Fix a false-positive inside const fn in comparison_chain

clippy_lints/src/comparison_chain.rs

@@ -1,7 +1,7 @@
 use clippy_utils::diagnostics::span_lint_and_help;
 use clippy_utils::ty::implements_trait;
 use clippy_utils::{get_trait_def_id, if_sequence, is_else_clause, paths, SpanlessEq};
-use rustc_hir::{BinOpKind, Expr, ExprKind};
+use rustc_hir::{BinOpKind, Expr, ExprKind, Node};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 
@@ -64,6 +64,10 @@ impl<'tcx> LateLintPass<'tcx> for ComparisonChain {
             return;
         }
 
+        if parent_node_is_if_const_fn(cx, expr) {
+            return;
+        }
+
         // Check that there exists at least one explicit else condition
         let (conds, _) = if_sequence(expr);
         if conds.len() < 2 {
@@ -123,3 +127,11 @@ impl<'tcx> LateLintPass<'tcx> for ComparisonChain {
 fn kind_is_cmp(kind: BinOpKind) -> bool {
     matches!(kind, BinOpKind::Lt | BinOpKind::Gt | BinOpKind::Eq)
 }
+
+fn parent_node_is_if_const_fn(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
+    match cx.tcx.hir().find(cx.tcx.hir().get_parent_item(expr.hir_id)) {
+        Some(Node::Item(item)) => rustc_mir::const_eval::is_const_fn(cx.tcx, item.def_id.to_def_id()),
+        Some(Node::ImplItem(impl_item)) => rustc_mir::const_eval::is_const_fn(cx.tcx, impl_item.def_id.to_def_id()),
+        _ => false,
+    }
+}

tests/ui/comparison_chain.rs

@@ -203,4 +203,32 @@ mod issue_5212 {
     }
 }
 
+enum Sign {
+    Negative,
+    Positive,
+    Zero,
+}
+
+impl Sign {
+    const fn sign_i8(n: i8) -> Self {
+        if n == 0 {
+            Sign::Zero
+        } else if n > 0 {
+            Sign::Positive
+        } else {
+            Sign::Negative
+        }
+    }
+}
+
+const fn sign_i8(n: i8) -> Sign {
+    if n == 0 {
+        Sign::Zero
+    } else if n > 0 {
+        Sign::Positive
+    } else {
+        Sign::Negative
+    }
+}
+
 fn main() {}