add extern "custom"#2300
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tgross35
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Jun 28, 2026
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Maybe it would be good to have an example at src/inline-assembly.md alongside naked functions?
| For other considerations and limitations regarding unwinding across FFI boundaries, see the [relevant section in the Panic documentation][panic-ffi]. | ||
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| 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]. |
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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].
tgross35
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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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We actually disallow them entirely
error: invalid signature for `extern "custom"` function
--> <source>:6:31
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6 | unsafe extern "custom" fn foo(a: i32) -> i32 {
| ^^^^^^ ^^^
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= note: functions with the "custom" ABI cannot have any parameters or return type
help: remove the parameters and return type
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6 - unsafe extern "custom" fn foo(a: i32) -> i32 {
6 + unsafe extern "custom" fn foo() {
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stabilization PR: rust-lang/rust#158504
This is a tricky one that is an exception to some rules. I've tried to put the raw information in, but this'll probably require some refinement.