Add a test to ensure that RawFd is the size we assume it is.

library/std/src/os/unix/io/fd.rs

@@ -2,6 +2,10 @@
 
 #![unstable(feature = "io_safety", issue = "87074")]
 
+// Tests for this module
+#[cfg(test)]
+mod tests;
+
 use super::raw::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
 use crate::fmt;
 use crate::fs;

library/std/src/os/unix/io/fd/tests.rs

@@ -0,0 +1,11 @@
+use crate::mem::size_of;
+use crate::os::unix::io::RawFd;
+
+#[test]
+fn test_raw_fd_layout() {
+    // `OwnedFd` and `BorrowedFd` use `rustc_layout_scalar_valid_range_start`
+    // and `rustc_layout_scalar_valid_range_end`, with values that depend on
+    // the bit width of `RawFd`. If this ever changes, those values will need
+    // to be updated.
+    assert_eq!(size_of::<RawFd>(), 4);
+}

library/std/src/os/wasi/io/fd.rs

@@ -2,6 +2,10 @@
 
 #![unstable(feature = "wasi_ext", issue = "71213")]
 
+// Tests for this module
+#[cfg(test)]
+mod tests;
+
 use super::raw::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
 use crate::fmt;
 use crate::fs;

library/std/src/os/wasi/io/fd/tests.rs

@@ -0,0 +1,11 @@
+use std::mem::size_of;
+use std::os::wasi::io::RawFd;
+
+#[test]
+fn test_raw_fd_layout() {
+    /// `OwnedFd` and `BorrowedFd` use `rustc_layout_scalar_valid_range_start`
+    /// and `rustc_layout_scalar_valid_range_end`, with values that depend on
+    /// the bit width of `RawFd`. If this ever changes, those values will need
+    /// to be updated.
+    assert_eq!(size_of::<RawFd>(), 4);
+}