be conservative in has_significant_drop

compiler/rustc_middle/src/ty/util.rs

@@ -816,6 +816,15 @@ pub fn has_significant_drop(
                     [component_ty] => component_ty,
                     _ => self,
                 };
+
+                // FIXME: We should be canonicalizing, or else moving this to a method of inference
+                // context, or *something* like that, but for now just avoid passing inference
+                // variables to queries that can't cope with them. Instead, conservatively
+                // return "true" (may change drop order).
+                if query_ty.needs_infer() {
+                    return true;
+                }
+
                 // This doesn't depend on regions, so try to minimize distinct
                 // query keys used.
                 let erased = tcx.normalize_erasing_regions(param_env, query_ty);

src/test/ui/closures/2229_closure_analysis/migrations/issue-86753.rs

+// edition:2018
+// check-pass
+
+#![warn(rust_2021_compatibility)]
+
+use std::future::Future;
+
+struct Runtime;
+
+impl Runtime {
+    pub fn block_on<F: Future>(&self, _future: F) -> F::Output {
+        unimplemented!()
+    }
+}
+
+pub fn http<F, Fut>(_func: F)
+where
+    F: Fn() -> Fut,
+    Fut: Future<Output = ()>,
+{
+    let rt = Runtime {};
+    let srv = rt.block_on(async move { serve(move || async move { unimplemented!() }) });
+    let _ = || rt.block_on(async { srv });
+}
+
+pub struct Server<S> {
+    _marker: std::marker::PhantomData<S>,
+}
+
+pub fn serve<S>(_new_service: S) -> Server<S> {
+    unimplemented!()
+}
+
+fn main() { }