Rollup merge of #90417 - lcnr:stabilize-relaxed-struct-unsizing, r=wesleywiser

stabilize `relaxed_struct_unsize`

closes #81793

the fcp is already complete.

Author: matthiaskrgr

Diff for: compiler/rustc_feature/src/accepted.rs

@@ -297,6 +297,8 @@ declare_features! (
     (accepted, macro_attributes_in_derive_output, "1.57.0", Some(81119), None),
     /// Allows panicking during const eval (producing compile-time errors).
     (accepted, const_panic, "1.57.0", Some(51999), None),
+    /// Lessens the requirements for structs to implement `Unsize`.
+    (accepted, relaxed_struct_unsize, "1.58.0", Some(81793), None),
 
     // -------------------------------------------------------------------------
     // feature-group-end: accepted features

Diff for: compiler/rustc_feature/src/active.rs

@@ -589,9 +589,6 @@ declare_features! (
     /// Allows `extern "C-cmse-nonsecure-call" fn()`.
     (active, abi_c_cmse_nonsecure_call, "1.51.0", Some(81391), None),
 
-    /// Lessens the requirements for structs to implement `Unsize`.
-    (active, relaxed_struct_unsize, "1.51.0", Some(81793), None),
-
     /// Allows associated types in inherent impls.
     (incomplete, inherent_associated_types, "1.52.0", Some(8995), None),
 

Diff for: compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -948,52 +948,24 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let tail_field_ty = tcx.type_of(tail_field.did);
 
                 let mut unsizing_params = GrowableBitSet::new_empty();
-                if tcx.features().relaxed_struct_unsize {
-                    for arg in tail_field_ty.walk(tcx) {
-                        if let Some(i) = maybe_unsizing_param_idx(arg) {
-                            unsizing_params.insert(i);
-                        }
-                    }
-
-                    // Ensure none of the other fields mention the parameters used
-                    // in unsizing.
-                    for field in prefix_fields {
-                        for arg in tcx.type_of(field.did).walk(tcx) {
-                            if let Some(i) = maybe_unsizing_param_idx(arg) {
-                                unsizing_params.remove(i);
-                            }
-                        }
+                for arg in tail_field_ty.walk(tcx) {
+                    if let Some(i) = maybe_unsizing_param_idx(arg) {
+                        unsizing_params.insert(i);
                     }
+                }
 
-                    if unsizing_params.is_empty() {
-                        return Err(Unimplemented);
-                    }
-                } else {
-                    let mut found = false;
-                    for arg in tail_field_ty.walk(tcx) {
+                // Ensure none of the other fields mention the parameters used
+                // in unsizing.
+                for field in prefix_fields {
+                    for arg in tcx.type_of(field.did).walk(tcx) {
                         if let Some(i) = maybe_unsizing_param_idx(arg) {
-                            unsizing_params.insert(i);
-                            found = true;
+                            unsizing_params.remove(i);
                         }
                     }
-                    if !found {
-                        return Err(Unimplemented);
-                    }
+                }
 
-                    // Ensure none of the other fields mention the parameters used
-                    // in unsizing.
-                    // FIXME(eddyb) cache this (including computing `unsizing_params`)
-                    // by putting it in a query; it would only need the `DefId` as it
-                    // looks at declared field types, not anything substituted.
-                    for field in prefix_fields {
-                        for arg in tcx.type_of(field.did).walk(tcx) {
-                            if let Some(i) = maybe_unsizing_param_idx(arg) {
-                                if unsizing_params.contains(i) {
-                                    return Err(Unimplemented);
-                                }
-                            }
-                        }
-                    }
+                if unsizing_params.is_empty() {
+                    return Err(Unimplemented);
                 }
 
                 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`.

Diff for: src/test/ui/feature-gates/feature-gate-relaxed_struct_unsize.rs

@@ -1,4 +1,3 @@
-#![feature(relaxed_struct_unsize)]
 // run-pass
 // Test that we allow unsizing even if there is an unchanged param in the
 // field getting unsized.