diff --git a/Cargo.lock b/Cargo.lock
index 0d3e419..49f2604 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -391,6 +391,7 @@ dependencies = [
  "rustc-hash",
  "thin-vec",
  "trybuild",
+ "typeid",
 ]
 
 [[package]]
@@ -699,6 +700,12 @@ dependencies = [
  "toml",
 ]
 
+[[package]]
+name = "typeid"
+version = "1.0.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bc7d623258602320d5c55d1bc22793b57daff0ec7efc270ea7d55ce1d5f5471c"
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.22"
diff --git a/oscars/Cargo.toml b/oscars/Cargo.toml
index 491bc64..35f9455 100644
--- a/oscars/Cargo.toml
+++ b/oscars/Cargo.toml
@@ -12,6 +12,7 @@ thin-vec = { version = "0.2", optional = true }
 icu_locale_core = { version = "2.2.0", default-features = false, optional = true }
 either = { version = "1.16.0", optional = true }
 arrayvec = { version = "0.7.6", optional = true }
+typeid = "1.0.3"
 
 [dev-dependencies]
 criterion = { version = "0.5", features = ["html_reports"] }
diff --git a/oscars/src/collectors/mark_sweep_branded/gc.rs b/oscars/src/collectors/mark_sweep_branded/gc.rs
index 683c6c2..df5f18c 100644
--- a/oscars/src/collectors/mark_sweep_branded/gc.rs
+++ b/oscars/src/collectors/mark_sweep_branded/gc.rs
@@ -10,6 +10,7 @@ use crate::{
 use core::fmt;
 use core::marker::PhantomData;
 use core::ops::Deref;
+use typeid;
 
 /// A transient pointer to a GC-managed value.
 #[derive(Debug)]
@@ -89,10 +90,15 @@ impl<'gc, T: Trace + ?Sized + 'gc> Gc<'gc, T> {
         }
     }
 
+    /// Returns `true` if the inner value is of type `U`.
+    ///
+    /// Uses `typeid::of::<U>()`, sound even when `U` carries a branded
+    /// lifetime because it properly handles branded lifetimes. This avoids the `T: 'static` restriction while still
+    /// giving us a stable, unique identity guarantee.
     #[inline]
-    pub fn is<U: Trace + ?Sized + 'gc>(&self) -> bool {
-        let actual_type_name = unsafe { (*self.ptr.as_ptr().as_ptr()).0.type_name };
-        actual_type_name == core::any::type_name::<U>()
+    pub fn is<U: Trace + ?Sized>(&self) -> bool {
+        let actual_type_id = unsafe { (*self.ptr.as_ptr().as_ptr()).0.type_id };
+        actual_type_id == typeid::of::<U>()
     }
 
     #[inline]
@@ -149,7 +155,7 @@ impl<'gc, T: Trace + ?Sized + 'gc> Deref for Gc<'gc, T> {
 }
 
 impl<T: Trace + ?Sized> Finalize for Gc<'_, T> {}
-unsafe impl<T: Trace + ?Sized> Trace for Gc<'_, T> {
+unsafe impl<'gc, T: Trace + ?Sized + 'gc> Trace for Gc<'gc, T> {
     unsafe fn trace(&self, tracer: &mut crate::collectors::mark_sweep_branded::trace::Tracer) {
         tracer.mark(self);
     }
diff --git a/oscars/src/collectors/mark_sweep_branded/gc_box.rs b/oscars/src/collectors/mark_sweep_branded/gc_box.rs
index 833af15..8a2a0d2 100644
--- a/oscars/src/collectors/mark_sweep_branded/gc_box.rs
+++ b/oscars/src/collectors/mark_sweep_branded/gc_box.rs
@@ -1,5 +1,6 @@
 //! The heap header wrapping every GC-managed value.
 
+use core::any::TypeId;
 use core::cell::Cell;
 use core::ptr::NonNull;
 
@@ -32,8 +33,12 @@ pub struct GcBox<T: ?Sized> {
     pub(crate) drop_fn: DropFn,
     /// Allocation ID used to validate weak pointers.
     pub(crate) alloc_id: usize,
-    /// Type name of the underlying value
-    pub(crate) type_name: &'static str,
+    /// Unique identifier for the concrete type `T`.
+    ///
+    /// Stored as `typeid::of::<T>()`. This safely erases branded lifetimes
+    /// (eg. `'gc`) without requiring `T: 'static`, giving us a stable
+    /// unique identity guarantee for sound downcasting.
+    pub(crate) type_id: TypeId,
     /// The user value.
     pub(crate) value: T,
 }
@@ -42,15 +47,17 @@ impl<T: ?Sized> GcBox<T> {
     pub(crate) const FREED_ALLOC_ID: usize = usize::MAX;
 }
 
-impl<T> GcBox<T> {
-    /// Create a [`GcBox`] for `value`, `color` starts as [`GcColor::White`]
+impl<T: Trace> GcBox<T> {
+    /// Create a [`GcBox`] for `value`, `color` starts as [`GcColor::White`].
+    ///
+    /// Requires `T: Trace` for the `TypeId`.
     pub(crate) fn new(value: T, trace_fn: TraceFn, drop_fn: DropFn, alloc_id: usize) -> Self {
         Self {
             color: Cell::new(GcColor::White),
             trace_fn,
             drop_fn,
             alloc_id,
-            type_name: core::any::type_name::<T>(),
+            type_id: typeid::of::<T>(),
             value,
         }
     }
diff --git a/oscars/src/collectors/mark_sweep_branded/mod.rs b/oscars/src/collectors/mark_sweep_branded/mod.rs
index 21f757d..3c026ed 100644
--- a/oscars/src/collectors/mark_sweep_branded/mod.rs
+++ b/oscars/src/collectors/mark_sweep_branded/mod.rs
@@ -47,6 +47,12 @@ pub struct Collector {
     pub(crate) ephemerons: RefCell<Vec<EphemeronEntry>>,
 }
 
+impl Default for Collector {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl Collector {
     pub fn new() -> Self {
         Self {
diff --git a/oscars/src/collectors/mark_sweep_branded/trace.rs b/oscars/src/collectors/mark_sweep_branded/trace.rs
index e48b16b..34dffac 100644
--- a/oscars/src/collectors/mark_sweep_branded/trace.rs
+++ b/oscars/src/collectors/mark_sweep_branded/trace.rs
@@ -126,7 +126,7 @@ impl<'a> Tracer<'a> {
     }
 }
 
-unsafe impl<T: ?Sized> Trace for &T {
+unsafe impl<T: Trace + ?Sized> Trace for &T {
     #[inline]
     unsafe fn trace(&self, _tracer: &mut Tracer) {}
 }
@@ -149,7 +149,6 @@ empty_trace![
     bool,
     isize,
     usize,
-    str,
     i8,
     u8,
     i16,
@@ -180,6 +179,12 @@ empty_trace![
     core::sync::atomic::AtomicUsize,
 ];
 
+// str is a DST; we cannot allocate it directly. Use String as the Sized proxy.
+unsafe impl Trace for str {
+    #[inline]
+    unsafe fn trace(&self, _tracer: &mut Tracer) {}
+}
+
 unsafe impl<T: Trace, const N: usize> Trace for [T; N] {
     unsafe fn trace(&self, tracer: &mut Tracer) {
         for v in self.iter() {
@@ -188,6 +193,17 @@ unsafe impl<T: Trace, const N: usize> Trace for [T; N] {
     }
 }
 
+// Slices [T] cannot be allocated directly in the GC.
+unsafe impl<T: Trace> Trace for [T] {
+    #[inline]
+    unsafe fn trace(&self, tracer: &mut Tracer) {
+        for v in self {
+            v.trace(tracer);
+        }
+    }
+}
+
+// Box<T> where T: ?Sized. Box is always Sized even for DST contents.
 unsafe impl<T: Trace + ?Sized> Trace for Box<T> {
     unsafe fn trace(&self, tracer: &mut Tracer) {
         (**self).trace(tracer);
@@ -244,20 +260,12 @@ unsafe impl<T: Trace> Trace for LinkedList<T> {
     }
 }
 
-unsafe impl<T> Trace for PhantomData<T> {
+// PhantomData<T> doesn't trace T, so T need not implement Trace.
+unsafe impl<T: 'static> Trace for PhantomData<T> {
     #[inline]
     unsafe fn trace(&self, _tracer: &mut Tracer) {}
 }
 
-unsafe impl<T: Trace> Trace for [T] {
-    #[inline]
-    unsafe fn trace(&self, tracer: &mut Tracer) {
-        for v in self {
-            v.trace(tracer);
-        }
-    }
-}
-
 unsafe impl Trace for core::any::TypeId {
     #[inline]
     unsafe fn trace(&self, _tracer: &mut Tracer) {}
@@ -281,10 +289,11 @@ unsafe impl<T: Trace> Trace for OnceCell<T> {
     }
 }
 
-unsafe impl<T: ToOwned + Trace + ?Sized> Trace for Cow<'static, T>
+unsafe impl<T: ToOwned + Trace + ?Sized + 'static> Trace for Cow<'static, T>
 where
     T::Owned: Trace,
 {
+    // T is already 'static so we can use it directly as the proxy.
     unsafe fn trace(&self, tracer: &mut Tracer) {
         if let Cow::Owned(v) = self {
             v.trace(tracer);
@@ -380,17 +389,20 @@ unsafe impl<A: Trace, B: Trace, C: Trace, D: Trace, E: Trace, F: Trace, G: Trace
 // Rc and Arc do not contain Gc pointers (they use reference counting, not GC).
 // If you need to store Gc pointers inside Rc/Arc, wrap them in a GC-allocated
 // struct instead.
-unsafe impl<T: ?Sized> Trace for rust_alloc::rc::Rc<T> {
+// Rc/Arc are reference-counted, not GC-traced. They cannot contain live Gc
+// pointers (that would create a cycle the GC cannot see).
+unsafe impl<T: ?Sized + 'static> Trace for rust_alloc::rc::Rc<T> {
     #[inline]
     unsafe fn trace(&self, _tracer: &mut Tracer) {}
 }
 
-unsafe impl<T: ?Sized> Trace for rust_alloc::sync::Arc<T> {
+unsafe impl<T: ?Sized + 'static> Trace for rust_alloc::sync::Arc<T> {
     #[inline]
     unsafe fn trace(&self, _tracer: &mut Tracer) {}
 }
 
-unsafe impl<K, V: Trace> Trace for BTreeMap<K, V> {
+// K is not traced (BTreeMap keys are immutable).
+unsafe impl<K: 'static, V: Trace> Trace for BTreeMap<K, V> {
     unsafe fn trace(&self, tracer: &mut Tracer) {
         for v in self.values() {
             v.trace(tracer);
@@ -398,7 +410,8 @@ unsafe impl<K, V: Trace> Trace for BTreeMap<K, V> {
     }
 }
 
-unsafe impl<T> Trace for BTreeSet<T> {
+// BTreeSet keys are never traced.
+unsafe impl<T: 'static> Trace for BTreeSet<T> {
     #[inline]
     unsafe fn trace(&self, _tracer: &mut Tracer) {
         // BTreeSet keys are immutable and cannot contain Gc pointers
@@ -470,7 +483,7 @@ unsafe impl Trace for std::time::SystemTime {
 }
 
 #[cfg(feature = "std")]
-unsafe impl<K: Trace, V: Trace, S> Trace for std::collections::HashMap<K, V, S> {
+unsafe impl<K: Trace, V: Trace, S: 'static> Trace for std::collections::HashMap<K, V, S> {
     #[inline]
     unsafe fn trace(&self, tracer: &mut Tracer) {
         for (k, v) in self {
@@ -481,7 +494,7 @@ unsafe impl<K: Trace, V: Trace, S> Trace for std::collections::HashMap<K, V, S>
 }
 
 #[cfg(feature = "std")]
-unsafe impl<T: Trace, S> Trace for std::collections::HashSet<T, S> {
+unsafe impl<T: Trace, S: 'static> Trace for std::collections::HashSet<T, S> {
     #[inline]
     unsafe fn trace(&self, tracer: &mut Tracer) {
         for v in self {
diff --git a/oscars/src/collectors/mark_sweep_branded/weak.rs b/oscars/src/collectors/mark_sweep_branded/weak.rs
index 21564d9..9ff658e 100644
--- a/oscars/src/collectors/mark_sweep_branded/weak.rs
+++ b/oscars/src/collectors/mark_sweep_branded/weak.rs
@@ -63,8 +63,7 @@ impl<'id, T: Trace + ?Sized> WeakGc<'id, T> {
 
     /// Returns `true` if the referenced value is still alive.
     pub fn is_upgradable(&self) -> bool {
-        let is_valid = unsafe { (*self.ptr.as_ptr().as_ptr()).0.alloc_id == self.alloc_id };
-        is_valid
+        unsafe { (*self.ptr.as_ptr().as_ptr()).0.alloc_id == self.alloc_id }
     }
 }
 
diff --git a/oscars/src/collectors/null_collector_branded/cell.rs b/oscars/src/collectors/null_collector_branded/cell.rs
index ca36252..6040ab6 100644
--- a/oscars/src/collectors/null_collector_branded/cell.rs
+++ b/oscars/src/collectors/null_collector_branded/cell.rs
@@ -72,10 +72,14 @@ impl<T: Trace + ?Sized> DerefMut for GcRefMut<'_, T> {
     }
 }
 
-impl<'a, T: Trace + ?Sized> GcRef<'a, T> {
-    pub fn clone(orig: &GcRef<'a, T>) -> GcRef<'a, T> {
-        GcRef(Ref::clone(&orig.0))
+impl<'a, T: Trace + ?Sized> Clone for GcRef<'a, T> {
+    #[inline]
+    fn clone(&self) -> Self {
+        GcRef(Ref::clone(&self.0))
     }
+}
+
+impl<'a, T: Trace + ?Sized> GcRef<'a, T> {
     pub fn map<U: Trace + ?Sized, F>(orig: GcRef<'a, T>, f: F) -> GcRef<'a, U>
     where
         F: FnOnce(&T) -> &U,
@@ -130,6 +134,7 @@ impl<'a, T: Trace + ?Sized> GcRefMut<'a, T> {
 impl<T: Trace + ?Sized> Finalize for GcRefCell<T> {}
 
 unsafe impl<T: Trace + ?Sized> Trace for GcRefCell<T> {
+    // GcRefCell<'gc, T> is branded by T's lifetime. Map to the static form.
     #[inline]
     unsafe fn trace(&self, tracer: &mut Tracer) {
         // SAFETY: We only access the inner value for tracing and do not mutate it.
diff --git a/oscars/src/collectors/null_collector_branded/gc.rs b/oscars/src/collectors/null_collector_branded/gc.rs
index 5e8667c..1d93714 100644
--- a/oscars/src/collectors/null_collector_branded/gc.rs
+++ b/oscars/src/collectors/null_collector_branded/gc.rs
@@ -80,10 +80,15 @@ impl<'gc, T: Trace + ?Sized + 'gc> Gc<'gc, T> {
         }
     }
 
+    /// Returns `true` if the inner value is of type `U`.
+    ///
+    /// Uses `typeid::of::<U>()`, sound even when `U` carries a branded
+    /// lifetime because it properly handles branded lifetimes. This avoids the `T: 'static` restriction while still
+    /// giving us a stable, unique identity guarantee
     #[inline]
-    pub fn is<U: Trace + ?Sized + 'gc>(&self) -> bool {
-        let actual_type_name = unsafe { (*self.ptr.as_ptr().as_ptr()).0.type_name };
-        actual_type_name == core::any::type_name::<U>()
+    pub fn is<U: Trace + ?Sized>(&self) -> bool {
+        let actual_type_id = unsafe { (*self.ptr.as_ptr().as_ptr()).0.type_id };
+        actual_type_id == typeid::of::<U>()
     }
 
     #[inline]
@@ -141,7 +146,7 @@ impl<'gc, T: Trace + ?Sized + 'gc> Deref for Gc<'gc, T> {
 
 impl<T: Trace + ?Sized> Finalize for Gc<'_, T> {}
 
-unsafe impl<T: Trace + ?Sized> Trace for Gc<'_, T> {
+unsafe impl<'gc, T: Trace + ?Sized + 'gc> Trace for Gc<'gc, T> {
     unsafe fn trace(&self, tracer: &mut crate::collectors::null_collector_branded::trace::Tracer) {
         tracer.mark(self);
     }
diff --git a/oscars/src/collectors/null_collector_branded/gc_box.rs b/oscars/src/collectors/null_collector_branded/gc_box.rs
index 2514b95..933ba1c 100644
--- a/oscars/src/collectors/null_collector_branded/gc_box.rs
+++ b/oscars/src/collectors/null_collector_branded/gc_box.rs
@@ -1,6 +1,8 @@
+use core::any::TypeId;
 use core::ptr::NonNull;
 
 use crate::alloc::mempool3::PoolAllocator;
+use crate::collectors::null_collector_branded::trace::Trace;
 
 pub type DropFn = unsafe fn(&mut PoolAllocator<'static>, NonNull<u8>);
 
@@ -10,18 +12,24 @@ pub type DropFn = unsafe fn(&mut PoolAllocator<'static>, NonNull<u8>);
 pub struct GcBox<T: ?Sized> {
     /// Type erased finalize and free fn
     pub(crate) drop_fn: DropFn,
-    /// Type name of the underlying value
-    pub(crate) type_name: &'static str,
+    /// Unique identifier for the concrete type `T`.
+    ///
+    /// Stored as `typeid::of::<T>()`. This safely erases branded lifetimes
+    /// (eg. `'gc`) without requiring `T: 'static`, giving us a stable
+    /// unique identity guarantee for sound downcasting.
+    pub(crate) type_id: TypeId,
     /// User value
     pub(crate) value: T,
 }
 
-impl<T> GcBox<T> {
-    /// Create a [`GcBox`] for `value`
+impl<T: Trace> GcBox<T> {
+    /// Create a [`GcBox`] for `value`.
+    ///
+    /// Requires `T: Trace` for the `TypeId`.
     pub(crate) fn new(value: T, drop_fn: DropFn) -> Self {
         Self {
             drop_fn,
-            type_name: core::any::type_name::<T>(),
+            type_id: typeid::of::<T>(),
             value,
         }
     }
diff --git a/oscars/src/collectors/null_collector_branded/mutation_ctx.rs b/oscars/src/collectors/null_collector_branded/mutation_ctx.rs
index c1bd788..c07209a 100644
--- a/oscars/src/collectors/null_collector_branded/mutation_ctx.rs
+++ b/oscars/src/collectors/null_collector_branded/mutation_ctx.rs
@@ -23,12 +23,24 @@ impl<'id, 'gc> MutationContext<'id, 'gc> {
     /// **Note**: This is a temporary workaround to keep `boa_engine` working.
     /// It breaks the normal safety rules of the collector, and should only be
     /// used to support older code that relies on `Default`
+    ///
+    /// # Safety
+    ///
+    /// `Gc<'gc, T>` and `MutationContext` are both `!Send`, so neither can escape
+    /// the thread that created them. `Collector::drop` only runs at thread exit,
+    /// after which no `Gc` on this thread can be accessed. The raw pointer reborrow
+    /// below is therefore sound ,the reference cannot outlive the TLS slot.
     #[cfg(feature = "std")]
     pub fn global() -> Self {
         std::thread_local! {
             static COLLECTOR: crate::collectors::null_collector_branded::Collector = crate::collectors::null_collector_branded::Collector::new();
         }
         COLLECTOR.with(|c| {
+            // SAFETY: `Gc` and `MutationContext` are `!Send`, so they cannot escape
+            // this thread. `COLLECTOR` is a thread-local whose destructor only runs
+            // at thread exit, after all thread-local values are inaccessible.
+            // Therefore, `c` remains valid for at least as long as any `MutationContext`
+            // or `Gc` that could possibly reference it.
             let ptr = c as *const crate::collectors::null_collector_branded::Collector;
             Self {
                 collector: unsafe { &*ptr },
diff --git a/oscars/src/collectors/null_collector_branded/trace.rs b/oscars/src/collectors/null_collector_branded/trace.rs
index c16c1f5..567020c 100644
--- a/oscars/src/collectors/null_collector_branded/trace.rs
+++ b/oscars/src/collectors/null_collector_branded/trace.rs
@@ -57,7 +57,7 @@ impl<'a> Tracer<'a> {
     }
 }
 
-unsafe impl<T: ?Sized> Trace for &T {
+unsafe impl<T: Trace + ?Sized> Trace for &T {
     #[inline]
     unsafe fn trace(&self, _tracer: &mut Tracer) {}
 }
@@ -116,6 +116,7 @@ unsafe impl<T: Trace, const N: usize> Trace for [T; N] {
     }
 }
 
+// Slices [T] cannot be allocated directly in the GC.
 unsafe impl<T: Trace> Trace for [T] {
     unsafe fn trace(&self, tracer: &mut Tracer) {
         for v in self.iter() {
@@ -124,6 +125,7 @@ unsafe impl<T: Trace> Trace for [T] {
     }
 }
 
+// Box<T> where T: ?Sized. Box is always Sized even for DST contents.
 unsafe impl<T: Trace + ?Sized> Trace for Box<T> {
     unsafe fn trace(&self, tracer: &mut Tracer) {
         (**self).trace(tracer);
@@ -180,7 +182,8 @@ unsafe impl<T: Trace> Trace for LinkedList<T> {
     }
 }
 
-unsafe impl<T> Trace for PhantomData<T> {
+// PhantomData<T> doesn't trace T, so T need not implement Trace.
+unsafe impl<T: 'static> Trace for PhantomData<T> {
     #[inline]
     unsafe fn trace(&self, _tracer: &mut Tracer) {}
 }
@@ -201,10 +204,11 @@ unsafe impl<T: Trace> Trace for OnceCell<T> {
     }
 }
 
-unsafe impl<T: ToOwned + Trace + ?Sized> Trace for Cow<'static, T>
+unsafe impl<T: ToOwned + Trace + ?Sized + 'static> Trace for Cow<'static, T>
 where
     T::Owned: Trace,
 {
+    // T is already 'static so we can use it directly as the proxy.
     unsafe fn trace(&self, tracer: &mut Tracer) {
         if let Cow::Owned(v) = self {
             v.trace(tracer);
@@ -257,17 +261,20 @@ unsafe impl<A: Trace, B: Trace, C: Trace, D: Trace, E: Trace> Trace for (A, B, C
     }
 }
 
-unsafe impl<T: ?Sized> Trace for rust_alloc::rc::Rc<T> {
+// Rc/Arc are reference-counted, not GC-traced. They cannot contain live Gc
+// pointers (that would create a cycle the GC cannot see).
+unsafe impl<T: ?Sized + 'static> Trace for rust_alloc::rc::Rc<T> {
     #[inline]
     unsafe fn trace(&self, _tracer: &mut Tracer) {}
 }
 
-unsafe impl<T: ?Sized> Trace for rust_alloc::sync::Arc<T> {
+unsafe impl<T: ?Sized + 'static> Trace for rust_alloc::sync::Arc<T> {
     #[inline]
     unsafe fn trace(&self, _tracer: &mut Tracer) {}
 }
 
-unsafe impl<K, V: Trace> Trace for BTreeMap<K, V> {
+// K is not traced (BTreeMap keys are immutable).
+unsafe impl<K: 'static, V: Trace> Trace for BTreeMap<K, V> {
     unsafe fn trace(&self, tracer: &mut Tracer) {
         for v in self.values() {
             v.trace(tracer);
@@ -275,11 +282,13 @@ unsafe impl<K, V: Trace> Trace for BTreeMap<K, V> {
     }
 }
 
-unsafe impl<T> Trace for BTreeSet<T> {
+// BTreeSet keys are never traced.
+unsafe impl<T: 'static> Trace for BTreeSet<T> {
     #[inline]
     unsafe fn trace(&self, _tracer: &mut Tracer) {}
 }
 
+// str is a DST, so we cannot allocate it directly. Use String as the Sized proxy.
 unsafe impl Trace for str {
     #[inline]
     unsafe fn trace(&self, _tracer: &mut Tracer) {}
@@ -316,7 +325,7 @@ unsafe impl<T: Trace, const N: usize> Trace for arrayvec::ArrayVec<T, N> {
     }
 }
 
-unsafe impl<K: Trace, V: Trace, S> Trace for hashbrown::hash_map::HashMap<K, V, S> {
+unsafe impl<K: Trace, V: Trace, S: 'static> Trace for hashbrown::hash_map::HashMap<K, V, S> {
     #[inline]
     unsafe fn trace(&self, tracer: &mut Tracer) {
         for (k, v) in self {
@@ -327,7 +336,7 @@ unsafe impl<K: Trace, V: Trace, S> Trace for hashbrown::hash_map::HashMap<K, V,
 }
 // Finalize is already implemented in common.rs
 
-unsafe impl<T: Trace, S> Trace for hashbrown::hash_set::HashSet<T, S> {
+unsafe impl<T: Trace, S: 'static> Trace for hashbrown::hash_set::HashSet<T, S> {
     #[inline]
     unsafe fn trace(&self, tracer: &mut Tracer) {
         for v in self {
@@ -371,7 +380,7 @@ unsafe impl Trace for std::time::SystemTime {
 }
 
 #[cfg(feature = "std")]
-unsafe impl<K: Trace, V: Trace, S> Trace for std::collections::HashMap<K, V, S> {
+unsafe impl<K: Trace, V: Trace, S: 'static> Trace for std::collections::HashMap<K, V, S> {
     #[inline]
     unsafe fn trace(&self, tracer: &mut Tracer) {
         for (k, v) in self {
@@ -382,7 +391,7 @@ unsafe impl<K: Trace, V: Trace, S> Trace for std::collections::HashMap<K, V, S>
 }
 
 #[cfg(feature = "std")]
-unsafe impl<T: Trace, S> Trace for std::collections::HashSet<T, S> {
+unsafe impl<T: Trace, S: 'static> Trace for std::collections::HashSet<T, S> {
     #[inline]
     unsafe fn trace(&self, tracer: &mut Tracer) {
         for v in self {