In https://rustc-dev-guide.rust-lang.org/macro-expansion.html I fear the sentence

Eager expansion generally performs a subset of the things that lazy (normal) expansion does.

is not clear. And I'd suggest, maybe, to give an example in order to clarify it.

I could imagine that you mean that, in the following example

macro bar($i: ident) { $i }
macro foo($i: ident) { $i + $i }
foo!(bar!(5))

an eager expansion would lead to a single expansion of bar while the normal expansion will lead to two expansions of bar .
However, the query system that was described in the previous section seems to indicate that the expansion would consist in a single query call with a cached result, so that would be very cheap.

Also, if we consider

macro bar($i: ident) { $i }
macro foo($i: ident) { }
foo!(bar!(5))

an eager expansion would lead to expending bar, while a normal expansion would avoid this expansion; showing that there are also case where the eager expansion do more work.

My best hypothesis, to be honest, right now, is that the sentence should be read with eager and lazy reversed