Rollup merge of #89359 - fee1-dead:const-it, r=oli-obk

Various fixes for const_trait_impl

A few problems I found while making `Iterator` easier to const-implement.

1. More generous `~const Drop` check.

We check for nested fields with caller bounds.

For example, an ADT type with fields of types `A`, `B`, `C`, check if all of them are either:
 - Bounded (`A: ~const Drop`, `B: Copy`)
 - Known to be able to destruct at compile time (`C = i32`, `struct C(i32)`, `C = some_fn`)

2. Don't treat trait functions marked with `#[default_method_body_is_const]` as stable const fns when checking `const_for` and `const_try` feature gates.

I think anyone can review this, so no r? this time.

compiler/rustc_passes/src/check_const.rs

@@ -173,6 +173,12 @@ impl<'tcx> CheckConstVisitor<'tcx> {
                 None => return true,
             };
 
+            // If the function belongs to a trait, then it must enable the const_trait_impl
+            // feature to use that trait function (with a const default body).
+            if tcx.trait_of_item(def_id).is_some() {
+                return true;
+            }
+
             // If this crate is not using stability attributes, or this function is not claiming to be a
             // stable `const fn`, that is all that is required.
             if !tcx.features().staged_api || tcx.has_attr(def_id, sym::rustc_const_unstable) {