diff --git a/build.rs b/build.rs
index b208a9a1c6fb0..25342e58c0f14 100644
--- a/build.rs
+++ b/build.rs
@@ -19,7 +19,6 @@ const ALLOWED_CFGS: &'static [&'static str] = &[
     "libc_const_extern_fn_unstable",
     "libc_core_cvoid",
     "libc_deny_warnings",
-    "libc_int128",
     "libc_long_array",
     "libc_non_exhaustive",
     "libc_packedN",
@@ -87,11 +86,6 @@ fn main() {
         set_cfg("libc_deny_warnings");
     }
 
-    // Rust >= 1.26 supports i128 and u128:
-    if rustc_minor_ver >= 26 || rustc_dep_of_std {
-        set_cfg("libc_int128");
-    }
-
     // Rust >= 1.30 supports `core::ffi::c_void`, so libc can just re-export it.
     // Otherwise, it defines an incompatible type to retaining
     // backwards-compatibility.
diff --git a/libc-test/build.rs b/libc-test/build.rs
index 01907dff90652..b60c4ac03a5e1 100644
--- a/libc-test/build.rs
+++ b/libc-test/build.rs
@@ -327,6 +327,9 @@ fn test_apple(target: &str) {
             // FIXME: Requires the macOS 14.4 SDK.
             "os_sync_wake_by_address_flags_t" | "os_sync_wait_on_address_flags_t" => true,
 
+            // FIXME: "'__uint128' undeclared" in C
+            "__uint128" => true,
+
             _ => false,
         }
     });
@@ -1858,6 +1861,10 @@ fn test_android(target: &str) {
             // These are intended to be opaque
             "posix_spawn_file_actions_t" => true,
             "posix_spawnattr_t" => true,
+
+            // FIXME: "'__uint128' undeclared" in C
+            "__uint128" => true,
+
             _ => false,
         }
     });
@@ -3702,6 +3709,9 @@ fn test_linux(target: &str) {
             "priority_t" if musl => true,
             "name_t" if musl => true,
 
+            // FIXME: "'__uint128' undeclared" in C
+            "__uint128" => true,
+
             t => {
                 if musl {
                     // LFS64 types have been removed in musl 1.2.4+
diff --git a/src/fixed_width_ints.rs b/src/fixed_width_ints.rs
index 999de8f54f194..980ff61ecdaf3 100644
--- a/src/fixed_width_ints.rs
+++ b/src/fixed_width_ints.rs
@@ -20,7 +20,7 @@ pub type uint32_t = u32;
 pub type uint64_t = u64;
 
 cfg_if! {
-    if #[cfg(all(libc_int128, target_arch = "aarch64", not(target_os = "windows")))] {
+    if #[cfg(all(target_arch = "aarch64", not(target_os = "windows")))] {
         // This introduces partial support for FFI with __int128 and
         // equivalent types on platforms where Rust's definition is validated
         // to match the standard C ABI of that platform.
diff --git a/src/unix/bsd/apple/b64/aarch64/mod.rs b/src/unix/bsd/apple/b64/aarch64/mod.rs
index 1d4a5590fbf36..dd22fcb2507b1 100644
--- a/src/unix/bsd/apple/b64/aarch64/mod.rs
+++ b/src/unix/bsd/apple/b64/aarch64/mod.rs
@@ -37,16 +37,8 @@ s! {
         pub __pad: u32,
     }
 
-    // This type natively uses a uint128, but for a while we hacked
-    // it in with repr(align) and `[u64; 2]`. uint128 isn't available
-    // all the way back to our earliest supported versions so we
-    // preserver the old shim.
-    #[cfg_attr(not(libc_int128), repr(align(16)))]
     pub struct __darwin_arm_neon_state64 {
-        #[cfg(libc_int128)]
         pub __v: [::__uint128_t; 32],
-        #[cfg(not(libc_int128))]
-        pub __v: [[u64; 2]; 32],
         pub __fpsr: u32,
         pub __fpcr: u32,
     }
diff --git a/src/unix/linux_like/android/b64/aarch64/int128.rs b/src/unix/linux_like/android/b64/aarch64/int128.rs
deleted file mode 100644
index 4535e73eeddf1..0000000000000
--- a/src/unix/linux_like/android/b64/aarch64/int128.rs
+++ /dev/null
@@ -1,7 +0,0 @@
-s! {
-    pub struct user_fpsimd_struct {
-        pub vregs: [::__uint128_t; 32],
-        pub fpsr: u32,
-        pub fpcr: u32,
-    }
-}
diff --git a/src/unix/linux_like/android/b64/aarch64/mod.rs b/src/unix/linux_like/android/b64/aarch64/mod.rs
index d063c022bfb7e..49b990fd8f1ac 100644
--- a/src/unix/linux_like/android/b64/aarch64/mod.rs
+++ b/src/unix/linux_like/android/b64/aarch64/mod.rs
@@ -75,6 +75,12 @@ s! {
         // auto-derive traits like Debug
         __reserved: [[u64; 32]; 16],
     }
+
+    pub struct user_fpsimd_struct {
+        pub vregs: [::__uint128_t; 32],
+        pub fpsr: u32,
+        pub fpcr: u32,
+    }
 }
 
 s_no_extra_traits! {
@@ -448,10 +454,3 @@ pub const PROT_MTE: ::c_int = 0x20;
 // From NDK's asm/auxvec.h
 pub const AT_SYSINFO_EHDR: ::c_ulong = 33;
 pub const AT_VECTOR_SIZE_ARCH: ::c_ulong = 2;
-
-cfg_if! {
-    if #[cfg(libc_int128)] {
-        mod int128;
-        pub use self::int128::*;
-    }
-}
diff --git a/src/unix/linux_like/linux/gnu/b64/aarch64/fallback.rs b/src/unix/linux_like/linux/gnu/b64/aarch64/fallback.rs
deleted file mode 100644
index 398fbb53755c8..0000000000000
--- a/src/unix/linux_like/linux/gnu/b64/aarch64/fallback.rs
+++ /dev/null
@@ -1,8 +0,0 @@
-s! {
-    #[repr(align(16))]
-    pub struct user_fpsimd_struct {
-        pub vregs: [[u64; 2]; 32],
-        pub fpsr: ::c_uint,
-        pub fpcr: ::c_uint,
-    }
-}
diff --git a/src/unix/linux_like/linux/gnu/b64/aarch64/int128.rs b/src/unix/linux_like/linux/gnu/b64/aarch64/int128.rs
deleted file mode 100644
index 4535e73eeddf1..0000000000000
--- a/src/unix/linux_like/linux/gnu/b64/aarch64/int128.rs
+++ /dev/null
@@ -1,7 +0,0 @@
-s! {
-    pub struct user_fpsimd_struct {
-        pub vregs: [::__uint128_t; 32],
-        pub fpsr: u32,
-        pub fpcr: u32,
-    }
-}
diff --git a/src/unix/linux_like/linux/gnu/b64/aarch64/mod.rs b/src/unix/linux_like/linux/gnu/b64/aarch64/mod.rs
index 783200a6d6e9c..5ee559190a8a2 100644
--- a/src/unix/linux_like/linux/gnu/b64/aarch64/mod.rs
+++ b/src/unix/linux_like/linux/gnu/b64/aarch64/mod.rs
@@ -217,6 +217,12 @@ s! {
         __reserved: [[u64; 32]; 16],
     }
 
+    pub struct user_fpsimd_struct {
+        pub vregs: [::__uint128_t; 32],
+        pub fpsr: ::c_uint,
+        pub fpcr: ::c_uint,
+    }
+
     #[repr(align(8))]
     pub struct clone_args {
         pub flags: ::c_ulonglong,
@@ -964,13 +970,3 @@ cfg_if! {
         pub use self::lp64::*;
     }
 }
-
-cfg_if! {
-    if #[cfg(libc_int128)] {
-        mod int128;
-        pub use self::int128::*;
-    } else {
-        mod fallback;
-        pub use self::fallback::*;
-    }
-}
diff --git a/src/unix/linux_like/linux/musl/b64/aarch64/int128.rs b/src/unix/linux_like/linux/musl/b64/aarch64/int128.rs
deleted file mode 100644
index 4535e73eeddf1..0000000000000
--- a/src/unix/linux_like/linux/musl/b64/aarch64/int128.rs
+++ /dev/null
@@ -1,7 +0,0 @@
-s! {
-    pub struct user_fpsimd_struct {
-        pub vregs: [::__uint128_t; 32],
-        pub fpsr: u32,
-        pub fpcr: u32,
-    }
-}
diff --git a/src/unix/linux_like/linux/musl/b64/aarch64/mod.rs b/src/unix/linux_like/linux/musl/b64/aarch64/mod.rs
index 7582676c3bfa6..f71d47effe85c 100644
--- a/src/unix/linux_like/linux/musl/b64/aarch64/mod.rs
+++ b/src/unix/linux_like/linux/musl/b64/aarch64/mod.rs
@@ -101,6 +101,12 @@ s! {
         pub set_tid_size: ::c_ulonglong,
         pub cgroup: ::c_ulonglong,
     }
+
+    pub struct user_fpsimd_struct {
+        pub vregs: [::__uint128_t; 32],
+        pub fpsr: u32,
+        pub fpcr: u32,
+    }
 }
 
 s_no_extra_traits! {
@@ -684,10 +690,3 @@ pub const VMIN: usize = 6;
 pub const IEXTEN: ::tcflag_t = 0x00008000;
 pub const TOSTOP: ::tcflag_t = 0x00000100;
 pub const FLUSHO: ::tcflag_t = 0x00001000;
-
-cfg_if! {
-    if #[cfg(libc_int128)] {
-        mod int128;
-        pub use self::int128::*;
-    }
-}