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Rust macros track syntax context so identifiers introduced by a macro do not casually collide with names at the call site. Use explicit paths such as $crate for helper items exported with a macro.
A macro can introduce identifiers in the caller's scope, and those names must resolve predictably. Rust's syntax contexts protect many identifiers introduced by a declarative macro from accidentally capturing a similarly named local value. This is hygiene, but it is not a reason to ignore names entirely: exported macros still need a stable way to refer to helper items from their defining crate. $crate::helper points to that crate even when a caller renames its dependency.
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