Implement RFC 2945: "C-unwind" ABI #76570

### 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: rust-lang-ci@fcf697f ### Footnotes [1]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md [2]: rust-lang#74990 [3]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md#other-unwind-abi-strings

### Changes This commit implements unwind ABI's, specified in RFC 2945. 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. ### Tests Unit tests, checking that the behavior is correct for `C-unwind`, `stdcall-unwind`, `system-unwind`, and `thiscall-unwind`, are included. These alternative `unwind` ABI strings are specified in RFC 2945, in the "_Other `unwind` ABI strings_" section. Additionally, a test case is included to assert that the LLVM IR generated for an external function defined with the `C-unwind` ABI will be appropriately labeled with the `nounwind` LLVM attribute when the `panic=abort` compilation flag is used. ### Ignore Directives This commit uses `ignore-*` directives 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`.

### Integration Tests This commit introduces some new fixtures to the `run-make-fulldeps` test suite. * c-unwind-abi-catch-panic: Exercise unwinding a panic. This catches a panic across an FFI boundary and downcasts it into an integer. * c-unwind-abi-catch-lib-panic: This is similar to the previous `*catch-panic` test, however in this case the Rust code that panics resides in a separate crate. ### Add `rust_eh_personality` to `#[no_std]` alloc tests This commit addresses some test failures that now occur in the following two tests: * no_std-alloc-error-handler-custom.rs * no_std-alloc-error-handler-default.rs Each test now defines a `rust_eh_personality` extern function, in the same manner as shown in the "Writing an executable without stdlib" section of the `lang_items` documentation here: https://doc.rust-lang.org/unstable-book/language-features/lang-items.html#writing-an-executable-without-stdlib Without this change, these tests would fail to compile due to a linking error explaining that there was an "undefined reference to `rust_eh_personality'." ### Updated hash * update 32-bit hash in `impl1` test ### Panics This commit uses `panic!` macro invocations that return a string, rather than using an integer as a panic payload. Doing so avoids the following warnings that were observed during rollup for the `*-msvc-1` targets: ``` warning: panic message is not a string literal --> panic.rs:10:16 | 10 | panic!(x); // That is too big! | ^ | = note: `#[warn(non_fmt_panic)]` on by default = note: this is no longer accepted in Rust 2021 help: add a "{}" format string to Display the message | 10 | panic!("{}", x); // That is too big! | ^^^^^ help: or use std::panic::panic_any instead | 10 | std::panic::panic_any(x); // That is too big! | ^^^^^^^^^^^^^^^^^^^^^ warning: 1 warning emitted ``` See: https://github.com/rust-lang-ci/rust/runs/1992118428 As these errors imply, panicking without a format string will be disallowed in Rust 2021, per rust-lang#78500.

### Add debug assertion to check `AbiDatas` ordering This makes a small alteration to `Abi::index`, so that we include a debug assertion to check that the index we are returning corresponds with the same abi in our data array. This will help prevent ordering bugs in the future, which can manifest in rather strange errors. ### Using exhaustive ABI matches This slightly modifies the changes from our previous commits, favoring exhaustive matches in place of `_ => ...` fall-through arms. This should help with maintenance in the future, when additional ABI's are added, or when existing ABI's are modified. ### List all `-unwind` ABI's in unstable book This updates the `c-unwind` page in the unstable book to list _all_ of the other ABI strings that are introduced by this feature gate. Now, all of the ABI's specified by RFC 2945 are shown. Co-authored-by: Amanieu d'Antras <amanieu@gmail.com> Co-authored-by: Niko Matsakis <niko@alum.mit.edu>

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Implement RFC 2945: "C-unwind" ABI #76570

Implement RFC 2945: "C-unwind" ABI #76570

Commits on Mar 9, 2021