add extern "custom" - #2300
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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].
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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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ehuss
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| * `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). | ||
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| r[items.extern.abi.custom] | ||
| * `unsafe extern "custom"` --- A custom ABI that is not known to the rust compiler. |
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| * `unsafe extern "custom"` --- A custom ABI that is not known to the rust compiler. | |
| * `unsafe extern "custom"` --- A custom ABI that is not known to the compiler. |
| 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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| r[items.fn.extern.custom.safety] | ||
| An `extern "custom"` function must be `unsafe`. | ||
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| r[items.fn.extern.custom.naked] | ||
| An `extern "custom"` function definition must be a [naked function]. | ||
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We need to add the rules here about what's allowed or not allowed in terms of parameters and return types.
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And we need to add text about the function pointer restrictions as well.
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Reminder, once the PR becomes ready for a review, use |
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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.