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document cfg conditions on inline assembly templates and operands #2063

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2 changes: 2 additions & 0 deletions src/attributes.md
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Expand Up @@ -108,6 +108,7 @@ Attributes may be applied to many forms in the language:
* [Function][functions], [closure] and [function pointer]
parameters accept outer attributes. This includes attributes on variadic parameters
denoted with `...` in function pointers and [external blocks][variadic functions].
* [Inline assembly] template strings and operands accept outer attributes.
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@petrochenkov petrochenkov Oct 25, 2025 •
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Again, it may be a mostly theoretical concern, but attributes are not supported here.
asm! rather have an attribute-like syntax in its DSL - rust-lang/rust#147736 (comment).

The "only cfg and cfg_attr" restriction is unique to all attribute positions and is a direct consequence of the above.

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This list is about allowed positions, and with asm_cfg attributes are parsed on template strings and operands, so saying that they are a valid position for attributes feels right.

But yeah they aren't really supported like in the other places listed here.


r[attributes.meta]
## Meta item attribute syntax
Expand Down Expand Up @@ -410,3 +411,4 @@ The following is an index of all built-in attributes.
[variadic functions]: items/external-blocks.html#variadic-functions
[`diagnostic::on_unimplemented`]: attributes/diagnostics.md#the-diagnosticon_unimplemented-attribute
[`diagnostic::do_not_recommend`]: attributes/diagnostics.md#the-diagnosticdo_not_recommend-attribute
[Inline assembly]: inline-assembly.md
27 changes: 26 additions & 1 deletion src/inline-assembly.md
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Expand Up @@ -49,15 +49,19 @@ r[asm.syntax]
The following grammar specifies the arguments that can be passed to the `asm!`, `global_asm!` and `naked_asm!` macros.

```grammar,assembly
@root AsmArgs -> FormatString (`,` FormatString)* (`,` AsmOperand)* `,`?
@root AsmArgs -> AsmAttrFormatString (`,` AsmAttrFormatString)* (`,` AsmAttrOperand)* `,`?

FormatString -> STRING_LITERAL | RAW_STRING_LITERAL | MacroInvocation

AsmAttrFormatString -> (OuterAttribute)* FormatString

AsmOperand ->
ClobberAbi
| AsmOptions
| RegOperand

AsmAttrOperand -> (OuterAttribute)* AsmOperand

ClobberAbi -> `clobber_abi` `(` Abi (`,` Abi)* `,`? `)`

AsmOptions ->
Expand Down Expand Up @@ -266,6 +270,27 @@ Further constraints on the directives used by inline assembly are indicated by [
[format-syntax]: std::fmt#syntax
[rfc-2795]: https://github.com/rust-lang/rfcs/pull/2795

r[asm.attributes]
## Attributes

r[asm.attributes.supported-attributes]
Only the [`cfg`] and [`cfg_attr`] attributes are accepted semantically on inline assembly template strings and operands. Other attributes are parsed, but rejected when the assembly macro is expanded.

r[asm.attributes.starts-with-template]
Syntactically there must be at least one template string before the first operand.

```rust, ignore
// This is rejected because `a = out(reg) x` does not parse as a template string.
core::arch::asm!(
#[cfg(false)]
a = out(reg) x, //~ ERROR expected token: `,`
"",
);
```

[`cfg`]: conditional-compilation.md#the-cfg-attribute
[`cfg_attr`]: conditional-compilation.md#the-cfg_attr-attribute

r[asm.operand-type]
## Operand type

Expand Down