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Generalize get_nullable_type to allow types where null is all-ones.

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Generalize get_nullable_type to accept types that have an all-ones bit
pattern as their sentry "null" value.

This will allow [`OwnedFd`], [`BorrowedFd`], [`OwnedSocket`], and
[`BorrowedSocket`] to be marked with
`#[rustc_nonnull_optimization_guaranteed]`, which will allow
`Option<OwnedFd>`, `Option<BorrowedFd>`, `Option<OwnedSocket>`, and
`Option<BorrowedSocket>` to be used in FFI declarations, as described
in the [I/O safety RFC].

For example, it will allow a function like `open` on Unix and `WSASocketW`
on Windows to be declared using `Option<OwnedFd>` and `Option<OwnedSocket>`
return types, respectively.

The actual change to add `#[rustc_nonnull_optimization_guaranteed]`
to the abovementioned types will be a separate PR, as it'll depend on
having this patch in the stage0 compiler.

Also, update the diagnostics to mention that "niche optimizations" are
used in libstd as well as libcore, as `rustc_layout_scalar_valid_range_start`
and `rustc_layout_scalar_valid_range_end` are already in use in libstd.

[`OwnedFd`]: c9dc44be24/library/std/src/os/fd/owned.rs (L49)
[`BorrowedFd`]: c9dc44be24/library/std/src/os/fd/owned.rs (L29)
[`OwnedSocket`]: c9dc44be24/library/std/src/os/windows/io/socket.rs (L51)
[`BorrowedSocket`]: c9dc44be24/library/std/src/os/windows/io/socket.rs (L29)
[I/O safety RFC]: https://github.com/rust-lang/rfcs/blob/master/text/3128-io-safety.md#ownedfd-and-borrowedfdfd-1
This commit is contained in:
Dan Gohman 2022-03-03 14:41:10 -08:00
parent 10913c0001
commit 86b4658c56
4 changed files with 8 additions and 6 deletions
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4
compiler/rustc_lint/src/types.rs
View file

@ -795,7 +795,9 @@ crate fn repr_nullable_ptr<'tcx>(
let field_ty_abi = &cx.layout_of(field_ty).unwrap().abi;
if let Abi::Scalar(field_ty_scalar) = field_ty_abi {
match (field_ty_scalar.valid_range.start, field_ty_scalar.valid_range.end) {
(0, _) => unreachable!("Non-null optimisation extended to a non-zero value."),
(0, x) if x == field_ty_scalar.value.size(&cx.tcx).unsigned_int_max() - 1 => {
return Some(get_nullable_type(cx, field_ty).unwrap());
}
(1, _) => {
return Some(get_nullable_type(cx, field_ty).unwrap());
}