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.

library/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 {

src/test/ui/print_type_sizes/niche-filling.rs

@@ -15,12 +15,19 @@
 // padding and overall computed sizes can be quite different.
 
 #![feature(start)]
+#![feature(rustc_attrs)]
 #![allow(dead_code)]
 
 use std::num::NonZeroU32;
 
 pub enum MyOption<T> { None, Some(T) }
 
+#[rustc_layout_scalar_valid_range_start(0)]
+#[rustc_layout_scalar_valid_range_end(0xFF_FF_FF_FE)]
+pub struct MyNotNegativeOne {
+  _i: i32,
+}
+
 impl<T> Default for MyOption<T> {
     fn default() -> Self { MyOption::None }
 }
@@ -77,17 +84,18 @@ fn start(_: isize, _: *const *const u8) -> isize {
     let _a: MyOption<bool> = Default::default();
     let _b: MyOption<char> = Default::default();
     let _c: MyOption<std::cmp::Ordering> = Default::default();
-    let _b: MyOption<MyOption<u8>> = Default::default();
+    let _d: MyOption<MyOption<u8>> = Default::default();
     let _e: Enum4<(), char, (), ()> = Enum4::One(());
     let _f: Enum4<(), (), bool, ()> = Enum4::One(());
     let _g: Enum4<(), (), (), MyOption<u8>> = Enum4::One(());
+    let _h: MyOption<MyNotNegativeOne> = Default::default();
 
     // Unions do not currently participate in niche filling.
-    let _h: MyOption<Union2<NonZeroU32, u32>> = Default::default();
+    let _i: MyOption<Union2<NonZeroU32, u32>> = Default::default();
 
     // ...even when theoretically possible.
-    let _i: MyOption<Union1<NonZeroU32>> = Default::default();
+    let _j: MyOption<Union1<NonZeroU32>> = Default::default();
-    let _j: MyOption<Union2<NonZeroU32, NonZeroU32>> = Default::default();
+    let _k: MyOption<Union2<NonZeroU32, NonZeroU32>> = Default::default();
 
     0
 }

src/test/ui/print_type_sizes/niche-filling.stdout

@@ -43,6 +43,12 @@ print-type-size     variant `Three`: 0 bytes
 print-type-size         field `.0`: 0 bytes
 print-type-size     variant `Four`: 0 bytes
 print-type-size         field `.0`: 0 bytes
+print-type-size type: `MyNotNegativeOne`: 4 bytes, alignment: 4 bytes
+print-type-size     field `._i`: 4 bytes
+print-type-size type: `MyOption<MyNotNegativeOne>`: 4 bytes, alignment: 4 bytes
+print-type-size     variant `Some`: 4 bytes
+print-type-size         field `.0`: 4 bytes
+print-type-size     variant `None`: 0 bytes
 print-type-size type: `MyOption<char>`: 4 bytes, alignment: 4 bytes
 print-type-size     variant `Some`: 4 bytes
 print-type-size         field `.0`: 4 bytes