From b94a29a25ff9ca4a8edfc0b6c7d1d309dc03d5cd Mon Sep 17 00:00:00 2001
From: clubby789 <jamie@hill-daniel.co.uk>
Date: Tue, 17 Jan 2023 15:42:53 +0000
Subject: [PATCH 1/2] Implement `alloc::vec::IsZero` for `Option<$NUM>` types

---
 library/alloc/src/lib.rs         |  1 +
 library/alloc/src/vec/is_zero.rs | 17 +++++++++++++++++
 tests/codegen/vec-calloc.rs      | 17 +++++++++++++++++
 3 files changed, 35 insertions(+)

diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index afc3a3dc6a8..ca75c3895f4 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -106,6 +106,7 @@
 #![feature(const_size_of_val)]
 #![feature(const_align_of_val)]
 #![feature(const_ptr_read)]
+#![feature(const_maybe_uninit_zeroed)]
 #![feature(const_maybe_uninit_write)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_refs_to_cell)]
diff --git a/library/alloc/src/vec/is_zero.rs b/library/alloc/src/vec/is_zero.rs
index 26120270c0c..2b6be14de79 100644
--- a/library/alloc/src/vec/is_zero.rs
+++ b/library/alloc/src/vec/is_zero.rs
@@ -147,6 +147,23 @@ impl_is_zero_option_of_nonzero!(
     NonZeroIsize,
 );
 
+macro_rules! impl_is_zero_option_of_num {
+    ($($t:ty,)+) => {$(
+        unsafe impl IsZero for Option<$t> {
+            #[inline]
+            fn is_zero(&self) -> bool {
+                const {
+                    let none: Self = unsafe { core::mem::MaybeUninit::zeroed().assume_init() };
+                    assert!(none.is_none());
+                }
+                self.is_none()
+            }
+        }
+    )+};
+}
+
+impl_is_zero_option_of_num!(u8, u16, u32, u64, u128, i8, i16, i32, i64, i128, usize, isize,);
+
 unsafe impl<T: IsZero> IsZero for Wrapping<T> {
     #[inline]
     fn is_zero(&self) -> bool {
diff --git a/tests/codegen/vec-calloc.rs b/tests/codegen/vec-calloc.rs
index ae6e448f172..9cc5bd4fd1a 100644
--- a/tests/codegen/vec-calloc.rs
+++ b/tests/codegen/vec-calloc.rs
@@ -161,6 +161,23 @@ pub fn vec_option_bool(n: usize) -> Vec<Option<bool>> {
     vec![Some(false); n]
 }
 
+// CHECK-LABEL: @vec_option_i32
+#[no_mangle]
+pub fn vec_option_i32(n: usize) -> Vec<Option<i32>> {
+    // CHECK-NOT: call {{.*}}alloc::vec::from_elem
+    // CHECK-NOT: call {{.*}}reserve
+    // CHECK-NOT: call {{.*}}__rust_alloc(
+
+    // CHECK: call {{.*}}__rust_alloc_zeroed(
+
+    // CHECK-NOT: call {{.*}}alloc::vec::from_elem
+    // CHECK-NOT: call {{.*}}reserve
+    // CHECK-NOT: call {{.*}}__rust_alloc(
+
+    // CHECK: ret void
+    vec![None; n]
+}
+
 // Ensure that __rust_alloc_zeroed gets the right attributes for LLVM to optimize it away.
 // CHECK: declare noalias ptr @__rust_alloc_zeroed(i64, i64 allocalign) unnamed_addr [[RUST_ALLOC_ZEROED_ATTRS:#[0-9]+]]
 

From 50e9f2e6e87911659b6ae63230a5797f4cdf28e1 Mon Sep 17 00:00:00 2001
From: clubby789 <jamie@hill-daniel.co.uk>
Date: Wed, 18 Jan 2023 15:48:53 +0000
Subject: [PATCH 2/2] Update `IsZero` documentation

---
 library/alloc/src/vec/is_zero.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/library/alloc/src/vec/is_zero.rs b/library/alloc/src/vec/is_zero.rs
index 2b6be14de79..cb9adf05c25 100644
--- a/library/alloc/src/vec/is_zero.rs
+++ b/library/alloc/src/vec/is_zero.rs
@@ -4,7 +4,8 @@ use crate::boxed::Box;
 
 #[rustc_specialization_trait]
 pub(super) unsafe trait IsZero {
-    /// Whether this value's representation is all zeros
+    /// Whether this value's representation is all zeros,
+    /// or can be represented with all zeroes.
     fn is_zero(&self) -> bool;
 }