Implement RFC 2945: "C-unwind" ABI
## Implement RFC 2945: "C-unwind" ABI
This branch implements [RFC 2945]. The tracking issue for this RFC is #74990.
The feature gate for the issue is `#![feature(c_unwind)]`.
This RFC was created as part of the ffi-unwind project group tracked at rust-lang/lang-team#19.
### Changes
Further details will be provided in commit messages, but a high-level overview
of the changes follows:
* A boolean `unwind` payload is added to the `C`, `System`, `Stdcall`,
and `Thiscall` variants, marking whether unwinding across FFI boundaries is
acceptable. The cases where each of these variants' `unwind` member is true
correspond with the `C-unwind`, `system-unwind`, `stdcall-unwind`, and
`thiscall-unwind` ABI strings introduced in RFC 2945 [3].
* This commit adds a `c_unwind` feature gate for the new ABI strings.
Tests for this feature gate are included in `src/test/ui/c-unwind/`, which
ensure that this feature gate works correctly for each of the new ABIs.
A new language features entry in the unstable book is added as well.
* We adjust the `rustc_middle::ty::layout::fn_can_unwind` function,
used to compute whether or not a `FnAbi` object represents a function that
should be able to unwind when `panic=unwind` is in use.
* Changes are also made to
`rustc_mir_build::build::should_abort_on_panic` so that the function ABI is
used to determind whether it should abort, assuming that the `panic=unwind`
strategy is being used, and no explicit unwind attribute was provided.
[RFC 2945]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md
### Overview
This commit begins the implementation work for RFC 2945. For more
information, see the rendered RFC [1] and tracking issue [2].
A boolean `unwind` payload is added to the `C`, `System`, `Stdcall`,
and `Thiscall` variants, marking whether unwinding across FFI
boundaries is acceptable. The cases where each of these variants'
`unwind` member is true correspond with the `C-unwind`,
`system-unwind`, `stdcall-unwind`, and `thiscall-unwind` ABI strings
introduced in RFC 2945 [3].
### Feature Gate and Unstable Book
This commit adds a `c_unwind` feature gate for the new ABI strings.
Tests for this feature gate are included in `src/test/ui/c-unwind/`,
which ensure that this feature gate works correctly for each of the
new ABIs.
A new language features entry in the unstable book is added as well.
### Further Work To Be Done
This commit does not proceed to implement the new unwinding ABIs,
and is intentionally scoped specifically to *defining* the ABIs and
their feature flag.
### One Note on Test Churn
This will lead to some test churn, in re-blessing hash tests, as the
deleted comment in `src/librustc_target/spec/abi.rs` mentioned,
because we can no longer guarantee the ordering of the `Abi`
variants.
While this is a downside, this decision was made bearing in mind
that RFC 2945 states the following, in the "Other `unwind` Strings"
section [3]:
> More unwind variants of existing ABI strings may be introduced,
> with the same semantics, without an additional RFC.
Adding a new variant for each of these cases, rather than specifying
a payload for a given ABI, would quickly become untenable, and make
working with the `Abi` enum prone to mistakes.
This approach encodes the unwinding information *into* a given ABI,
to account for the future possibility of other `-unwind` ABI
strings.
### Ignore Directives
`ignore-*` directives are used in two of our `*-unwind` ABI test
cases.
Specifically, the `stdcall-unwind` and `thiscall-unwind` test cases
ignore architectures that do not support `stdcall` and
`thiscall`, respectively.
These directives are cribbed from
`src/test/ui/c-variadic/variadic-ffi-1.rs` for `stdcall`, and
`src/test/ui/extern/extern-thiscall.rs` for `thiscall`.
This would otherwise fail on some targets, see:
fcf697f902
### Footnotes
[1]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md
[2]: https://github.com/rust-lang/rust/issues/74990
[3]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md#other-unwind-abi-strings
Add #[rustc_legacy_const_generics]
This is the first step towards removing `#[rustc_args_required_const]`: a new attribute is added which rewrites function calls of the form `func(a, b, c)` to `func::<{b}>(a, c)`. This allows previously stabilized functions in `stdarch` which use `rustc_args_required_const` to use const generics instead.
This new attribute is not intended to ever be stabilized, it is only intended for use in `stdarch` as a replacement for `#[rustc_args_required_const]`.
```rust
#[rustc_legacy_const_generics(1)]
pub fn foo<const Y: usize>(x: usize, z: usize) -> [usize; 3] {
[x, Y, z]
}
fn main() {
assert_eq!(foo(0 + 0, 1 + 1, 2 + 2), [0, 2, 4]);
assert_eq!(foo::<{1 + 1}>(0 + 0, 2 + 2), [0, 2, 4]);
}
```
r? `@oli-obk`
Crate root is sufficiently different from `mod` items, at least at syntactic level.
Also remove customization point for "`mod` item or crate root" from AST visitors.
Only store a LocalDefId in some HIR nodes
Some HIR nodes are guaranteed to be HIR owners: Item, TraitItem, ImplItem, ForeignItem and MacroDef.
As a consequence, we do not need to store the `HirId`'s `local_id`, and we can directly store a `LocalDefId`.
This allows to avoid a bit of the dance with `tcx.hir().local_def_id` and `tcx.hir().local_def_id_to_hir_id` mappings.
This renames the variants in HIR UnOp from
enum UnOp {
UnDeref,
UnNot,
UnNeg,
}
to
enum UnOp {
Deref,
Not,
Neg,
}
Motivations:
- This is more consistent with the rest of the code base where most enum
variants don't have a prefix.
- These variants are never used without the `UnOp` prefix so the extra
`Un` prefix doesn't help with readability. E.g. we don't have any
`UnDeref`s in the code, we only have `UnOp::UnDeref`.
- MIR `UnOp` type variants don't have a prefix so this is more
consistent with MIR types.
- "un" prefix reads like "inverse" or "reverse", so as a beginner in
rustc code base when I see "UnDeref" what comes to my mind is
something like "&*" instead of just "*".
Box the biggest ast::ItemKind variants
This PR is a different approach on https://github.com/rust-lang/rust/pull/81400, aiming to save memory in humongous ASTs.
The three affected item kind enums are:
- `ast::ItemKind` (208 -> 112 bytes)
- `ast::AssocItemKind` (176 -> 72 bytes)
- `ast::ForeignItemKind` (176 -> 72 bytes)
Stabilize by-value `[T; N]` iterator `core::array::IntoIter`
Tracking issue: https://github.com/rust-lang/rust/issues/65798
This is unblocked now that `min_const_generics` has been stabilized in https://github.com/rust-lang/rust/pull/79135.
This PR does *not* include the corresponding `IntoIterator` impl, which is https://github.com/rust-lang/rust/pull/65819. Instead, an iterator can be constructed through the `new` method.
`new` would become unnecessary when `IntoIterator` is implemented and might be deprecated then, although it will stay stable.
