Rollup merge of #80726 - lcnr:unsize-query, r=oli-obk

relax adt unsizing requirements

Changes unsizing of structs in case the last struct field shares generic params with other adt fields which do not change.
This change is currently insta stable and changes the language, so it at least requires a lang fcp. I feel like the current state is fairly unintuitive.

An example for what's now allowed would be https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=6dd331d23f5c9ffc8c978175aae2e967
```rust
struct A<T, U: ?Sized>(T, B<T, U>); // previously ERR
// struct A<T, U: ?Sized>(T, B<[u32; 1], U>); // ok
struct B<T, U: ?Sized>(T, U);

fn main() {
    let x = A([0; 1], B([0; 1], [0; 1]));
    let y: &A<[u32; 1], [u32]> = &x;
    assert_eq!(y.1.1.len(), 1);
}
```

4 files changed, 60 insertions, 18 deletions

diff --git a/compiler/rustc_feature/src/active.rs b/compiler/rustc_feature/src/active.rs
index 818f6c70de0..4f38e060023 100644
--- a/compiler/rustc_feature/src/active.rs
+++ b/compiler/rustc_feature/src/active.rs
@@ -631,6 +631,9 @@ 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(1), None),
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
diff --git a/compiler/rustc_index/src/bit_set.rs b/compiler/rustc_index/src/bit_set.rs
index 0b501da7cd9..100824f4b94 100644
--- a/compiler/rustc_index/src/bit_set.rs
+++ b/compiler/rustc_index/src/bit_set.rs
@@ -707,6 +707,18 @@ impl<T: Idx> GrowableBitSet<T> {
         self.bit_set.insert(elem)
     }
 
+    /// Returns `true` if the set has changed.
+    #[inline]
+    pub fn remove(&mut self, elem: T) -> bool {
+        self.ensure(elem.index() + 1);
+        self.bit_set.remove(elem)
+    }
+
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.bit_set.is_empty()
+    }
+
     #[inline]
     pub fn contains(&self, elem: T) -> bool {
         let (word_index, mask) = word_index_and_mask(elem);
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index df23b4006b3..86f8061a24a 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -907,6 +907,7 @@ symbols! {
         register_attr,
         register_tool,
         relaxed_adts,
+        relaxed_struct_unsize,
         rem,
         rem_assign,
         repr,
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 3015188fd44..ed3e117fcfa 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -823,33 +823,59 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     },
                 };
 
+                // 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.
+
                 // The last field of the structure has to exist and contain type/const parameters.
                 let (tail_field, prefix_fields) =
                     def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?;
                 let tail_field_ty = tcx.type_of(tail_field.did);
 
                 let mut unsizing_params = GrowableBitSet::new_empty();
-                let mut found = false;
-                for arg in tail_field_ty.walk() {
-                    if let Some(i) = maybe_unsizing_param_idx(arg) {
-                        unsizing_params.insert(i);
-                        found = true;
+                if tcx.features().relaxed_struct_unsize {
+                    for arg in tail_field_ty.walk() {
+                        if let Some(i) = maybe_unsizing_param_idx(arg) {
+                            unsizing_params.insert(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() {
+                    // 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() {
+                            if let Some(i) = maybe_unsizing_param_idx(arg) {
+                                unsizing_params.remove(i);
+                            }
+                        }
+                    }
+
+                    if unsizing_params.is_empty() {
+                        return Err(Unimplemented);
+                    }
+                } else {
+                    let mut found = false;
+                    for arg in tail_field_ty.walk() {
                         if let Some(i) = maybe_unsizing_param_idx(arg) {
-                            if unsizing_params.contains(i) {
-                                return Err(Unimplemented);
+                            unsizing_params.insert(i);
+                            found = true;
+                        }
+                    }
+                    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() {
+                            if let Some(i) = maybe_unsizing_param_idx(arg) {
+                                if unsizing_params.contains(i) {
+                                    return Err(Unimplemented);
+                                }
                             }
                         }
                     }