Add extern "custom" - #2300
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Co-authored-by: Travis Cross <tc@traviscross.com>
The `extern "custom"` rules were falling under the *Unwinding* section. Let's fix that.
This had said that an `extern "custom"` function *does* not have any parameters, but it's clearer to say it *must* not have any parameters.
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On the lang side, we decided the signature rules for `extern "custom"` function pointers should match those for `extern "custom"` function items. Let's consolidate the signature rules on function items into a single rule then cite that rule normatively from the function pointer types chapter.
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In review, it was suggested that we add an example showing how `extern "custom"` functions are used in practice. Let's do that.
Often naked functions should use `extern "custom"`. Let's add an admonition to note that.
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| - Be `unsafe`. | ||
| - Not have any parameters. | ||
| - Return the [unit type]. |
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Needs some consideration for generics, rust-lang/rust#158504 (comment) and the comment below
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I guess that generics are probably okay and allowed by default if not disallowed?
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Looking through the list at https://doc.rust-lang.org/reference/items/functions.html#attributes-on-functions, also noticed that #[cold] may make no sense. Everything else seems fine
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It doesn't do harm either, so should we really make an exception here? I'm not against it per se, just wondering if we should make this calling convention even more special.
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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.