I do not know if this is a bug or if it is intended, but I ran cargo-expand with the latest (cargo-expand 1.0.70 + prettyplease 0.2.15), and it worked just fine without installing the nightly compiler. I checked and nightly wasn't installed. Here is the output of rustup show:

stable-x86_64-unknown-linux-gnu (default)
rustc 1.72.0 (5680fa18f 2023-08-23)

I did not test this extensively, so I do not know if it will always work. It did expand a couple declarative macros without issue.

If cargo expand works without nightly in certain cases but not others, a warning saying so, or the documentation explaining that, would be nice! Otherwise I guess it might be a bug because it didn't fail without nightly.

If it is intended to work without nightly, this part of the README is inaccurate:

Cargo expand relies on unstable compiler flags so it requires a nightly toolchain to be installed, though does not require nightly to be the default toolchain or the one with which cargo expand itself is executed. If the default toolchain is one other than nightly, running cargo expand will find and use nightly anyway.