diff --git a/src/Init/Data/Array/Basic.lean b/src/Init/Data/Array/Basic.lean
index 1584ca13bc..89b73875e0 100644
--- a/src/Init/Data/Array/Basic.lean
+++ b/src/Init/Data/Array/Basic.lean
@@ -50,6 +50,13 @@ instance : Inhabited (Array α) where
 def singleton (v : α) : Array α :=
   mkArray 1 v
 
+/-- Low-level version of `size` that directly queries the C array object cached size.
+    While this is not provable, `usize` always returns the exact size of the array since
+    the implementation only supports arrays of size less than `USize.size`.
+-/
+@[extern "lean_array_size", simp]
+def usize (a : @& Array α) : USize := a.size.toUSize
+
 /-- Low-level version of `fget` which is as fast as a C array read.
    `Fin` values are represented as tag pointers in the Lean runtime. Thus,
    `fget` may be slightly slower than `uget`. -/
@@ -174,7 +181,7 @@ def modifyOp (self : Array α) (idx : Nat) (f : α → α) : Array α :=
 
   This kind of low level trick can be removed with a little bit of compiler support. For example, if the compiler simplifies `as.size < usizeSz` to true. -/
 @[inline] unsafe def forInUnsafe {α : Type u} {β : Type v} {m : Type v → Type w} [Monad m] (as : Array α) (b : β) (f : α → β → m (ForInStep β)) : m β :=
-  let sz := USize.ofNat as.size
+  let sz := as.size.toUSize -- TODO: use usize
   let rec @[specialize] loop (i : USize) (b : β) : m β := do
     if i < sz then
       let a := as.uget i lcProof
@@ -280,7 +287,7 @@ def foldrM {α : Type u} {β : Type v} {m : Type v → Type w} [Monad m] (f : α
 /-- See comment at `forInUnsafe` -/
 @[inline]
 unsafe def mapMUnsafe {α : Type u} {β : Type v} {m : Type v → Type w} [Monad m] (f : α → m β) (as : Array α) : m (Array β) :=
-  let sz := USize.ofNat as.size
+  let sz := as.size.toUSize -- TODO: use usize
   let rec @[specialize] map (i : USize) (r : Array NonScalar) : m (Array PNonScalar.{v}) := do
     if i < sz then
      let v    := r.uget i lcProof
diff --git a/src/Init/Data/ByteArray/Basic.lean b/src/Init/Data/ByteArray/Basic.lean
index 1397652795..6171e3a86a 100644
--- a/src/Init/Data/ByteArray/Basic.lean
+++ b/src/Init/Data/ByteArray/Basic.lean
@@ -37,6 +37,10 @@ def push : ByteArray → UInt8 → ByteArray
 def size : (@& ByteArray) → Nat
   | ⟨bs⟩ => bs.size
 
+@[extern "lean_sarray_size", simp]
+def usize (a : @& ByteArray) : USize :=
+  a.size.toUSize
+
 @[extern "lean_byte_array_uget"]
 def uget : (a : @& ByteArray) → (i : USize) → i.toNat < a.size → UInt8
   | ⟨bs⟩, i, h => bs[i]
@@ -119,7 +123,7 @@ def toList (bs : ByteArray) : List UInt8 :=
   TODO: avoid code duplication in the future after we improve the compiler.
 -/
 @[inline] unsafe def forInUnsafe {β : Type v} {m : Type v → Type w} [Monad m] (as : ByteArray) (b : β) (f : UInt8 → β → m (ForInStep β)) : m β :=
-  let sz := USize.ofNat as.size
+  let sz := as.size.toUSize -- TODO: use usize
   let rec @[specialize] loop (i : USize) (b : β) : m β := do
     if i < sz then
       let a := as.uget i lcProof
diff --git a/src/Init/Data/FloatArray/Basic.lean b/src/Init/Data/FloatArray/Basic.lean
index 0c47834383..5540e07ee9 100644
--- a/src/Init/Data/FloatArray/Basic.lean
+++ b/src/Init/Data/FloatArray/Basic.lean
@@ -37,6 +37,10 @@ def push : FloatArray → Float → FloatArray
 def size : (@& FloatArray) → Nat
   | ⟨ds⟩ => ds.size
 
+@[extern "lean_sarray_size", simp]
+def usize (a : @& FloatArray) : USize :=
+  a.size.toUSize
+
 @[extern "lean_float_array_uget"]
 def uget : (a : @& FloatArray) → (i : USize) → i.toNat < a.size → Float
   | ⟨ds⟩, i, h => ds[i]
@@ -90,7 +94,7 @@ partial def toList (ds : FloatArray) : List Float :=
 -/
 -- TODO: avoid code duplication in the future after we improve the compiler.
 @[inline] unsafe def forInUnsafe {β : Type v} {m : Type v → Type w} [Monad m] (as : FloatArray) (b : β) (f : Float → β → m (ForInStep β)) : m β :=
-  let sz := USize.ofNat as.size
+  let sz := as.size.toUSize -- TODO: use usize
   let rec @[specialize] loop (i : USize) (b : β) : m β := do
     if i < sz then
       let a := as.uget i lcProof
diff --git a/stage0/src/stdlib_flags.h b/stage0/src/stdlib_flags.h
index 0699845ba4..f636bca698 100644
--- a/stage0/src/stdlib_flags.h
+++ b/stage0/src/stdlib_flags.h
@@ -18,3 +18,5 @@ options get_default_options() {
     return opts;
 }
 }
+
+// please update stage0