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Elision handles the common shapes, so you write annotations only when the relationship is genuinely ambiguous — typically when several references go in and one comes out. A struct holding a reference needs one too, because the struct cannot outlive what it borrows.
A reference field makes a struct dependent on data owned elsewhere. Excerpt<'a> says its part cannot be used after the borrowed text becomes invalid. The annotation documents and checks a relationship; it does not allocate, copy, or lengthen the source string. If an Excerpt must outlive the source, the model should own a String instead of holding &str.
In the example, part(&self) returns a view of the existing borrowed text.
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