1 files changed, 49 insertions, 0 deletions
diff --git a/clippy_lints/src/methods/is_empty.rs b/clippy_lints/src/methods/is_empty.rs
new file mode 100644
index 00000000000..7fe66062251
--- /dev/null
+++ b/clippy_lints/src/methods/is_empty.rs
@@ -0,0 +1,49 @@
+use clippy_utils::consts::constant_is_empty;
+use clippy_utils::diagnostics::span_lint;
+use clippy_utils::{find_binding_init, path_to_local};
+use rustc_hir::{Expr, HirId};
+use rustc_lint::{LateContext, LintContext};
+use rustc_middle::lint::in_external_macro;
+use rustc_span::sym;
+
+use super::CONST_IS_EMPTY;
+
+/// Expression whose initialization depend on a constant conditioned by a `#[cfg(…)]` directive will
+/// not trigger the lint.
+pub(super) fn check(cx: &LateContext<'_>, expr: &'_ Expr<'_>, receiver: &Expr<'_>) {
+    if in_external_macro(cx.sess(), expr.span) || !receiver.span.eq_ctxt(expr.span) {
+        return;
+    }
+    let init_expr = expr_or_init(cx, receiver);
+    if !receiver.span.eq_ctxt(init_expr.span) {
+        return;
+    }
+    if let Some(init_is_empty) = constant_is_empty(cx, init_expr) {
+        span_lint(
+            cx,
+            CONST_IS_EMPTY,
+            expr.span,
+            &format!("this expression always evaluates to {init_is_empty:?}"),
+        );
+    }
+}
+
+fn is_under_cfg(cx: &LateContext<'_>, id: HirId) -> bool {
+    cx.tcx
+        .hir()
+        .parent_id_iter(id)
+        .any(|id| cx.tcx.hir().attrs(id).iter().any(|attr| attr.has_name(sym::cfg)))
+}
+
+/// Similar to [`clippy_utils::expr_or_init`], but does not go up the chain if the initialization
+/// value depends on a `#[cfg(…)]` directive.
+fn expr_or_init<'a, 'b, 'tcx: 'b>(cx: &LateContext<'tcx>, mut expr: &'a Expr<'b>) -> &'a Expr<'b> {
+    while let Some(init) = path_to_local(expr)
+        .and_then(|id| find_binding_init(cx, id))
+        .filter(|init| cx.typeck_results().expr_adjustments(init).is_empty())
+        .filter(|init| !is_under_cfg(cx, init.hir_id))
+    {
+        expr = init;
+    }
+    expr
+}