg(TRUE)]
+//~^ WARNING unexpected `cfg` condition name: `TRUE`
+//~| HELP: to expect this configuration use
+//~| HELP: you may have meant to use `true` (notice the capitalization).
+pub fn f() {}
+
+#[cfg(True)]
+//~^ WARNING unexpected `cfg` condition name: `True`
+//~| HELP: to expect this configuration use
+//~| HELP: you may have meant to use `true` (notice the capitalization).
+pub fn g() {}
+
+#[cfg(r#true)]
+//~^ WARNING unexpected `cfg` condition name: `r#true`
+//~| HELP: to expect this configuration use
+// No capitalization help for r#true
+pub fn h() {}
+
+#[cfg(r#True)]
+//~^ WARNING unexpected `cfg` condition name: `True`
+//~| HELP: to expect this configuration use
+// No capitalization help for r#True
+pub fn i() {}
+
+#[cfg(true)]
+pub fn j() {}
+
+pub fn main() {}

tests/ui/check-cfg/false.stderr

@@ -0,0 +1,95 @@
+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: you may have meant to use `false` (notice the capitalization). Doing so makes this predicate evaluate to `false` unconditionally
+   |
+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: you may have meant to use `false` (notice the capitalization). Doing so makes this predicate evaluate to `false` unconditionally (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: unexpected `cfg` condition name: `False`
+  --> $DIR/false.rs:25:7
+   |
+LL | #[cfg(r#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
+
+warning: unexpected `cfg` condition name: `TRUE`
+  --> $DIR/false.rs:34:7
+   |
+LL | #[cfg(TRUE)]
+   |       ^^^^
+   |
+   = help: to expect this configuration use `--check-cfg=cfg(TRUE)`
+   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
+help: you may have meant to use `true` (notice the capitalization). Doing so makes this predicate evaluate to `true` unconditionally
+   |
+LL - #[cfg(TRUE)]
+LL + #[cfg(true)]
+   |
+
+warning: unexpected `cfg` condition name: `True`
+  --> $DIR/false.rs:40:7
+   |
+LL | #[cfg(True)]
+   |       ^^^^
+   |
+   = help: to expect this configuration use `--check-cfg=cfg(True)`
+   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
+help: you may have meant to use `true` (notice the capitalization). Doing so makes this predicate evaluate to `true` unconditionally
+   |
+LL - #[cfg(True)]
+LL + #[cfg(true)]
+   |
+
+warning: unexpected `cfg` condition name: `r#true`
+  --> $DIR/false.rs:46:7
+   |
+LL | #[cfg(r#true)]
+   |       ^^^^^^
+   |
+   = help: to expect this configuration use `--check-cfg=cfg(r#true)`
+   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
+
+warning: unexpected `cfg` condition name: `True`
+  --> $DIR/false.rs:52:7
+   |
+LL | #[cfg(r#True)]
+   |       ^^^^^^
+   |
+   = help: to expect this configuration use `--check-cfg=cfg(True)`
+   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
+
+warning: 8 warnings emitted
+