diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index 0025dc9033c57..d72053ca98533 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -1144,13 +1144,7 @@ fn should_encode_type(tcx: TyCtxt<'_>, def_id: LocalDefId, def_kind: DefKind) ->
             let assoc_item = tcx.associated_item(def_id);
             match assoc_item.container {
                 ty::AssocItemContainer::ImplContainer => true,
-                // Always encode RPITITs, since we need to be able to project
-                // from an RPITIT associated item to an opaque when installing
-                // the default projection predicates in default trait methods
-                // with RPITITs.
-                ty::AssocItemContainer::TraitContainer => {
-                    assoc_item.defaultness(tcx).has_value() || assoc_item.is_impl_trait_in_trait()
-                }
+                ty::AssocItemContainer::TraitContainer => assoc_item.defaultness(tcx).has_value(),
             }
         }
         DefKind::TyParam => {
diff --git a/compiler/rustc_ty_utils/src/ty.rs b/compiler/rustc_ty_utils/src/ty.rs
index 2055852eda070..4c896712b1b92 100644
--- a/compiler/rustc_ty_utils/src/ty.rs
+++ b/compiler/rustc_ty_utils/src/ty.rs
@@ -129,7 +129,9 @@ fn param_env(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ParamEnv<'_> {
     // sure that this will succeed without errors anyway.
 
     if tcx.def_kind(def_id) == DefKind::AssocFn
-        && tcx.associated_item(def_id).container == ty::AssocItemContainer::TraitContainer
+        && let assoc_item = tcx.associated_item(def_id)
+        && assoc_item.container == ty::AssocItemContainer::TraitContainer
+        && assoc_item.defaultness(tcx).has_value()
     {
         let sig = tcx.fn_sig(def_id).instantiate_identity();
         // We accounted for the binder of the fn sig, so skip the binder.
diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index bf01b2082eda2..60b07485c3a8e 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -2719,7 +2719,7 @@ impl<T> Weak<T> {
     /// ```
     #[inline]
     #[stable(feature = "downgraded_weak", since = "1.10.0")]
-    #[rustc_const_unstable(feature = "const_weak_new", issue = "95091", reason = "recently added")]
+    #[rustc_const_stable(feature = "const_weak_new", since = "CURRENT_RUSTC_VERSION")]
     #[must_use]
     pub const fn new() -> Weak<T> {
         Weak {
diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index c2202f2fce553..6c701225a848f 100644
--- a/library/alloc/src/sync.rs
+++ b/library/alloc/src/sync.rs
@@ -2503,7 +2503,7 @@ impl<T> Weak<T> {
     /// ```
     #[inline]
     #[stable(feature = "downgraded_weak", since = "1.10.0")]
-    #[rustc_const_unstable(feature = "const_weak_new", issue = "95091", reason = "recently added")]
+    #[rustc_const_stable(feature = "const_weak_new", since = "CURRENT_RUSTC_VERSION")]
     #[must_use]
     pub const fn new() -> Weak<T> {
         Weak {
diff --git a/library/std/src/backtrace.rs b/library/std/src/backtrace.rs
index 7543ffadd4140..f0e199fac737c 100644
--- a/library/std/src/backtrace.rs
+++ b/library/std/src/backtrace.rs
@@ -89,12 +89,11 @@ mod tests;
 // a backtrace or actually symbolizing it.
 
 use crate::backtrace_rs::{self, BytesOrWideString};
-use crate::cell::UnsafeCell;
 use crate::env;
 use crate::ffi::c_void;
 use crate::fmt;
 use crate::sync::atomic::{AtomicUsize, Ordering::Relaxed};
-use crate::sync::Once;
+use crate::sync::LazyLock;
 use crate::sys_common::backtrace::{lock, output_filename};
 use crate::vec::Vec;
 
@@ -133,12 +132,11 @@ pub enum BacktraceStatus {
 enum Inner {
     Unsupported,
     Disabled,
-    Captured(LazilyResolvedCapture),
+    Captured(LazyLock<Capture, LazyResolve>),
 }
 
 struct Capture {
     actual_start: usize,
-    resolved: bool,
     frames: Vec<BacktraceFrame>,
 }
 
@@ -179,7 +177,7 @@ impl fmt::Debug for Backtrace {
         let capture = match &self.inner {
             Inner::Unsupported => return fmt.write_str("<unsupported>"),
             Inner::Disabled => return fmt.write_str("<disabled>"),
-            Inner::Captured(c) => c.force(),
+            Inner::Captured(c) => &**c,
         };
 
         let frames = &capture.frames[capture.actual_start..];
@@ -347,11 +345,10 @@ impl Backtrace {
         let inner = if frames.is_empty() {
             Inner::Unsupported
         } else {
-            Inner::Captured(LazilyResolvedCapture::new(Capture {
+            Inner::Captured(LazyLock::new(lazy_resolve(Capture {
                 actual_start: actual_start.unwrap_or(0),
                 frames,
-                resolved: false,
-            }))
+            })))
         };
 
         Backtrace { inner }
@@ -376,7 +373,7 @@ impl<'a> Backtrace {
     #[must_use]
     #[unstable(feature = "backtrace_frames", issue = "79676")]
     pub fn frames(&'a self) -> &'a [BacktraceFrame] {
-        if let Inner::Captured(c) = &self.inner { &c.force().frames } else { &[] }
+        if let Inner::Captured(c) = &self.inner { &c.frames } else { &[] }
     }
 }
 
@@ -386,7 +383,7 @@ impl fmt::Display for Backtrace {
         let capture = match &self.inner {
             Inner::Unsupported => return fmt.write_str("unsupported backtrace"),
             Inner::Disabled => return fmt.write_str("disabled backtrace"),
-            Inner::Captured(c) => c.force(),
+            Inner::Captured(c) => &**c,
         };
 
         let full = fmt.alternate();
@@ -430,46 +427,15 @@ impl fmt::Display for Backtrace {
     }
 }
 
-struct LazilyResolvedCapture {
-    sync: Once,
-    capture: UnsafeCell<Capture>,
-}
-
-impl LazilyResolvedCapture {
-    fn new(capture: Capture) -> Self {
-        LazilyResolvedCapture { sync: Once::new(), capture: UnsafeCell::new(capture) }
-    }
-
-    fn force(&self) -> &Capture {
-        self.sync.call_once(|| {
-            // SAFETY: This exclusive reference can't overlap with any others
-            // `Once` guarantees callers will block until this closure returns
-            // `Once` also guarantees only a single caller will enter this closure
-            unsafe { &mut *self.capture.get() }.resolve();
-        });
-
-        // SAFETY: This shared reference can't overlap with the exclusive reference above
-        unsafe { &*self.capture.get() }
-    }
-}
-
-// SAFETY: Access to the inner value is synchronized using a thread-safe `Once`
-// So long as `Capture` is `Sync`, `LazilyResolvedCapture` is too
-unsafe impl Sync for LazilyResolvedCapture where Capture: Sync {}
-
-impl Capture {
-    fn resolve(&mut self) {
-        // If we're already resolved, nothing to do!
-        if self.resolved {
-            return;
-        }
-        self.resolved = true;
+type LazyResolve = impl (FnOnce() -> Capture) + Send + Sync;
 
+fn lazy_resolve(mut capture: Capture) -> LazyResolve {
+    move || {
         // Use the global backtrace lock to synchronize this as it's a
         // requirement of the `backtrace` crate, and then actually resolve
         // everything.
         let _lock = lock();
-        for frame in self.frames.iter_mut() {
+        for frame in capture.frames.iter_mut() {
             let symbols = &mut frame.symbols;
             let frame = match &frame.frame {
                 RawFrame::Actual(frame) => frame,
@@ -490,6 +456,8 @@ impl Capture {
                 });
             }
         }
+
+        capture
     }
 }
 
diff --git a/library/std/src/backtrace/tests.rs b/library/std/src/backtrace/tests.rs
index 4dfbf88e83ebc..ef419806dccbd 100644
--- a/library/std/src/backtrace/tests.rs
+++ b/library/std/src/backtrace/tests.rs
@@ -43,9 +43,8 @@ fn generate_fake_frames() -> Vec<BacktraceFrame> {
 #[test]
 fn test_debug() {
     let backtrace = Backtrace {
-        inner: Inner::Captured(LazilyResolvedCapture::new(Capture {
+        inner: Inner::Captured(LazyLock::preinit(Capture {
             actual_start: 1,
-            resolved: true,
             frames: generate_fake_frames(),
         })),
     };
@@ -66,9 +65,8 @@ fn test_debug() {
 #[test]
 fn test_frames() {
     let backtrace = Backtrace {
-        inner: Inner::Captured(LazilyResolvedCapture::new(Capture {
+        inner: Inner::Captured(LazyLock::preinit(Capture {
             actual_start: 1,
-            resolved: true,
             frames: generate_fake_frames(),
         })),
     };
diff --git a/library/std/src/io/util.rs b/library/std/src/io/util.rs
index 202c0a9a69cec..3840ffe7eecaa 100644
--- a/library/std/src/io/util.rs
+++ b/library/std/src/io/util.rs
@@ -100,7 +100,7 @@ impl SizeHint for Empty {
     }
 }
 
-#[stable(feature = "empty_write", since = "1.64.0")]
+#[stable(feature = "empty_write", since = "CURRENT_RUSTC_VERSION")]
 impl Write for Empty {
     #[inline]
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
@@ -124,7 +124,7 @@ impl Write for Empty {
     }
 }
 
-#[stable(feature = "empty_write", since = "1.64.0")]
+#[stable(feature = "empty_write", since = "CURRENT_RUSTC_VERSION")]
 impl Write for &Empty {
     #[inline]
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index 4cd251d0ac26d..238c74229cc49 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -272,6 +272,7 @@
 #![feature(staged_api)]
 #![feature(thread_local)]
 #![feature(try_blocks)]
+#![feature(type_alias_impl_trait)]
 #![feature(utf8_chunks)]
 // tidy-alphabetical-end
 //
diff --git a/library/std/src/sync/lazy_lock.rs b/library/std/src/sync/lazy_lock.rs
index 37bdec5abcc74..1aeed40656221 100644
--- a/library/std/src/sync/lazy_lock.rs
+++ b/library/std/src/sync/lazy_lock.rs
@@ -89,6 +89,15 @@ impl<T, F: FnOnce() -> T> LazyLock<T, F> {
         LazyLock { once: Once::new(), data: UnsafeCell::new(Data { f: ManuallyDrop::new(f) }) }
     }
 
+    /// Creates a new lazy value that is already initialized.
+    #[inline]
+    #[cfg(test)]
+    pub(crate) fn preinit(value: T) -> LazyLock<T, F> {
+        let once = Once::new();
+        once.call_once(|| {});
+        LazyLock { once, data: UnsafeCell::new(Data { value: ManuallyDrop::new(value) }) }
+    }
+
     /// Consumes this `LazyLock` returning the stored value.
     ///
     /// Returns `Ok(value)` if `Lazy` is initialized and `Err(f)` otherwise.
diff --git a/tests/ui/impl-trait/in-trait/check-wf-on-non-defaulted-rpitit.rs b/tests/ui/impl-trait/in-trait/check-wf-on-non-defaulted-rpitit.rs
new file mode 100644
index 0000000000000..742537ffcc43b
--- /dev/null
+++ b/tests/ui/impl-trait/in-trait/check-wf-on-non-defaulted-rpitit.rs
@@ -0,0 +1,10 @@
+#![feature(return_position_impl_trait_in_trait)]
+
+struct Wrapper<G: Send>(G);
+
+trait Foo {
+    fn bar() -> Wrapper<impl Sized>;
+    //~^ ERROR `impl Sized` cannot be sent between threads safely
+}
+
+fn main() {}
diff --git a/tests/ui/impl-trait/in-trait/check-wf-on-non-defaulted-rpitit.stderr b/tests/ui/impl-trait/in-trait/check-wf-on-non-defaulted-rpitit.stderr
new file mode 100644
index 0000000000000..dee87d0823864
--- /dev/null
+++ b/tests/ui/impl-trait/in-trait/check-wf-on-non-defaulted-rpitit.stderr
@@ -0,0 +1,16 @@
+error[E0277]: `impl Sized` cannot be sent between threads safely
+  --> $DIR/check-wf-on-non-defaulted-rpitit.rs:6:17
+   |
+LL |     fn bar() -> Wrapper<impl Sized>;
+   |                 ^^^^^^^^^^^^^^^^^^^ `impl Sized` cannot be sent between threads safely
+   |
+   = help: the trait `Send` is not implemented for `impl Sized`
+note: required by a bound in `Wrapper`
+  --> $DIR/check-wf-on-non-defaulted-rpitit.rs:3:19
+   |
+LL | struct Wrapper<G: Send>(G);
+   |                   ^^^^ required by this bound in `Wrapper`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.