Auto merge of #106209 - fee1-dead-contrib:rollup-47ysdcu, r=fee1-dead

Rollup of 9 pull requests

Successful merges:

 - #94145 (Test leaking of BinaryHeap Drain iterators)
 - #103945 (Remove `iter::Empty` hack)
 - #104024 (Fix `unused_must_use` warning for `Box::from_raw`)
 - #104708 (Fix backoff doc to match implementation)
 - #105347 (Account for `match` expr in single line)
 - #105484 (Implement allow-by-default `multiple_supertrait_upcastable` lint)
 - #106184 (Fix `core::any` docs)
 - #106201 (Emit fewer errors on invalid `#[repr(transparent)]` on `enum`)
 - #106205 (Remove some totally duplicated files in `rustc_infer`)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

compiler/rustc_feature/src/active.rs

@@ -160,6 +160,8 @@ declare_features! (
     (active, intrinsics, "1.0.0", None, None),
     /// Allows using `#[lang = ".."]` attribute for linking items to special compiler logic.
     (active, lang_items, "1.0.0", None, None),
+    /// Allows the `multiple_supertrait_upcastable` lint.
+    (active, multiple_supertrait_upcastable, "CURRENT_RUSTC_VERSION", None, None),
     /// Allows using `#[omit_gdb_pretty_printer_section]`.
     (active, omit_gdb_pretty_printer_section, "1.5.0", None, None),
     /// Allows using `#[prelude_import]` on glob `use` items.

compiler/rustc_hir_analysis/src/check/check.rs

@@ -1060,10 +1060,8 @@ pub(super) fn check_transparent<'tcx>(tcx: TyCtxt<'tcx>, adt: ty::AdtDef<'tcx>)
 
     if adt.variants().len() != 1 {
         bad_variant_count(tcx, adt, tcx.def_span(adt.did()), adt.did());
-        if adt.variants().is_empty() {
-            // Don't bother checking the fields. No variants (and thus no fields) exist.
-            return;
-        }
+        // Don't bother checking the fields.
+        return;
     }
 
     // For each field, figure out if it's known to be a ZST and align(1), with "known"

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -729,15 +729,15 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                             format!("this and all prior arms are found to be of type `{}`", t),
                         );
                     }
-                    let outer_error_span = if any_multiline_arm {
+                    let outer = if any_multiline_arm || !source_map.is_multiline(cause.span) {
                         // Cover just `match` and the scrutinee expression, not
                         // the entire match body, to reduce diagram noise.
                         cause.span.shrink_to_lo().to(scrut_span)
                     } else {
                         cause.span
                     };
                     let msg = "`match` arms have incompatible types";
-                    err.span_label(outer_error_span, msg);
+                    err.span_label(outer, msg);
                     self.suggest_remove_semi_or_return_binding(
                         err,
                         prior_arm_block_id,