Macro find_attr 

macro_rules! find_attr {
    ($tcx: expr, crate, $pattern: pat $(if $guard: expr)?) => { ... };
    ($tcx: expr, crate, $pattern: pat $(if $guard: expr)? => $e: expr) => { ... };
    ($tcx: expr, $id: expr, $pattern: pat $(if $guard: expr)?) => { ... };
    ($tcx: expr, $id: expr, $pattern: pat $(if $guard: expr)? => $e: expr) => { ... };
    ($attributes_list: expr, $pattern: pat $(if $guard: expr)?) => { ... };
    ($attributes_list: expr, $pattern: pat $(if $guard: expr)? => $e: expr) => { ... };
}

Finds attributes by pattern matching.

A little like matches but for attributes.

Note that this macro accepts several “id” types: DefId, LocalDefId, OwnerId and HirId.

Examples

It is most commonly used to check whether something has an attribute or to get its contents if it is present:

let is_naked: bool = find_attr!(tcx, def_id, Naked(..));

let is_visible: bool = find_attr!(tcx, def_id, Doc(doc) if doc.hidden.is_none());

let link_name: Option<Symbol> = find_attr!(tcx, def_id, LinkName { name, .. } => *name);

Another common case is finding attributes applied to the root of the current crate. For that, use the shortcut:

find_attr!(tcx, crate, <pattern>)

If you already have a list of attributes in scope, you can also use that:

let attrs = <list of attributes>;

// finds the repr attribute
if let Some(r) = find_attr!(attrs, Repr(r) => r) {

}

// checks if one has matched
if find_attr!(attrs, Repr(_)) {

}