Auto merge of #141366 - matthiaskrgr:rollup-utvtyy3, r=matthiaskrgr

Rollup of 8 pull requests

Successful merges:

 - #140526 (docs: Specify that common sort functions sort in an ascending direction)
 - #141230 (std: fix doctest and explain for `as_slices` and `as_mut_slices` in `VecDeque`)
 - #141341 (limit impls of `VaArgSafe` to just types that are actually safe)
 - #141347 (incorrectly prefer builtin `dyn` impls :3)
 - #141351 (Move -Zcrate-attr injection to just after crate root parsing)
 - #141356 (lower bodies' params to thir before the body's value)
 - #141357 (`unpretty=thir-tree`: don't require the final expr to be the body's value)
 - #141363 (Document why we allow escaping bound vars in LTA norm)

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

15 files changed, 203 insertions, 81 deletions

diff --git a/compiler/rustc_interface/src/passes.rs b/compiler/rustc_interface/src/passes.rs
index 8f6c5b47ee2..75d92ae7a2e 100644
--- a/compiler/rustc_interface/src/passes.rs
+++ b/compiler/rustc_interface/src/passes.rs
@@ -45,7 +45,7 @@ use crate::interface::Compiler;
 use crate::{errors, limits, proc_macro_decls, util};
 
 pub fn parse<'a>(sess: &'a Session) -> ast::Crate {
-    let krate = sess
+    let mut krate = sess
         .time("parse_crate", || {
             let mut parser = unwrap_or_emit_fatal(match &sess.io.input {
                 Input::File(file) => new_parser_from_file(&sess.psess, file, None),
@@ -64,6 +64,12 @@ pub fn parse<'a>(sess: &'a Session) -> ast::Crate {
         input_stats::print_ast_stats(&krate, "PRE EXPANSION AST STATS", "ast-stats-1");
     }
 
+    rustc_builtin_macros::cmdline_attrs::inject(
+        &mut krate,
+        &sess.psess,
+        &sess.opts.unstable_opts.crate_attr,
+    );
+
     krate
 }
 
@@ -805,17 +811,11 @@ pub static DEFAULT_QUERY_PROVIDERS: LazyLock<Providers> = LazyLock::new(|| {
 
 pub fn create_and_enter_global_ctxt<T, F: for<'tcx> FnOnce(TyCtxt<'tcx>) -> T>(
     compiler: &Compiler,
-    mut krate: rustc_ast::Crate,
+    krate: rustc_ast::Crate,
     f: F,
 ) -> T {
     let sess = &compiler.sess;
 
-    rustc_builtin_macros::cmdline_attrs::inject(
-        &mut krate,
-        &sess.psess,
-        &sess.opts.unstable_opts.crate_attr,
-    );
-
     let pre_configured_attrs = rustc_expand::config::pre_configure_attrs(sess, &krate.attrs);
 
     let crate_name = get_crate_name(sess, &pre_configured_attrs);
diff --git a/compiler/rustc_mir_build/src/builder/mod.rs b/compiler/rustc_mir_build/src/builder/mod.rs
index 8400709740f..127b191e335 100644
--- a/compiler/rustc_mir_build/src/builder/mod.rs
+++ b/compiler/rustc_mir_build/src/builder/mod.rs
@@ -450,10 +450,6 @@ fn construct_fn<'tcx>(
     let span = tcx.def_span(fn_def);
     let fn_id = tcx.local_def_id_to_hir_id(fn_def);
 
-    // The representation of thir for `-Zunpretty=thir-tree` relies on
-    // the entry expression being the last element of `thir.exprs`.
-    assert_eq!(expr.as_usize(), thir.exprs.len() - 1);
-
     // Figure out what primary body this item has.
     let body = tcx.hir_body_owned_by(fn_def);
     let span_with_body = tcx.hir_span_with_body(fn_id);
diff --git a/compiler/rustc_mir_build/src/thir/cx/mod.rs b/compiler/rustc_mir_build/src/thir/cx/mod.rs
index 2f593b9a0a7..8c817605847 100644
--- a/compiler/rustc_mir_build/src/thir/cx/mod.rs
+++ b/compiler/rustc_mir_build/src/thir/cx/mod.rs
@@ -27,8 +27,8 @@ pub(crate) fn thir_body(
     if let Some(reported) = cx.typeck_results.tainted_by_errors {
         return Err(reported);
     }
-    let expr = cx.mirror_expr(body.value);
 
+    // Lower the params before the body's expression so errors from params are shown first.
     let owner_id = tcx.local_def_id_to_hir_id(owner_def);
     if let Some(fn_decl) = tcx.hir_fn_decl_by_hir_id(owner_id) {
         let closure_env_param = cx.closure_env_param(owner_def, owner_id);
@@ -48,6 +48,7 @@ pub(crate) fn thir_body(
         }
     }
 
+    let expr = cx.mirror_expr(body.value);
     Ok((tcx.alloc_steal_thir(cx.thir), expr))
 }
 
diff --git a/compiler/rustc_mir_build/src/thir/print.rs b/compiler/rustc_mir_build/src/thir/print.rs
index 37248941e2c..db9547a481f 100644
--- a/compiler/rustc_mir_build/src/thir/print.rs
+++ b/compiler/rustc_mir_build/src/thir/print.rs
@@ -8,10 +8,10 @@ use rustc_span::def_id::LocalDefId;
 /// Create a THIR tree for debugging.
 pub fn thir_tree(tcx: TyCtxt<'_>, owner_def: LocalDefId) -> String {
     match super::cx::thir_body(tcx, owner_def) {
-        Ok((thir, _)) => {
+        Ok((thir, expr)) => {
             let thir = thir.steal();
             let mut printer = ThirPrinter::new(&thir);
-            printer.print();
+            printer.print(expr);
             printer.into_buffer()
         }
         Err(_) => "error".into(),
@@ -58,7 +58,7 @@ impl<'a, 'tcx> ThirPrinter<'a, 'tcx> {
         }
     }
 
-    fn print(&mut self) {
+    fn print(&mut self, body_expr: ExprId) {
         print_indented!(self, "params: [", 0);
         for param in self.thir.params.iter() {
             self.print_param(param, 1);
@@ -66,8 +66,7 @@ impl<'a, 'tcx> ThirPrinter<'a, 'tcx> {
         print_indented!(self, "]", 0);
 
         print_indented!(self, "body:", 0);
-        let expr = ExprId::from_usize(self.thir.exprs.len() - 1);
-        self.print_expr(expr, 1);
+        self.print_expr(body_expr, 1);
     }
 
     fn into_buffer(self) -> String {
diff --git a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
index 966d5422fbb..7c4e1dc2c12 100644
--- a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
@@ -9,7 +9,7 @@ use rustc_type_ir::{
     self as ty, Interner, Movability, TraitPredicate, TypeVisitableExt as _, TypingMode,
     Upcast as _, elaborate,
 };
-use tracing::{instrument, trace};
+use tracing::{debug, instrument, trace};
 
 use crate::delegate::SolverDelegate;
 use crate::solve::assembly::structural_traits::{self, AsyncCallableRelevantTypes};
@@ -17,7 +17,7 @@ use crate::solve::assembly::{self, AllowInferenceConstraints, AssembleCandidates
 use crate::solve::inspect::ProbeKind;
 use crate::solve::{
     BuiltinImplSource, CandidateSource, Certainty, EvalCtxt, Goal, GoalSource, MaybeCause,
-    NoSolution, ParamEnvSource, QueryResult,
+    NoSolution, ParamEnvSource, QueryResult, has_only_region_constraints,
 };
 
 impl<D, I> assembly::GoalKind<D> for TraitPredicate<I>
@@ -1253,6 +1253,45 @@ where
     D: SolverDelegate<Interner = I>,
     I: Interner,
 {
+    /// FIXME(#57893): For backwards compatability with the old trait solver implementation,
+    /// we need to handle overlap between builtin and user-written impls for trait objects.
+    ///
+    /// This overlap is unsound in general and something which we intend to fix separately.
+    /// To avoid blocking the stabilization of the trait solver, we add this hack to avoid
+    /// breakage in cases which are *mostly fine*™. Importantly, this preference is strictly
+    /// weaker than the old behavior.
+    ///
+    /// We only prefer builtin over user-written impls if there are no inference constraints.
+    /// Importantly, we also only prefer the builtin impls for trait goals, and not during
+    /// normalization. This means the only case where this special-case results in exploitable
+    /// unsoundness should be lifetime dependent user-written impls.
+    pub(super) fn unsound_prefer_builtin_dyn_impl(&mut self, candidates: &mut Vec<Candidate<I>>) {
+        match self.typing_mode() {
+            TypingMode::Coherence => return,
+            TypingMode::Analysis { .. }
+            | TypingMode::Borrowck { .. }
+            | TypingMode::PostBorrowckAnalysis { .. }
+            | TypingMode::PostAnalysis => {}
+        }
+
+        if candidates
+            .iter()
+            .find(|c| {
+                matches!(c.source, CandidateSource::BuiltinImpl(BuiltinImplSource::Object(_)))
+            })
+            .is_some_and(|c| has_only_region_constraints(c.result))
+        {
+            candidates.retain(|c| {
+                if matches!(c.source, CandidateSource::Impl(_)) {
+                    debug!(?c, "unsoundly dropping impl in favor of builtin dyn-candidate");
+                    false
+                } else {
+                    true
+                }
+            });
+        }
+    }
+
     #[instrument(level = "debug", skip(self), ret)]
     pub(super) fn merge_trait_candidates(
         &mut self,
@@ -1313,6 +1352,7 @@ where
         }
 
         self.filter_specialized_impls(AllowInferenceConstraints::No, &mut candidates);
+        self.unsound_prefer_builtin_dyn_impl(&mut candidates);
 
         // If there are *only* global where bounds, then make sure to return that this
         // is still reported as being proven-via the param-env so that rigid projections
diff --git a/compiler/rustc_trait_selection/src/traits/normalize.rs b/compiler/rustc_trait_selection/src/traits/normalize.rs
index 88a0c402702..eb6d5c8a60a 100644
--- a/compiler/rustc_trait_selection/src/traits/normalize.rs
+++ b/compiler/rustc_trait_selection/src/traits/normalize.rs
@@ -299,12 +299,21 @@ impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
             );
         }
 
+        // We don't replace bound vars in the generic arguments of the free alias with
+        // placeholders. This doesn't cause any issues as instantiating parameters with
+        // bound variables is special-cased to rewrite the debruijn index to be higher
+        // whenever we fold through a binder.
+        //
+        // However, we do replace any escaping bound vars in the resulting goals with
+        // placeholders as the trait solver does not expect to encounter escaping bound
+        // vars in obligations.
+        //
+        // FIXME(lazy_type_alias): Check how much this actually matters for perf before
+        // stabilization. This is a bit weird and generally not how we handle binders in
+        // the compiler so ideally we'd do the same boundvar->placeholder->boundvar dance
+        // that other kinds of normalization do.
         let infcx = self.selcx.infcx;
         self.obligations.extend(
-            // FIXME(BoxyUwU):
-            // FIXME(lazy_type_alias):
-            // It seems suspicious to instantiate the predicates with arguments that might be bound vars,
-            // we might wind up instantiating one of these bound vars underneath a hrtb.
             infcx.tcx.predicates_of(free.def_id).instantiate_own(infcx.tcx, free.args).map(
                 |(mut predicate, span)| {
                     if free.has_escaping_bound_vars() {
diff --git a/library/alloc/src/collections/vec_deque/mod.rs b/library/alloc/src/collections/vec_deque/mod.rs
index 712f38a76c0..08b1828ff00 100644
--- a/library/alloc/src/collections/vec_deque/mod.rs
+++ b/library/alloc/src/collections/vec_deque/mod.rs
@@ -1312,6 +1312,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
     ///
     /// If [`make_contiguous`] was previously called, all elements of the
     /// deque will be in the first slice and the second slice will be empty.
+    /// Otherwise, the exact split point depends on implementation details
+    /// and is not guaranteed.
     ///
     /// [`make_contiguous`]: VecDeque::make_contiguous
     ///
@@ -1326,12 +1328,18 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// deque.push_back(1);
     /// deque.push_back(2);
     ///
-    /// assert_eq!(deque.as_slices(), (&[0, 1, 2][..], &[][..]));
+    /// let expected = [0, 1, 2];
+    /// let (front, back) = deque.as_slices();
+    /// assert_eq!(&expected[..front.len()], front);
+    /// assert_eq!(&expected[front.len()..], back);
     ///
     /// deque.push_front(10);
     /// deque.push_front(9);
     ///
-    /// assert_eq!(deque.as_slices(), (&[9, 10][..], &[0, 1, 2][..]));
+    /// let expected = [9, 10, 0, 1, 2];
+    /// let (front, back) = deque.as_slices();
+    /// assert_eq!(&expected[..front.len()], front);
+    /// assert_eq!(&expected[front.len()..], back);
     /// ```
     #[inline]
     #[stable(feature = "deque_extras_15", since = "1.5.0")]
@@ -1347,6 +1355,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
     ///
     /// If [`make_contiguous`] was previously called, all elements of the
     /// deque will be in the first slice and the second slice will be empty.
+    /// Otherwise, the exact split point depends on implementation details
+    /// and is not guaranteed.
     ///
     /// [`make_contiguous`]: VecDeque::make_contiguous
     ///
@@ -1363,9 +1373,22 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// deque.push_front(10);
     /// deque.push_front(9);
     ///
-    /// deque.as_mut_slices().0[0] = 42;
-    /// deque.as_mut_slices().1[0] = 24;
-    /// assert_eq!(deque.as_slices(), (&[42, 10][..], &[24, 1][..]));
+    /// // Since the split point is not guaranteed, we may need to update
+    /// // either slice.
+    /// let mut update_nth = |index: usize, val: u32| {
+    ///     let (front, back) = deque.as_mut_slices();
+    ///     if index > front.len() - 1 {
+    ///         back[index - front.len()] = val;
+    ///     } else {
+    ///         front[index] = val;
+    ///     }
+    /// };
+    ///
+    /// update_nth(0, 42);
+    /// update_nth(2, 24);
+    ///
+    /// let v: Vec<_> = deque.into();
+    /// assert_eq!(v, [42, 10, 24, 1]);
     /// ```
     #[inline]
     #[stable(feature = "deque_extras_15", since = "1.5.0")]
diff --git a/library/alloc/src/slice.rs b/library/alloc/src/slice.rs
index 7c5d22e1ee9..b49b3f41a76 100644
--- a/library/alloc/src/slice.rs
+++ b/library/alloc/src/slice.rs
@@ -69,7 +69,7 @@ use crate::boxed::Box;
 use crate::vec::Vec;
 
 impl<T> [T] {
-    /// Sorts the slice, preserving initial order of equal elements.
+    /// Sorts the slice in ascending order, preserving initial order of equal elements.
     ///
     /// This sort is stable (i.e., does not reorder equal elements) and *O*(*n* \* log(*n*))
     /// worst-case.
@@ -137,7 +137,8 @@ impl<T> [T] {
         stable_sort(self, T::lt);
     }
 
-    /// Sorts the slice with a comparison function, preserving initial order of equal elements.
+    /// Sorts the slice in ascending order with a comparison function, preserving initial order of
+    /// equal elements.
     ///
     /// This sort is stable (i.e., does not reorder equal elements) and *O*(*n* \* log(*n*))
     /// worst-case.
@@ -197,7 +198,8 @@ impl<T> [T] {
         stable_sort(self, |a, b| compare(a, b) == Less);
     }
 
-    /// Sorts the slice with a key extraction function, preserving initial order of equal elements.
+    /// Sorts the slice in ascending order with a key extraction function, preserving initial order
+    /// of equal elements.
     ///
     /// This sort is stable (i.e., does not reorder equal elements) and *O*(*m* \* *n* \* log(*n*))
     /// worst-case, where the key function is *O*(*m*).
@@ -252,7 +254,8 @@ impl<T> [T] {
         stable_sort(self, |a, b| f(a).lt(&f(b)));
     }
 
-    /// Sorts the slice with a key extraction function, preserving initial order of equal elements.
+    /// Sorts the slice in ascending order with a key extraction function, preserving initial order
+    /// of equal elements.
     ///
     /// This sort is stable (i.e., does not reorder equal elements) and *O*(*m* \* *n* + *n* \*
     /// log(*n*)) worst-case, where the key function is *O*(*m*).
diff --git a/library/core/src/ffi/mod.rs b/library/core/src/ffi/mod.rs
index 288d0df0d05..0bc98e2ea86 100644
--- a/library/core/src/ffi/mod.rs
+++ b/library/core/src/ffi/mod.rs
@@ -28,7 +28,7 @@ pub mod c_str;
     issue = "44930",
     reason = "the `c_variadic` feature has not been properly tested on all supported platforms"
 )]
-pub use self::va_list::{VaList, VaListImpl};
+pub use self::va_list::{VaArgSafe, VaList, VaListImpl};
 
 #[unstable(
     feature = "c_variadic",
diff --git a/library/core/src/ffi/va_list.rs b/library/core/src/ffi/va_list.rs
index 1ad8038cbf6..f12bd289f27 100644
--- a/library/core/src/ffi/va_list.rs
+++ b/library/core/src/ffi/va_list.rs
@@ -223,39 +223,57 @@ impl<'a, 'f: 'a> DerefMut for VaList<'a, 'f> {
     }
 }
 
-// The VaArgSafe trait needs to be used in public interfaces, however, the trait
-// itself must not be allowed to be used outside this module. Allowing users to
-// implement the trait for a new type (thereby allowing the va_arg intrinsic to
-// be used on a new type) is likely to cause undefined behavior.
-//
-// FIXME(dlrobertson): In order to use the VaArgSafe trait in a public interface
-// but also ensure it cannot be used elsewhere, the trait needs to be public
-// within a private module. Once RFC 2145 has been implemented look into
-// improving this.
-mod sealed_trait {
-    /// Trait which permits the allowed types to be used with [super::VaListImpl::arg].
-    pub unsafe trait VaArgSafe {}
-}
+mod sealed {
+    pub trait Sealed {}
 
-macro_rules! impl_va_arg_safe {
-    ($($t:ty),+) => {
-        $(
-            unsafe impl sealed_trait::VaArgSafe for $t {}
-        )+
-    }
+    impl Sealed for i32 {}
+    impl Sealed for i64 {}
+    impl Sealed for isize {}
+
+    impl Sealed for u32 {}
+    impl Sealed for u64 {}
+    impl Sealed for usize {}
+
+    impl Sealed for f64 {}
+
+    impl<T> Sealed for *mut T {}
+    impl<T> Sealed for *const T {}
 }
 
-impl_va_arg_safe! {i8, i16, i32, i64, usize}
-impl_va_arg_safe! {u8, u16, u32, u64, isize}
-impl_va_arg_safe! {f64}
+/// Trait which permits the allowed types to be used with [`VaListImpl::arg`].
+///
+/// # Safety
+///
+/// This trait must only be implemented for types that C passes as varargs without implicit promotion.
+///
+/// In C varargs, integers smaller than [`c_int`] and floats smaller than [`c_double`]
+/// are implicitly promoted to [`c_int`] and [`c_double`] respectively. Implementing this trait for
+/// types that are subject to this promotion rule is invalid.
+///
+/// [`c_int`]: core::ffi::c_int
+/// [`c_double`]: core::ffi::c_double
+pub unsafe trait VaArgSafe: sealed::Sealed {}
+
+// i8 and i16 are implicitly promoted to c_int in C, and cannot implement `VaArgSafe`.
+unsafe impl VaArgSafe for i32 {}
+unsafe impl VaArgSafe for i64 {}
+unsafe impl VaArgSafe for isize {}
+
+// u8 and u16 are implicitly promoted to c_int in C, and cannot implement `VaArgSafe`.
+unsafe impl VaArgSafe for u32 {}
+unsafe impl VaArgSafe for u64 {}
+unsafe impl VaArgSafe for usize {}
+
+// f32 is implicitly promoted to c_double in C, and cannot implement `VaArgSafe`.
+unsafe impl VaArgSafe for f64 {}
 
-unsafe impl<T> sealed_trait::VaArgSafe for *mut T {}
-unsafe impl<T> sealed_trait::VaArgSafe for *const T {}
+unsafe impl<T> VaArgSafe for *mut T {}
+unsafe impl<T> VaArgSafe for *const T {}
 
 impl<'f> VaListImpl<'f> {
     /// Advance to the next arg.
     #[inline]
-    pub unsafe fn arg<T: sealed_trait::VaArgSafe>(&mut self) -> T {
+    pub unsafe fn arg<T: VaArgSafe>(&mut self) -> T {
         // SAFETY: the caller must uphold the safety contract for `va_arg`.
         unsafe { va_arg(self) }
     }
@@ -317,4 +335,4 @@ unsafe fn va_copy<'f>(dest: *mut VaListImpl<'f>, src: &VaListImpl<'f>);
 /// argument `ap` points to.
 #[rustc_intrinsic]
 #[rustc_nounwind]
-unsafe fn va_arg<T: sealed_trait::VaArgSafe>(ap: &mut VaListImpl<'_>) -> T;
+unsafe fn va_arg<T: VaArgSafe>(ap: &mut VaListImpl<'_>) -> T;
diff --git a/library/core/src/slice/mod.rs b/library/core/src/slice/mod.rs
index c9b8231e856..26520123a8d 100644
--- a/library/core/src/slice/mod.rs
+++ b/library/core/src/slice/mod.rs
@@ -2986,7 +2986,7 @@ impl<T> [T] {
         self.binary_search_by(|k| f(k).cmp(b))
     }
 
-    /// Sorts the slice **without** preserving the initial order of equal elements.
+    /// Sorts the slice in ascending order **without** preserving the initial order of equal elements.
     ///
     /// This sort is unstable (i.e., may reorder equal elements), in-place (i.e., does not
     /// allocate), and *O*(*n* \* log(*n*)) worst-case.
@@ -3047,8 +3047,8 @@ impl<T> [T] {
         sort::unstable::sort(self, &mut T::lt);
     }
 
-    /// Sorts the slice with a comparison function, **without** preserving the initial order of
-    /// equal elements.
+    /// Sorts the slice in ascending order with a comparison function, **without** preserving the
+    /// initial order of equal elements.
     ///
     /// This sort is unstable (i.e., may reorder equal elements), in-place (i.e., does not
     /// allocate), and *O*(*n* \* log(*n*)) worst-case.
@@ -3102,8 +3102,8 @@ impl<T> [T] {
         sort::unstable::sort(self, &mut |a, b| compare(a, b) == Ordering::Less);
     }
 
-    /// Sorts the slice with a key extraction function, **without** preserving the initial order of
-    /// equal elements.
+    /// Sorts the slice in ascending order with a key extraction function, **without** preserving
+    /// the initial order of equal elements.
     ///
     /// This sort is unstable (i.e., may reorder equal elements), in-place (i.e., does not
     /// allocate), and *O*(*n* \* log(*n*)) worst-case.
diff --git a/library/std/src/ffi/mod.rs b/library/std/src/ffi/mod.rs
index 56791609910..024cb71b915 100644
--- a/library/std/src/ffi/mod.rs
+++ b/library/std/src/ffi/mod.rs
@@ -172,7 +172,7 @@ pub use core::ffi::c_void;
               all supported platforms",
     issue = "44930"
 )]
-pub use core::ffi::{VaList, VaListImpl};
+pub use core::ffi::{VaArgSafe, VaList, VaListImpl};
 #[stable(feature = "core_ffi_c", since = "1.64.0")]
 pub use core::ffi::{
     c_char, c_double, c_float, c_int, c_long, c_longlong, c_schar, c_short, c_uchar, c_uint,
diff --git a/tests/run-make/c-link-to-rust-va-list-fn/checkrust.rs b/tests/run-make/c-link-to-rust-va-list-fn/checkrust.rs
index bcaef33344e..7e4344f1c69 100644
--- a/tests/run-make/c-link-to-rust-va-list-fn/checkrust.rs
+++ b/tests/run-make/c-link-to-rust-va-list-fn/checkrust.rs
@@ -30,9 +30,9 @@ pub unsafe extern "C" fn check_list_0(mut ap: VaList) -> usize {
 #[no_mangle]
 pub unsafe extern "C" fn check_list_1(mut ap: VaList) -> usize {
     continue_if!(ap.arg::<c_int>() == -1);
-    continue_if!(ap.arg::<c_char>() == 'A' as c_char);
-    continue_if!(ap.arg::<c_char>() == '4' as c_char);
-    continue_if!(ap.arg::<c_char>() == ';' as c_char);
+    continue_if!(ap.arg::<c_int>() == 'A' as c_int);
+    continue_if!(ap.arg::<c_int>() == '4' as c_int);
+    continue_if!(ap.arg::<c_int>() == ';' as c_int);
     continue_if!(ap.arg::<c_int>() == 0x32);
     continue_if!(ap.arg::<c_int>() == 0x10000001);
     continue_if!(compare_c_str(ap.arg::<*const c_char>(), "Valid!"));
@@ -43,7 +43,7 @@ pub unsafe extern "C" fn check_list_1(mut ap: VaList) -> usize {
 pub unsafe extern "C" fn check_list_2(mut ap: VaList) -> usize {
     continue_if!(ap.arg::<c_double>().floor() == 3.14f64.floor());
     continue_if!(ap.arg::<c_long>() == 12);
-    continue_if!(ap.arg::<c_char>() == 'a' as c_char);
+    continue_if!(ap.arg::<c_int>() == 'a' as c_int);
     continue_if!(ap.arg::<c_double>().floor() == 6.18f64.floor());
     continue_if!(compare_c_str(ap.arg::<*const c_char>(), "Hello"));
     continue_if!(ap.arg::<c_int>() == 42);
@@ -55,7 +55,7 @@ pub unsafe extern "C" fn check_list_2(mut ap: VaList) -> usize {
 pub unsafe extern "C" fn check_list_copy_0(mut ap: VaList) -> usize {
     continue_if!(ap.arg::<c_double>().floor() == 6.28f64.floor());
     continue_if!(ap.arg::<c_int>() == 16);
-    continue_if!(ap.arg::<c_char>() == 'A' as c_char);
+    continue_if!(ap.arg::<c_int>() == 'A' as c_int);
     continue_if!(compare_c_str(ap.arg::<*const c_char>(), "Skip Me!"));
     ap.with_copy(
         |mut ap| {
@@ -75,7 +75,7 @@ pub unsafe extern "C" fn check_varargs_0(_: c_int, mut ap: ...) -> usize {
 pub unsafe extern "C" fn check_varargs_1(_: c_int, mut ap: ...) -> usize {
     continue_if!(ap.arg::<c_double>().floor() == 3.14f64.floor());
     continue_if!(ap.arg::<c_long>() == 12);
-    continue_if!(ap.arg::<c_char>() == 'A' as c_char);
+    continue_if!(ap.arg::<c_int>() == 'A' as c_int);
     continue_if!(ap.arg::<c_longlong>() == 1);
     0
 }
diff --git a/tests/ui/associated-consts/associated-const-type-parameter-pattern.stderr b/tests/ui/associated-consts/associated-const-type-parameter-pattern.stderr
index a8256f775a6..19b63a041d6 100644
--- a/tests/ui/associated-consts/associated-const-type-parameter-pattern.stderr
+++ b/tests/ui/associated-consts/associated-const-type-parameter-pattern.stderr
@@ -27,7 +27,7 @@ LL |         B::X => println!("B::X"),
    |         ^^^^ `const` depends on a generic parameter
 
 error[E0158]: constant pattern cannot depend on generic parameters
-  --> $DIR/associated-const-type-parameter-pattern.rs:30:9
+  --> $DIR/associated-const-type-parameter-pattern.rs:28:48
    |
 LL | pub trait Foo {
    | -------------
@@ -35,13 +35,12 @@ LL |     const X: EFoo;
    |     ------------- constant defined here
 ...
 LL | pub fn test_let_pat<A: Foo, B: Foo>(arg: EFoo, A::X: EFoo) {
-   |                     - constant depends on this generic parameter
-LL |
-LL |     let A::X = arg;
-   |         ^^^^ `const` depends on a generic parameter
+   |                     -                          ^^^^ `const` depends on a generic parameter
+   |                     |
+   |                     constant depends on this generic parameter
 
 error[E0158]: constant pattern cannot depend on generic parameters
-  --> $DIR/associated-const-type-parameter-pattern.rs:28:48
+  --> $DIR/associated-const-type-parameter-pattern.rs:30:9
    |
 LL | pub trait Foo {
    | -------------
@@ -49,9 +48,10 @@ LL |     const X: EFoo;
    |     ------------- constant defined here
 ...
 LL | pub fn test_let_pat<A: Foo, B: Foo>(arg: EFoo, A::X: EFoo) {
-   |                     -                          ^^^^ `const` depends on a generic parameter
-   |                     |
-   |                     constant depends on this generic parameter
+   |                     - constant depends on this generic parameter
+LL |
+LL |     let A::X = arg;
+   |         ^^^^ `const` depends on a generic parameter
 
 error: aborting due to 4 previous errors
 
diff --git a/tests/ui/traits/next-solver/assembly/better_any-backcompat.rs b/tests/ui/traits/next-solver/assembly/better_any-backcompat.rs
new file mode 100644
index 00000000000..8b153833e3a
--- /dev/null
+++ b/tests/ui/traits/next-solver/assembly/better_any-backcompat.rs
@@ -0,0 +1,33 @@
+//@ check-pass
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicit revisions)
+
+// A regression test for trait-system-refactor-initiative#183. While
+// this concrete instance is likely not practically unsound, the general
+// pattern is, see #57893.
+
+use std::any::TypeId;
+
+unsafe trait TidAble<'a>: Tid<'a> {}
+trait TidExt<'a>: Tid<'a> {
+    fn downcast_box(self: Box<Self>) {
+        loop {}
+    }
+}
+
+impl<'a, X: ?Sized + Tid<'a>> TidExt<'a> for X {}
+
+unsafe trait Tid<'a>: 'a {}
+
+unsafe impl<'a, T: ?Sized + TidAble<'a>> Tid<'a> for T {}
+
+impl<'a> dyn Tid<'a> + 'a {
+    fn downcast_any_box(self: Box<Self>) {
+        self.downcast_box();
+    }
+}
+
+unsafe impl<'a> TidAble<'a> for dyn Tid<'a> + 'a {}
+
+fn main() {}