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Rework the definitions of the extern ABIs #2003

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Merged
merged 6 commits into from
Oct 8, 2025
Merged

Rework the definitions of the extern ABIs #2003

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4047870
Rework the definitions of the extern ABIs
ehuss Sep 17, 2025
e8969f7
Add more ABI correspondence with other compilers
ehuss Sep 29, 2025
a78b6c2
Clarify default when not using `extern fn`
traviscross Oct 8, 2025
5ef9f4f
Clarify about Windows functions with explicit ABI
traviscross Oct 8, 2025
45de0c5
Convert ABI "details" links to lists
traviscross Oct 8, 2025
cc463db
Strengthen wording on "C" ABI
traviscross Oct 8, 2025
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76 changes: 66 additions & 10 deletions src/items/external-blocks.md
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Expand Up @@ -118,13 +118,16 @@ r[items.extern.abi.standard]
The following ABI strings are supported on all platforms:

r[items.extern.abi.rust]
* `unsafe extern "Rust"` --- The default ABI when you write a normal `fn foo()` in any Rust code.
* `unsafe extern "Rust"` --- The native calling convention for Rust functions and closures. This is the default when a function is declared without using [`extern fn`]. The Rust ABI offers no stability guarantees.

r[items.extern.abi.c]
* `unsafe extern "C"` --- This is the same as `extern fn foo()`; whatever the default your C compiler supports.
* `unsafe extern "C"` --- The "C" ABI matches the default ABI chosen by the dominant C compiler for the target.

r[items.extern.abi.system]
* `unsafe extern "system"` --- Usually the same as `extern "C"`, except on Win32, in which case it's `"stdcall"`, or what you should use to link to the Windows API itself.
* `unsafe extern "system"` --- This is equivalent to `extern "C"` except on Windows x86_32 where it is equivalent to `"stdcall"`.

> [!NOTE]
> As the correct underlying ABI on Windows is target-specific, it's best to use `extern "system"` when attempting to link Windows API functions that don't use an explicitly defined ABI.

r[items.extern.abi.unwind]
* `extern "C-unwind"` and `extern "system-unwind"` --- Identical to `"C"` and `"system"`, respectively, but with [different behavior][unwind-behavior] when the callee unwinds (by panicking or throwing a C++ style exception).
Expand All @@ -133,32 +136,83 @@ r[items.extern.abi.platform]
There are also some platform-specific ABI strings:

r[items.extern.abi.cdecl]
* `unsafe extern "cdecl"` --- The default for x86_32 C code.
* `unsafe extern "cdecl"` --- The calling convention typically used with x86_32 C code.
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Does this correspond to __scdecl or __attribute__((cdecl)) or anything like that on any other compiler?
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I believe the answer here is also "yes" (assuming the s is a typo).
* Only available on x86_32 targets.
* Corresponds to MSVC's `__cdecl` and GCC and clang's `__attribute__((cdecl))`.

> [!NOTE]
> For details, see:
>
> - <https://learn.microsoft.com/en-us/cpp/cpp/cdecl>
> - <https://en.wikipedia.org/wiki/X86_calling_conventions#cdecl>

r[items.extern.abi.stdcall]
* `unsafe extern "stdcall"` --- The default for the Win32 API on x86_32.
* `unsafe extern "stdcall"` --- The calling convention typically used by the [Win32 API] on x86_32.
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Does this correspond to __stdcall or __attribute__((stdcall)) or anything like that on any other compiler?
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I believe the answer is "yes". Are you suggesting this should explicitly say that like the entry for fastcall does?
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Seems worthwhile, doesn't it?
* Only available on x86_32 targets.
* Corresponds to MSVC's `__stdcall` and GCC and clang's `__attribute__((stdcall))`.

> [!NOTE]
> For details, see:
>
> - <https://learn.microsoft.com/en-us/cpp/cpp/stdcall>
> - <https://en.wikipedia.org/wiki/X86_calling_conventions#stdcall>

r[items.extern.abi.win64]
* `unsafe extern "win64"` --- The default for C code on x86_64 Windows.
* `unsafe extern "win64"` --- The Windows x64 ABI.
* Only available on x86_64 targets.
* "win64" is the same as the "C" ABI on Windows x86_64 targets.
* Corresponds to GCC and clang's `__attribute__((ms_abi))`.

> [!NOTE]
> For details, see:
>
> - <https://learn.microsoft.com/en-us/cpp/build/x64-software-conventions>
> - <https://en.wikipedia.org/wiki/X86_calling_conventions#Microsoft_x64_calling_convention>

r[items.extern.abi.sysv64]
* `unsafe extern "sysv64"` --- The default for C code on non-Windows x86_64.
* `unsafe extern "sysv64"` --- The System V ABI.
* Only available on x86_64 targets.
* "sysv64" is the same as the "C" ABI on non-Windows x86_64 targets.
* Corresponds to GCC and clang's `__attribute__((sysv_abi))`.

> [!NOTE]
> For details, see:
>
> - <https://wiki.osdev.org/System_V_ABI>
> - <https://en.wikipedia.org/wiki/X86_calling_conventions#System_V_AMD64_ABI>

r[items.extern.abi.aapcs]
* `unsafe extern "aapcs"` --- The default for ARM.
* `unsafe extern "aapcs"` --- The soft-float ABI for ARM.
* Only available on ARM32 targets.
* "aapcs" is the same as the "C" ABI on soft-float ARM32.
* Corresponds to clang's `__attribute__((pcs("aapcs")))`.

> [!NOTE]
> For details, see:
>
> - [Arm Procedure Call Standard](https://developer.arm.com/documentation/107656/0101/Getting-started-with-Armv8-M-based-systems/Procedure-Call-Standard-for-Arm-Architecture--AAPCS-)

r[items.extern.abi.fastcall]
* `unsafe extern "fastcall"` --- The `fastcall` ABI --- corresponds to MSVC's `__fastcall` and GCC and clang's `__attribute__((fastcall))`.
* `unsafe extern "fastcall"` --- A "fast" variant of stdcall that passes some arguments in registers.
* Only available on x86_32 targets.
* Corresponds to MSVC's `__fastcall` and GCC and clang's `__attribute__((fastcall))`.

> [!NOTE]
> For details, see:
>
> - <https://learn.microsoft.com/en-us/cpp/cpp/fastcall>
> - <https://en.wikipedia.org/wiki/X86_calling_conventions#Microsoft_fastcall>

r[items.extern.abi.thiscall]
* `unsafe extern "thiscall"` --- The default for C++ member functions on x86_32 MSVC --- corresponds to MSVC's `__thiscall` and GCC and clang's `__attribute__((thiscall))`.
* `unsafe extern "thiscall"` --- The calling convention typically used on C++ class member functions on x86_32 MSVC.
* Only available on x86_32 targets.
* Corresponds to MSVC's `__thiscall` and GCC and clang's `__attribute__((thiscall))`.

> [!NOTE]
> For details, see:
>
> - <https://en.wikipedia.org/wiki/X86_calling_conventions#thiscall>
> - <https://learn.microsoft.com/en-us/cpp/cpp/thiscall>

r[items.extern.abi.efiapi]
* `unsafe extern "efiapi"` --- The ABI used for [UEFI] functions.
Expand Down Expand Up @@ -485,6 +539,7 @@ restrictions as [regular function parameters].
[WebAssembly module]: https://webassembly.github.io/spec/core/syntax/modules.html
[`bundle` documentation for rustc]: ../../rustc/command-line-arguments.html#linking-modifiers-bundle
[`dylib` versus `raw-dylib`]: #dylib-versus-raw-dylib
[`extern fn`]: items.fn.extern
[`unsafe` context]: ../unsafe-keyword.md
[`verbatim` documentation for rustc]: ../../rustc/command-line-arguments.html#linking-modifiers-verbatim
[`whole-archive` documentation for rustc]: ../../rustc/command-line-arguments.html#linking-modifiers-whole-archive
Expand All @@ -494,4 +549,5 @@ restrictions as [regular function parameters].
[statics]: static-items.md
[unwind-behavior]: functions.md#unwinding
[value namespace]: ../names/namespaces.md
[win32 api]: https://learn.microsoft.com/en-us/windows/win32/api/
[`link_ordinal`]: items.extern.attributes.link_ordinal