Mark -1 as an available niche for file descriptors

Based on discussion from https://internals.rust-lang.org/t/can-the-standard-library-shrink-option-file/12768,
the file descriptor -1 is chosen based on the POSIX API designs that use it as a sentinel to report errors.
A bigger niche could've been chosen, particularly on Linux, but would not necessarily be portable.

This PR also adds a test case to ensure that the -1 niche
(which is kind of hacky and has no obvious test case) works correctly.
It requires the "upper" bound, which is actually -1, to be expressed in two's complement.

Author: notriddle

std/src/sys/unix/fd.rs

@@ -12,6 +12,11 @@ use crate::sys_common::AsInner;
 use libc::{c_int, c_void};
 
 #[derive(Debug)]
+#[rustc_layout_scalar_valid_range_start(0)]
+// libstd/os/raw/mod.rs assures me that every libstd-supported platform has a
+// 32-bit c_int. Below is -2, in two's complement, but that only works out
+// because c_int is 32 bits.
+#[rustc_layout_scalar_valid_range_end(0xFF_FF_FF_FE)]
 pub struct FileDesc {
     fd: c_int,
 }
@@ -63,7 +68,8 @@ const fn max_iov() -> usize {
 
 impl FileDesc {
     pub fn new(fd: c_int) -> FileDesc {
-        FileDesc { fd }
+        assert_ne!(fd, -1);
+        unsafe { FileDesc { fd } }
     }
 
     pub fn raw(&self) -> c_int {