Attributes

An attribute is a #Name written before what it describes, with its arguments in parentheses when it takes any. It instructs the compiler — link a foreign library, fix a calling convention, flag a function at every call — without changing the code of the declaration it annotates. Rux has a fixed set of attributes; a program cannot define new ones.

attribute  = "#" identifier [ "(" argument { "," argument } ")" ]
attributes = { attribute }
declaration-with-attributes = attributes [ "pub" ] declaration
parameter-with-attributes   = attributes parameter

Attributes come before pub, one after another. An attribute that takes no arguments is written without parentheses:

import Core::Panic;

#NoReturn
#Warn("Abort skips the cleanup that Exit runs")
pub func Abort() {
    Panic("aborted");
}

The attributes

#Format is the one parameter attribute; it is written before a parameter inside the parameter list. All the others are declaration attributes, written before a declaration at the package root, inside a module, or before a member of an extend block. Members of an extern { } block take no attributes of their own — the attributes on the block apply to all of them:

error: expected 'func' or a variable declaration in the external block before '#'

#Error and #Warn also have a second form, as directives inside a function body or a when arm — see Build diagnostics.

Rules

Each attribute checks where it stands and what it is given, and the errors name the rule:

Restricting code to one platform is not an attribute. Use conditional compilation: when #target.os == .Windows { … } removes the other platforms' code before it is ever resolved.

See also

• Foreign function interface — where #Link and #Abi are used
• Build diagnostics — #Error and #Warn as directives
• Functions — the declarations most attributes annotate
• #Test — unit tests that rux test runs
• Learn: Compile error, ABI