Auto merge of #74699 - notriddle:fd-non-negative, r=m-ou-se

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.

2 files changed, 18 insertions, 4 deletions

diff --git a/src/test/ui/print_type_sizes/niche-filling.rs b/src/test/ui/print_type_sizes/niche-filling.rs
index 37ac45f7e05..0716cee21c6 100644
--- a/src/test/ui/print_type_sizes/niche-filling.rs
+++ b/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<Union2<NonZeroU32, NonZeroU32>> = Default::default();
+    let _j: MyOption<Union1<NonZeroU32>> = Default::default();
+    let _k: MyOption<Union2<NonZeroU32, NonZeroU32>> = Default::default();
 
     0
 }
diff --git a/src/test/ui/print_type_sizes/niche-filling.stdout b/src/test/ui/print_type_sizes/niche-filling.stdout
index 1894cd218ee..d1753c26ca8 100644
--- a/src/test/ui/print_type_sizes/niche-filling.stdout
+++ b/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