Add new UNIQUE_CFG_CONDITION lint

Author: GuillaumeGomez

Diff for: clippy_lints/src/attrs.rs

@@ -292,6 +292,30 @@ declare_clippy_lint! {
     "ensures that all `allow` and `expect` attributes have a reason"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for `any` and `all` combinators in `cfg` with only one condition.
+    ///
+    /// ### Why is this bad?
+    /// If there is only one condition, no need to wrap it into `any` or `all` combinators.
+    ///
+    /// ### Example
+    /// ```rust
+    /// #[cfg(any(unix))]
+    /// pub struct Bar;
+    /// ```
+    ///
+    /// Use instead:
+    /// ```rust
+    /// #[cfg(unix)]
+    /// pub struct Bar;
+    /// ```
+    #[clippy::version = "1.71.0"]
+    pub UNIQUE_CFG_CONDITION,
+    style,
+    "ensure that all `cfg(any())` and `cfg(all())` have more than one condition"
+}
+
 declare_lint_pass!(Attributes => [
     ALLOW_ATTRIBUTES_WITHOUT_REASON,
     INLINE_ALWAYS,
@@ -604,6 +628,7 @@ impl_lint_pass!(EarlyAttributes => [
     DEPRECATED_CFG_ATTR,
     MISMATCHED_TARGET_OS,
     EMPTY_LINE_AFTER_OUTER_ATTR,
+    UNIQUE_CFG_CONDITION,
 ]);
 
 impl EarlyLintPass for EarlyAttributes {
@@ -614,6 +639,7 @@ impl EarlyLintPass for EarlyAttributes {
     fn check_attribute(&mut self, cx: &EarlyContext<'_>, attr: &Attribute) {
         check_deprecated_cfg_attr(cx, attr, &self.msrv);
         check_mismatched_target_os(cx, attr);
+        check_unique_cfg_condition(cx, attr);
     }
 
     extract_msrv_attr!(EarlyContext);
@@ -690,6 +716,40 @@ fn check_deprecated_cfg_attr(cx: &EarlyContext<'_>, attr: &Attribute, msrv: &Msr
     }
 }
 
+fn check_nested_cfg(cx: &EarlyContext<'_>, items: &[NestedMetaItem]) {
+    for item in items.iter() {
+        if let NestedMetaItem::MetaItem(meta) = item {
+            if !meta.has_name(sym::any) && !meta.has_name(sym::all) {
+                continue;
+            }
+            if let MetaItemKind::List(list) = &meta.kind {
+                check_nested_cfg(cx, list);
+                if list.len() == 1 {
+                    span_lint_and_then(
+                        cx,
+                        UNIQUE_CFG_CONDITION,
+                        meta.span,
+                        "unneeded sub `cfg` when there is only one condition",
+                        |diag| {
+                            if let Some(snippet) = snippet_opt(cx, list[0].span()) {
+                                diag.span_suggestion(meta.span, "try", snippet, Applicability::MaybeIncorrect);
+                            }
+                        },
+                    );
+                }
+            }
+        }
+    }
+}
+
+fn check_unique_cfg_condition(cx: &EarlyContext<'_>, attr: &Attribute) {
+    if attr.has_name(sym::cfg) &&
+        let Some(items) = attr.meta_item_list()
+    {
+        check_nested_cfg(cx, &items);
+    }
+}
+
 fn check_mismatched_target_os(cx: &EarlyContext<'_>, attr: &Attribute) {
     fn find_os(name: &str) -> Option<&'static str> {
         UNIX_SYSTEMS

Diff for: clippy_lints/src/declared_lints.rs

@@ -51,6 +51,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::attrs::EMPTY_LINE_AFTER_OUTER_ATTR_INFO,
     crate::attrs::INLINE_ALWAYS_INFO,
     crate::attrs::MISMATCHED_TARGET_OS_INFO,
+    crate::attrs::UNIQUE_CFG_CONDITION_INFO,
     crate::attrs::USELESS_ATTRIBUTE_INFO,
     crate::await_holding_invalid::AWAIT_HOLDING_INVALID_TYPE_INFO,
     crate::await_holding_invalid::AWAIT_HOLDING_LOCK_INFO,