revert change removing has_infer check. Commit conservatively patches for now, but more development proceeding. Also contains a more concise test

Author: tnuha

compiler/rustc_middle/src/ty/util.rs

@@ -1368,7 +1368,6 @@ impl<'tcx> Ty<'tcx> {
     /// 2229 drop reorder migration analysis.
     #[inline]
     pub fn has_significant_drop(self, tcx: TyCtxt<'tcx>, typing_env: ty::TypingEnv<'tcx>) -> bool {
-        assert!(!self.has_non_region_infer());
         // Avoid querying in simple cases.
         match needs_drop_components(tcx, self) {
             Err(AlwaysRequiresDrop) => true,
@@ -1381,6 +1380,16 @@ impl<'tcx> Ty<'tcx> {
                     _ => 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.has_infer() {
+                    return true;
+                }
+
                 // This doesn't depend on regions, so try to minimize distinct
                 // query keys used.
                 let erased = tcx.normalize_erasing_regions(typing_env, query_ty);

tests/ui/type-inference/box_has_sigdrop.rs

@@ -0,0 +1,9 @@
+//@ should-fail
+//@ compile-flags: -Wrust-2021-incompatible-closure-captures
+// Inference, canonicalization, and significant drops should work nicely together.
+// Related issue: #86868
+
+fn main() {
+    let mut state = 0;
+    Box::new(move || state)
+}

tests/ui/type-inference/box_has_sigdrop.stderr

@@ -0,0 +1,17 @@
+error[E0308]: mismatched types
+  --> $DIR/box_has_sigdrop.rs:8:5
+   |
+LL | fn main() {
+   |          - expected `()` because of default return type
+LL |     let mut state = 0;
+LL |     Box::new(move || state)
+   |     ^^^^^^^^^^^^^^^^^^^^^^^- help: consider using a semicolon here: `;`
+   |     |
+   |     expected `()`, found `Box<{closure@box_has_sigdrop.rs:8:14}>`
+   |
+   = note: expected unit type `()`
+                 found struct `Box<{closure@$DIR/box_has_sigdrop.rs:8:14: 8:21}>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.