add extern "custom" functions
#140770
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add extern "custom" functions
#140770
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These commits modify compiler targets. |
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Some changes occurred in compiler/rustc_codegen_cranelift cc @bjorn3 Some changes occurred in compiler/rustc_codegen_gcc |
bjorn3
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May 7, 2025
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tgross35
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May 7, 2025
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mejrs
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traviscross
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May 8, 2025
tests/ui/abi/custom.rs
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| #[unsafe(naked)] | ||
| unsafe extern "custom" fn double(a: u64) -> u64 { | ||
| naked_asm!("add rax, rax", "ret"); | ||
| } |
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For now, I think I'd suggest just not allowing parameters and a return type -- or putting this under a separate feature gate.
We could always allow this later. It seems like one of the few question marks here, so it'd be better to do the minimal thing to start.
tgross35
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May 8, 2025
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Did you forget to press escape before quitting vim? |
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ah, yeah. Well it'll probably get squashed before this is ready |
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I've made a proper tracking issue at #140829 |
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tracking issue: #140829
previous discussion: #140566
In short, an
extern "custom"function is a function with a custom ABI, that rust does not know about. Therefore, such functions can only be defined with#[unsafe(naked)]andnaked_asm!, or via anextern "C" { /* ... */ }block. These functions cannot be called using normal rust syntax: calling them can only be done from inline assembly.The motivation is low-level scenarios where a custom calling convention is used. Currently, we often pick
extern "C", but that is a lie because the function does not actually respect the C calling convention.At the moment
"custom"seems to be the name with the most support. That name is not final, but we need to pick something to actually implement this.r? @traviscross
cc @tgross35