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add extern "custom" #2300

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5 changes: 4 additions & 1 deletion src/items/external-blocks.md
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@tgross35 tgross35 Jun 28, 2026 •
edited by rustbot
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Maybe it would be good to have an example at src/inline-assembly.md alongside naked functions?

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@tgross35 tgross35 Jun 28, 2026 •
edited by rustbot
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Somewhere in the reference it would probably be good to clarify the purpose of parameters and return types in extern "custom" definitions, since they don't directly serve any purpose. I feel like we discussed this somewhere but can't seem to find it.

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@folkertdev folkertdev Jun 28, 2026

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We actually disallow them entirely

error: invalid signature for `extern "custom"` function
 --> <source>:6:31
  |
6 | unsafe extern "custom" fn foo(a: i32) -> i32 {
  |                               ^^^^^^     ^^^
  |
  = note: functions with the "custom" ABI cannot have any parameters or return type
help: remove the parameters and return type
  |
6 - unsafe extern "custom" fn foo(a: i32) -> i32 {
6 + unsafe extern "custom" fn foo() {
  |

Original file line number Diff line number Diff line change
Expand Up @@ -48,7 +48,7 @@ r[items.extern.fn.param-patterns]
Patterns are not allowed in parameters, only [IDENTIFIER] or `_` may be used.

r[items.extern.fn.qualifiers]
The `safe` and `unsafe` function qualifiers are allowed, but other function qualifiers (e.g. `const`, `async`, `extern`) are not.
The `safe` and `unsafe` function qualifiers are allowed, but other function qualifiers (e.g. `const`, `async`, `extern`) are not. The `safe` qualifier is rejected in `extern "custom"` blocks.

r[items.extern.fn.foreign-abi]
Functions within external blocks may be called by Rust code, just like functions defined in Rust. The Rust compiler automatically translates between the Rust ABI and the foreign ABI.
Expand Down Expand Up @@ -112,6 +112,9 @@ r[items.extern.abi.system]
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).

r[items.extern.abi.custom]
* `unsafe extern "custom"` --- A custom ABI that is not known to the rust compiler.

r[items.extern.abi.platform]
There are also some platform-specific ABI strings:

Expand Down
11 changes: 11 additions & 0 deletions src/items/functions.md
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Expand Up @@ -239,6 +239,15 @@ With `panic=unwind`, when a `panic` is turned into an abort by a non-unwinding A

For other considerations and limitations regarding unwinding across FFI boundaries, see the [relevant section in the Panic documentation][panic-ffi].

r[items.fn.extern.custom]
An `extern "custom"` function has an unknown, custom ABI. The only way to call such a function is via [inline assembly].

@tgross35 tgross35 Jun 28, 2026 •
edited by rustbot
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Maybe a usage hint:

... unknown, custom ABI. This can be used to implement calling conventions unsupported by rustc. The only way to call such a function is via [inline assembly].

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r[items.fn.extern.custom.safety]
An `extern "custom"` function must be `unsafe`.

r[items.fn.extern.custom.naked]
An `extern "custom"` function definition must be a [naked function].

[forced-unwinding]: https://rust-lang.github.io/rfcs/2945-c-unwind-abi.html#forced-unwinding
[panic handler]: ../panic.md#the-panic_handler-attribute
[panic-ffi]: ../panic.md#unwinding-across-ffi-boundaries
Expand Down Expand Up @@ -411,6 +420,7 @@ fn foo_oof(#[some_inert_attribute] arg: u8) {
[testing attributes]: ../attributes/testing.md
[`cold`]: ../attributes/codegen.md#the-cold-attribute
[`inline`]: ../attributes/codegen.md#the-inline-attribute
[naked function]: ../attributes/codegen.md#the-naked-attribute
[`deprecated`]: ../attributes/diagnostics.md#the-deprecated-attribute
[`doc`]: ../../rustdoc/the-doc-attribute.html
[`must_use`]: ../attributes/diagnostics.md#the-must_use-attribute
Expand All @@ -427,3 +437,4 @@ fn foo_oof(#[some_inert_attribute] arg: u8) {
[variadic function]: external-blocks.md#variadic-functions
[`extern` block]: external-blocks.md
[zero-sized]: glossary.zst
[inline assembly]: ../inline-assembly.md
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