Fix missing_panics_doc warning on unreachable!.

clippy_lints/src/doc.rs

@@ -1,6 +1,6 @@
 use crate::utils::{
-    implements_trait, is_entrypoint_fn, is_type_diagnostic_item, match_panic_def_id, method_chain_args, return_ty,
-    span_lint, span_lint_and_note,
+    implements_trait, is_entrypoint_fn, is_expn_of, is_type_diagnostic_item, match_panic_def_id, method_chain_args,
+    return_ty, span_lint, span_lint_and_note,
 };
 use if_chain::if_chain;
 use itertools::Itertools;
@@ -699,6 +699,7 @@ fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
             if let ExprKind::Path(QPath::Resolved(_, ref path)) = func_expr.kind;
             if let Some(path_def_id) = path.res.opt_def_id();
             if match_panic_def_id(self.cx, path_def_id);
+            if is_expn_of(expr.span, "unreachable").is_none();
             then {
                 self.panic_span = Some(expr.span);
             }

tests/ui/doc_panics.rs

@@ -93,3 +93,8 @@ fn inner_body_private(opt: Option<u32>) {
         }
     });
 }
+
+/// This is okay because unreachable
+pub fn unreachable() {
+    unreachable!("This function panics")
+}