Designators
The arguments of a macro are prefixed by a dollar sign $
and type annotated
with a designator:
These are some of the available designators:
block
expr
is used for expressionsident
is used for variable/function namesitem
literal
is used for literal constantspat
(pattern)path
stmt
(statement)tt
(token tree)ty
(type)vis
(visibility qualifier)
For a complete list, see the Rust Reference.