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Mark -1 as an available niche for file descriptors #74699

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merged 4 commits into from
Dec 20, 2020
+27 −6
Merged

Mark -1 as an available niche for file descriptors #74699

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59abdb6
Mark `-1` as an available niche for file descriptors
notriddle Jul 28, 2020
a50811a
Add safety note to library/std/src/sys/unix/fd.rs
notriddle Jul 30, 2020
08b70ed
Fix fd test case
notriddle Dec 10, 2020
094b1da
Check that c_int is i32 in FileDesc::new.
m-ou-se Dec 20, 2020
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9 changes: 8 additions & 1 deletion library/std/src/sys/unix/fd.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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,
}
Expand Down Expand Up @@ -63,7 +68,9 @@ const fn max_iov() -> usize {

impl FileDesc {
pub fn new(fd: c_int) -> FileDesc {
FileDesc { fd }
assert_ne!(fd, -1i32);
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@vi vi Dec 10, 2020 •
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Will there be unsafe fn new_unchecked, to avoid including panicking code in a program that otherwise avoid it?

If this is a private type, not directly available in public API, will panicking and string formatting code suddenly appear in places where it was absent before, breaking things like no-panic?

// SAFETY: we just asserted that the value is in the valid range and isn't `-1` (the only value bigger than `0xFF_FF_FF_FE` unsigned)
unsafe { FileDesc { fd } }
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}

pub fn raw(&self) -> c_int {
Expand Down
2 changes: 1 addition & 1 deletion library/std/src/sys/unix/fd/tests.rs
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Expand Up @@ -3,7 +3,7 @@ use core::mem::ManuallyDrop;

#[test]
fn limit_vector_count() {
let stdout = ManuallyDrop::new(FileDesc { fd: 1 });
let stdout = ManuallyDrop::new(unsafe { FileDesc { fd: 1 } });
let bufs = (0..1500).map(|_| IoSlice::new(&[])).collect::<Vec<_>>();
assert!(stdout.write_vectored(&bufs).is_ok());
}
16 changes: 12 additions & 4 deletions src/test/ui/print_type_sizes/niche-filling.rs
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Expand Up @@ -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 }
}
Expand Down Expand Up @@ -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
}
6 changes: 6 additions & 0 deletions src/test/ui/print_type_sizes/niche-filling.stdout
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Expand Up @@ -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
Expand Down