Rollup merge of rust-lang#132136 - RalfJung:target-feature-abi-compat…

…, r=Mark-Simulacrum

ABI compatibility: remove section on target features

Once rust-lang#127731 lands, we will properly diagnose ABI issues caused by target feature mismatch (at least on tier 1 targets). So I'd say we can remove the corresponding part of the docs here -- this is now something the compiler can take care of, so programmers don't need to be concerned. For now this is just a lint, but that's just a transition period, like in prior cases where we fix I-unsound bugs by adding a new check that goes through the "future incompatibility" stages. We have decided that it's actually a bug that we have ABI risks around target features, and we shouldn't document that bug as-if it was intended behavior.

Cc `@rust-lang/opsem` `@chorman0773` `@veluca93`

library/core/src/primitive_docs.rs

@@ -1746,11 +1746,10 @@ mod prim_ref {}
 /// alignment, they might be passed in different registers and hence not be ABI-compatible.
 ///
 /// ABI compatibility as a concern only arises in code that alters the type of function pointers,
-/// code that imports functions via `extern` blocks, and in code that combines `#[target_feature]`
-/// with `extern fn`. Altering the type of function pointers is wildly unsafe (as in, a lot more
-/// unsafe than even [`transmute_copy`][mem::transmute_copy]), and should only occur in the most
-/// exceptional circumstances. Most Rust code just imports functions via `use`. `#[target_feature]`
-/// is also used rarely. So, most likely you do not have to worry about ABI compatibility.
+/// and code that imports functions via `extern` blocks. Altering the type of function pointers is
+/// wildly unsafe (as in, a lot more unsafe than even [`transmute_copy`][mem::transmute_copy]), and
+/// should only occur in the most exceptional circumstances. Most Rust code just imports functions
+/// via `use`. So, most likely you do not have to worry about ABI compatibility.
 ///
 /// But assuming such circumstances, what are the rules? For this section, we are only considering
 /// the ABI of direct Rust-to-Rust calls (with both definition and callsite visible to the
@@ -1762,9 +1761,8 @@ mod prim_ref {}
 /// types from `core::ffi` or `libc`**.
 ///
 /// For two signatures to be considered *ABI-compatible*, they must use a compatible ABI string,
-/// must take the same number of arguments, the individual argument types and the return types must
-/// be ABI-compatible, and the target feature requirements must be met (see the subsection below for
-/// the last point). The ABI string is declared via `extern "ABI" fn(...) -> ...`; note that
+/// must take the same number of arguments, and the individual argument types and the return types
+/// must be ABI-compatible. The ABI string is declared via `extern "ABI" fn(...) -> ...`; note that
 /// `fn name(...) -> ...` implicitly uses the `"Rust"` ABI string and `extern fn name(...) -> ...`
 /// implicitly uses the `"C"` ABI string.
 ///
@@ -1834,24 +1832,6 @@ mod prim_ref {}
 /// Behavior since transmuting `None::<NonZero<i32>>` to `NonZero<i32>` violates the non-zero
 /// requirement.
 ///
-/// #### Requirements concerning target features
-///
-/// Under some conditions, the signature used by the caller and the callee can be ABI-incompatible
-/// even if the exact same ABI string and types are being used. As an example, the
-/// `std::arch::x86_64::__m256` type has a different `extern "C"` ABI when the `avx` feature is
-/// enabled vs when it is not enabled.
-///
-/// Therefore, to ensure ABI compatibility when code using different target features is combined
-/// (such as via `#[target_feature]`), we further require that one of the following conditions is
-/// met:
-///
-/// - The function uses the `"Rust"` ABI string (which is the default without `extern`).
-/// - Caller and callee are using the exact same set of target features. For the callee we consider
-///   the features enabled (via `#[target_feature]` and `-C target-feature`/`-C target-cpu`) at the
-///   declaration site; for the caller we consider the features enabled at the call site.
-/// - Neither any argument nor the return value involves a SIMD type (`#[repr(simd)]`) that is not
-///   behind a pointer indirection (i.e., `*mut __m256` is fine, but `(i32, __m256)` is not).
-///
 /// ### Trait implementations
 ///
 /// In this documentation the shorthand `fn(T₁, T₂, …, Tₙ)` is used to represent non-variadic