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Stabilize `asm_cfg` tracking issue: https://github.com/rust-lang/rust/issues/140364 closes https://github.com/rust-lang/rust/issues/140364 Reference PR: - https://github.com/rust-lang/reference/pull/2063 # Request for Stabilization ## Summary The `cfg_asm` feature allows `#[cfg(...)]` and `#[cfg_attr(...)]` on the arguments of the assembly macros, for instance: ```rust asm!( // or global_asm! or naked_asm! "nop", #[cfg(target_feature = "sse2")] "nop", // ... #[cfg(target_feature = "sse2")] a = const 123, // only used on sse2 ); ``` ## Semantics Templates, operands, `options` and `clobber_abi` in the assembly macros (`asm!`, `naked_asm!` and `global_asm!`) can be annotated with `#[cfg(...)]` and `#[cfg_attr(...)]`. When the condition evaluates to true, the annotated argument has no effect, and is completely ignored when expanding the assembly macro. ## Documentation reference PR: https://github.com/rust-lang/reference/pull/2063 ## Tests - [tests/ui/asm/cfg.rs](https://github.com/rust-lang/rust/blob/master/tests/ui/asm/cfg.rs) checks that `cfg`'d arguments where the condition evaluates to false have no effect - [tests/ui/asm/cfg-parse-error.rs](https://github.com/rust-lang/rust/blob/master/tests/ui/asm/cfg.rs) checks the parsing rules (parsing effectively assumes that the cfg conditions are all true) ## History - https://github.com/rust-lang/rust/issues/140279 - https://github.com/rust-lang/rust/pull/140367 # Resolved questions **how are other attributes handled** Other attributes are parsed, but explicitly rejected. # unresolved questions **operand before template** The current implementation expects at least one template string before any operands. In the example below, if the `cfg` condition evaluates to true, the assembly block is ill-formed. But even when it evaluates to `false` this block is rejected, because the parser still expects just a template (a template is parsed as an expression and then validated to ensure that it is or expands to a string literal). Changing how this works is difficult. ```rust // This is rejected because `a = out(reg) x` does not parse as an expresion. asm!( #[cfg(false)] a = out(reg) x, //~ ERROR expected token: `,` "", ); ``` **lint on positional arguments?** Adding a lint to warn on the definition or use of positional arguments being `cfg`'d out was discussed in https://github.com/rust-lang/rust/issues/140279#issuecomment-2832237372 and subsequent comments. Such a lint is not currently implemented, but that may not be a blocker based on the comments there. r? `@traviscross` (I'm assuming you'll reassign as needed) |
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