document cfg conditions on inline assembly templates and operands

Author: folkertdev

src/attributes.md

@@ -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.
 
 r[attributes.meta]
 ## Meta item attribute syntax
@@ -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)

src/inline-assembly.md

@@ -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 ->
@@ -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