The name for an item declaration conflicts with an external crate's name.
Erroneous code example:
extern crate core; struct core; fn main() {}
There are two possible solutions:
Solution #1: Rename the item.
extern crate core; struct xyz;
Solution #2: Import the crate with a different name.
extern crate core as xyz; struct abc;
See the Declaration Statements section of the reference for more information about what constitutes an Item declaration and what does not: