diff --git a/library/alloc/tests/string.rs b/library/alloc/tests/string.rs
index b6836fdc88ee8..99d1296a4c925 100644
--- a/library/alloc/tests/string.rs
+++ b/library/alloc/tests/string.rs
@@ -693,12 +693,6 @@ fn test_try_reserve() {
     const MAX_CAP: usize = isize::MAX as usize;
     const MAX_USIZE: usize = usize::MAX;
 
-    // On 16/32-bit, we check that allocations don't exceed isize::MAX,
-    // on 64-bit, we assume the OS will give an OOM for such a ridiculous size.
-    // Any platform that succeeds for these requests is technically broken with
-    // ptr::offset because LLVM is the worst.
-    let guards_against_isize = usize::BITS < 64;
-
     {
         // Note: basic stuff is checked by test_reserve
         let mut empty_string: String = String::new();
@@ -712,35 +706,19 @@ fn test_try_reserve() {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
 
-        if guards_against_isize {
-            // Check isize::MAX + 1 does count as overflow
-            assert_matches!(
-                empty_string.try_reserve(MAX_CAP + 1).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-
-            // Check usize::MAX does count as overflow
-            assert_matches!(
-                empty_string.try_reserve(MAX_USIZE).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "usize::MAX should trigger an overflow!"
-            );
-        } else {
-            // Check isize::MAX + 1 is an OOM
-            assert_matches!(
-                empty_string.try_reserve(MAX_CAP + 1).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-
-            // Check usize::MAX is an OOM
-            assert_matches!(
-                empty_string.try_reserve(MAX_USIZE).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "usize::MAX should trigger an OOM!"
-            );
-        }
+        // Check isize::MAX + 1 does count as overflow
+        assert_matches!(
+            empty_string.try_reserve(MAX_CAP + 1).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
+        // Check usize::MAX does count as overflow
+        assert_matches!(
+            empty_string.try_reserve(MAX_USIZE).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "usize::MAX should trigger an overflow!"
+        );
     }
 
     {
@@ -753,19 +731,13 @@ fn test_try_reserve() {
         if let Err(CapacityOverflow) = ten_bytes.try_reserve(MAX_CAP - 10).map_err(|e| e.kind()) {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
-        if guards_against_isize {
-            assert_matches!(
-                ten_bytes.try_reserve(MAX_CAP - 9).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-        } else {
-            assert_matches!(
-                ten_bytes.try_reserve(MAX_CAP - 9).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-        }
+
+        assert_matches!(
+            ten_bytes.try_reserve(MAX_CAP - 9).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
         // Should always overflow in the add-to-len
         assert_matches!(
             ten_bytes.try_reserve(MAX_USIZE).map_err(|e| e.kind()),
@@ -785,8 +757,6 @@ fn test_try_reserve_exact() {
     const MAX_CAP: usize = isize::MAX as usize;
     const MAX_USIZE: usize = usize::MAX;
 
-    let guards_against_isize = usize::BITS < 64;
-
     {
         let mut empty_string: String = String::new();
 
@@ -799,31 +769,17 @@ fn test_try_reserve_exact() {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
 
-        if guards_against_isize {
-            assert_matches!(
-                empty_string.try_reserve_exact(MAX_CAP + 1).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-
-            assert_matches!(
-                empty_string.try_reserve_exact(MAX_USIZE).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "usize::MAX should trigger an overflow!"
-            );
-        } else {
-            assert_matches!(
-                empty_string.try_reserve_exact(MAX_CAP + 1).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-
-            assert_matches!(
-                empty_string.try_reserve_exact(MAX_USIZE).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "usize::MAX should trigger an OOM!"
-            );
-        }
+        assert_matches!(
+            empty_string.try_reserve_exact(MAX_CAP + 1).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
+        assert_matches!(
+            empty_string.try_reserve_exact(MAX_USIZE).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "usize::MAX should trigger an overflow!"
+        );
     }
 
     {
@@ -839,19 +795,13 @@ fn test_try_reserve_exact() {
         {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
-        if guards_against_isize {
-            assert_matches!(
-                ten_bytes.try_reserve_exact(MAX_CAP - 9).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-        } else {
-            assert_matches!(
-                ten_bytes.try_reserve_exact(MAX_CAP - 9).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-        }
+
+        assert_matches!(
+            ten_bytes.try_reserve_exact(MAX_CAP - 9).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
         assert_matches!(
             ten_bytes.try_reserve_exact(MAX_USIZE).map_err(|e| e.kind()),
             Err(CapacityOverflow),
diff --git a/library/alloc/tests/vec.rs b/library/alloc/tests/vec.rs
index ca0fcc855c7b8..2f93250bce24a 100644
--- a/library/alloc/tests/vec.rs
+++ b/library/alloc/tests/vec.rs
@@ -1489,12 +1489,6 @@ fn test_try_reserve() {
     const MAX_CAP: usize = isize::MAX as usize;
     const MAX_USIZE: usize = usize::MAX;
 
-    // On 16/32-bit, we check that allocations don't exceed isize::MAX,
-    // on 64-bit, we assume the OS will give an OOM for such a ridiculous size.
-    // Any platform that succeeds for these requests is technically broken with
-    // ptr::offset because LLVM is the worst.
-    let guards_against_isize = usize::BITS < 64;
-
     {
         // Note: basic stuff is checked by test_reserve
         let mut empty_bytes: Vec<u8> = Vec::new();
@@ -1508,35 +1502,19 @@ fn test_try_reserve() {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
 
-        if guards_against_isize {
-            // Check isize::MAX + 1 does count as overflow
-            assert_matches!(
-                empty_bytes.try_reserve(MAX_CAP + 1).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-
-            // Check usize::MAX does count as overflow
-            assert_matches!(
-                empty_bytes.try_reserve(MAX_USIZE).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "usize::MAX should trigger an overflow!"
-            );
-        } else {
-            // Check isize::MAX + 1 is an OOM
-            assert_matches!(
-                empty_bytes.try_reserve(MAX_CAP + 1).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-
-            // Check usize::MAX is an OOM
-            assert_matches!(
-                empty_bytes.try_reserve(MAX_USIZE).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "usize::MAX should trigger an OOM!"
-            );
-        }
+        // Check isize::MAX + 1 does count as overflow
+        assert_matches!(
+            empty_bytes.try_reserve(MAX_CAP + 1).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
+        // Check usize::MAX does count as overflow
+        assert_matches!(
+            empty_bytes.try_reserve(MAX_USIZE).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "usize::MAX should trigger an overflow!"
+        );
     }
 
     {
@@ -1549,19 +1527,13 @@ fn test_try_reserve() {
         if let Err(CapacityOverflow) = ten_bytes.try_reserve(MAX_CAP - 10).map_err(|e| e.kind()) {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
-        if guards_against_isize {
-            assert_matches!(
-                ten_bytes.try_reserve(MAX_CAP - 9).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-        } else {
-            assert_matches!(
-                ten_bytes.try_reserve(MAX_CAP - 9).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-        }
+
+        assert_matches!(
+            ten_bytes.try_reserve(MAX_CAP - 9).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
         // Should always overflow in the add-to-len
         assert_matches!(
             ten_bytes.try_reserve(MAX_USIZE).map_err(|e| e.kind()),
@@ -1582,19 +1554,13 @@ fn test_try_reserve() {
         {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
-        if guards_against_isize {
-            assert_matches!(
-                ten_u32s.try_reserve(MAX_CAP / 4 - 9).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-        } else {
-            assert_matches!(
-                ten_u32s.try_reserve(MAX_CAP / 4 - 9).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-        }
+
+        assert_matches!(
+            ten_u32s.try_reserve(MAX_CAP / 4 - 9).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
         // Should fail in the mul-by-size
         assert_matches!(
             ten_u32s.try_reserve(MAX_USIZE - 20).map_err(|e| e.kind()),
@@ -1614,8 +1580,6 @@ fn test_try_reserve_exact() {
     const MAX_CAP: usize = isize::MAX as usize;
     const MAX_USIZE: usize = usize::MAX;
 
-    let guards_against_isize = size_of::<usize>() < 8;
-
     {
         let mut empty_bytes: Vec<u8> = Vec::new();
 
@@ -1628,31 +1592,17 @@ fn test_try_reserve_exact() {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
 
-        if guards_against_isize {
-            assert_matches!(
-                empty_bytes.try_reserve_exact(MAX_CAP + 1).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-
-            assert_matches!(
-                empty_bytes.try_reserve_exact(MAX_USIZE).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "usize::MAX should trigger an overflow!"
-            );
-        } else {
-            assert_matches!(
-                empty_bytes.try_reserve_exact(MAX_CAP + 1).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-
-            assert_matches!(
-                empty_bytes.try_reserve_exact(MAX_USIZE).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "usize::MAX should trigger an OOM!"
-            );
-        }
+        assert_matches!(
+            empty_bytes.try_reserve_exact(MAX_CAP + 1).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
+        assert_matches!(
+            empty_bytes.try_reserve_exact(MAX_USIZE).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "usize::MAX should trigger an overflow!"
+        );
     }
 
     {
@@ -1668,19 +1618,13 @@ fn test_try_reserve_exact() {
         {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
-        if guards_against_isize {
-            assert_matches!(
-                ten_bytes.try_reserve_exact(MAX_CAP - 9).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-        } else {
-            assert_matches!(
-                ten_bytes.try_reserve_exact(MAX_CAP - 9).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-        }
+
+        assert_matches!(
+            ten_bytes.try_reserve_exact(MAX_CAP - 9).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
         assert_matches!(
             ten_bytes.try_reserve_exact(MAX_USIZE).map_err(|e| e.kind()),
             Err(CapacityOverflow),
@@ -1701,19 +1645,13 @@ fn test_try_reserve_exact() {
         {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
-        if guards_against_isize {
-            assert_matches!(
-                ten_u32s.try_reserve_exact(MAX_CAP / 4 - 9).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-        } else {
-            assert_matches!(
-                ten_u32s.try_reserve_exact(MAX_CAP / 4 - 9).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-        }
+
+        assert_matches!(
+            ten_u32s.try_reserve_exact(MAX_CAP / 4 - 9).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
         assert_matches!(
             ten_u32s.try_reserve_exact(MAX_USIZE - 20).map_err(|e| e.kind()),
             Err(CapacityOverflow),
diff --git a/library/alloc/tests/vec_deque.rs b/library/alloc/tests/vec_deque.rs
index 89cc7f905be2c..019d73c0b16bb 100644
--- a/library/alloc/tests/vec_deque.rs
+++ b/library/alloc/tests/vec_deque.rs
@@ -2,7 +2,6 @@ use std::assert_matches::assert_matches;
 use std::collections::TryReserveErrorKind::*;
 use std::collections::{vec_deque::Drain, VecDeque};
 use std::fmt::Debug;
-use std::mem::size_of;
 use std::ops::Bound::*;
 use std::panic::{catch_unwind, AssertUnwindSafe};
 
@@ -1161,12 +1160,6 @@ fn test_try_reserve() {
     const MAX_CAP: usize = (isize::MAX as usize + 1) / 2 - 1;
     const MAX_USIZE: usize = usize::MAX;
 
-    // On 16/32-bit, we check that allocations don't exceed isize::MAX,
-    // on 64-bit, we assume the OS will give an OOM for such a ridiculous size.
-    // Any platform that succeeds for these requests is technically broken with
-    // ptr::offset because LLVM is the worst.
-    let guards_against_isize = size_of::<usize>() < 8;
-
     {
         // Note: basic stuff is checked by test_reserve
         let mut empty_bytes: VecDeque<u8> = VecDeque::new();
@@ -1180,31 +1173,19 @@ fn test_try_reserve() {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
 
-        if guards_against_isize {
-            // Check isize::MAX + 1 does count as overflow
-            assert_matches!(
-                empty_bytes.try_reserve(MAX_CAP + 1).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-
-            // Check usize::MAX does count as overflow
-            assert_matches!(
-                empty_bytes.try_reserve(MAX_USIZE).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "usize::MAX should trigger an overflow!"
-            );
-        } else {
-            // Check isize::MAX is an OOM
-            // VecDeque starts with capacity 7, always adds 1 to the capacity
-            // and also rounds the number to next power of 2 so this is the
-            // furthest we can go without triggering CapacityOverflow
-            assert_matches!(
-                empty_bytes.try_reserve(MAX_CAP).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-        }
+        // Check isize::MAX + 1 does count as overflow
+        assert_matches!(
+            empty_bytes.try_reserve(MAX_CAP + 1).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
+        // Check usize::MAX does count as overflow
+        assert_matches!(
+            empty_bytes.try_reserve(MAX_USIZE).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "usize::MAX should trigger an overflow!"
+        );
     }
 
     {
@@ -1217,19 +1198,13 @@ fn test_try_reserve() {
         if let Err(CapacityOverflow) = ten_bytes.try_reserve(MAX_CAP - 10).map_err(|e| e.kind()) {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
-        if guards_against_isize {
-            assert_matches!(
-                ten_bytes.try_reserve(MAX_CAP - 9).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-        } else {
-            assert_matches!(
-                ten_bytes.try_reserve(MAX_CAP - 9).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-        }
+
+        assert_matches!(
+            ten_bytes.try_reserve(MAX_CAP - 9).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
         // Should always overflow in the add-to-len
         assert_matches!(
             ten_bytes.try_reserve(MAX_USIZE).map_err(|e| e.kind()),
@@ -1250,19 +1225,13 @@ fn test_try_reserve() {
         {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
-        if guards_against_isize {
-            assert_matches!(
-                ten_u32s.try_reserve(MAX_CAP / 4 - 9).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-        } else {
-            assert_matches!(
-                ten_u32s.try_reserve(MAX_CAP / 4 - 9).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-        }
+
+        assert_matches!(
+            ten_u32s.try_reserve(MAX_CAP / 4 - 9).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
         // Should fail in the mul-by-size
         assert_matches!(
             ten_u32s.try_reserve(MAX_USIZE - 20).map_err(|e| e.kind()),
@@ -1282,8 +1251,6 @@ fn test_try_reserve_exact() {
     const MAX_CAP: usize = (isize::MAX as usize + 1) / 2 - 1;
     const MAX_USIZE: usize = usize::MAX;
 
-    let guards_against_isize = size_of::<usize>() < 8;
-
     {
         let mut empty_bytes: VecDeque<u8> = VecDeque::new();
 
@@ -1296,29 +1263,17 @@ fn test_try_reserve_exact() {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
 
-        if guards_against_isize {
-            assert_matches!(
-                empty_bytes.try_reserve_exact(MAX_CAP + 1).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-
-            assert_matches!(
-                empty_bytes.try_reserve_exact(MAX_USIZE).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "usize::MAX should trigger an overflow!"
-            );
-        } else {
-            // Check isize::MAX is an OOM
-            // VecDeque starts with capacity 7, always adds 1 to the capacity
-            // and also rounds the number to next power of 2 so this is the
-            // furthest we can go without triggering CapacityOverflow
-            assert_matches!(
-                empty_bytes.try_reserve_exact(MAX_CAP).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-        }
+        assert_matches!(
+            empty_bytes.try_reserve_exact(MAX_CAP + 1).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
+        assert_matches!(
+            empty_bytes.try_reserve_exact(MAX_USIZE).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "usize::MAX should trigger an overflow!"
+        );
     }
 
     {
@@ -1334,19 +1289,13 @@ fn test_try_reserve_exact() {
         {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
-        if guards_against_isize {
-            assert_matches!(
-                ten_bytes.try_reserve_exact(MAX_CAP - 9).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-        } else {
-            assert_matches!(
-                ten_bytes.try_reserve_exact(MAX_CAP - 9).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-        }
+
+        assert_matches!(
+            ten_bytes.try_reserve_exact(MAX_CAP - 9).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
         assert_matches!(
             ten_bytes.try_reserve_exact(MAX_USIZE).map_err(|e| e.kind()),
             Err(CapacityOverflow),
@@ -1367,19 +1316,13 @@ fn test_try_reserve_exact() {
         {
             panic!("isize::MAX shouldn't trigger an overflow!");
         }
-        if guards_against_isize {
-            assert_matches!(
-                ten_u32s.try_reserve_exact(MAX_CAP / 4 - 9).map_err(|e| e.kind()),
-                Err(CapacityOverflow),
-                "isize::MAX + 1 should trigger an overflow!"
-            );
-        } else {
-            assert_matches!(
-                ten_u32s.try_reserve_exact(MAX_CAP / 4 - 9).map_err(|e| e.kind()),
-                Err(AllocError { .. }),
-                "isize::MAX + 1 should trigger an OOM!"
-            );
-        }
+
+        assert_matches!(
+            ten_u32s.try_reserve_exact(MAX_CAP / 4 - 9).map_err(|e| e.kind()),
+            Err(CapacityOverflow),
+            "isize::MAX + 1 should trigger an overflow!"
+        );
+
         assert_matches!(
             ten_u32s.try_reserve_exact(MAX_USIZE - 20).map_err(|e| e.kind()),
             Err(CapacityOverflow),
diff --git a/library/core/src/alloc/layout.rs b/library/core/src/alloc/layout.rs
index ea639268652c3..e43fd29d94396 100644
--- a/library/core/src/alloc/layout.rs
+++ b/library/core/src/alloc/layout.rs
@@ -53,8 +53,8 @@ impl Layout {
     /// * `align` must be a power of two,
     ///
     /// * `size`, when rounded up to the nearest multiple of `align`,
-    ///    must not overflow (i.e., the rounded value must be less than
-    ///    or equal to `usize::MAX`).
+    ///    must not overflow isize (i.e., the rounded value must be
+    ///    less than or equal to `isize::MAX`).
     #[stable(feature = "alloc_layout", since = "1.28.0")]
     #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
     #[inline]
@@ -77,7 +77,7 @@ impl Layout {
         //
         // Above implies that checking for summation overflow is both
         // necessary and sufficient.
-        if size > usize::MAX - (align - 1) {
+        if size > isize::MAX as usize - (align - 1) {
             return Err(LayoutError);
         }
 
@@ -277,8 +277,8 @@ impl Layout {
         let pad = self.padding_needed_for(self.align());
         // This cannot overflow. Quoting from the invariant of Layout:
         // > `size`, when rounded up to the nearest multiple of `align`,
-        // > must not overflow (i.e., the rounded value must be less than
-        // > `usize::MAX`)
+        // > must not overflow isize (i.e., the rounded value must be
+        // > less than or equal to `isize::MAX`)
         let new_size = self.size() + pad;
 
         // SAFETY: self.align is already known to be valid and new_size has been
@@ -299,14 +299,13 @@ impl Layout {
     pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutError> {
         // This cannot overflow. Quoting from the invariant of Layout:
         // > `size`, when rounded up to the nearest multiple of `align`,
-        // > must not overflow (i.e., the rounded value must be less than
-        // > `usize::MAX`)
+        // > must not overflow isize (i.e., the rounded value must be
+        // > less than or equal to `isize::MAX`)
         let padded_size = self.size() + self.padding_needed_for(self.align());
         let alloc_size = padded_size.checked_mul(n).ok_or(LayoutError)?;
 
-        // SAFETY: self.align is already known to be valid and alloc_size has been
-        // padded already.
-        unsafe { Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size)) }
+        // The safe constructor is called here to enforce the isize size limit.
+        Layout::from_size_align(alloc_size, self.align()).map(|layout| (layout, padded_size))
     }
 
     /// Creates a layout describing the record for `self` followed by
@@ -363,6 +362,7 @@ impl Layout {
         let offset = self.size().checked_add(pad).ok_or(LayoutError)?;
         let new_size = offset.checked_add(next.size()).ok_or(LayoutError)?;
 
+        // The safe constructor is called here to enforce the isize size limit.
         let layout = Layout::from_size_align(new_size, new_align)?;
         Ok((layout, offset))
     }
@@ -383,6 +383,7 @@ impl Layout {
     #[inline]
     pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutError> {
         let size = self.size().checked_mul(n).ok_or(LayoutError)?;
+        // The safe constructor is called here to enforce the isize size limit.
         Layout::from_size_align(size, self.align())
     }
 
@@ -396,6 +397,7 @@ impl Layout {
     #[inline]
     pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutError> {
         let new_size = self.size().checked_add(next.size()).ok_or(LayoutError)?;
+        // The safe constructor is called here to enforce the isize size limit.
         Layout::from_size_align(new_size, self.align())
     }
 
@@ -406,16 +408,8 @@ impl Layout {
     #[inline]
     pub fn array<T>(n: usize) -> Result<Self, LayoutError> {
         let array_size = mem::size_of::<T>().checked_mul(n).ok_or(LayoutError)?;
-
-        // SAFETY:
-        // - Size: `array_size` cannot be too big because `size_of::<T>()` must
-        //   be a multiple of `align_of::<T>()`. Therefore, `array_size`
-        //   rounded up to the nearest multiple of `align_of::<T>()` is just
-        //   `array_size`. And `array_size` cannot be too big because it was
-        //   just checked by the `checked_mul()`.
-        // - Alignment: `align_of::<T>()` will always give an acceptable
-        //   (non-zero, power of two) alignment.
-        Ok(unsafe { Layout::from_size_align_unchecked(array_size, mem::align_of::<T>()) })
+        // The safe constructor is called here to enforce the isize size limit.
+        Layout::from_size_align(array_size, mem::align_of::<T>())
     }
 }