How to install the latest nightly Rust that supports rustfmt (or any other component)

tl;dr: rustup toolchain install nightly --allow-downgrade -c rustfmt

I recently had to debug a Rust macro in one of my personal projects. I wanted to use the excellent cargo expand command to dump the expanded macro code, but ran into a dependency issue: cargo expand can only run on nightly Rust and the version of nightly that I had installed didn’t support rustfmt (which is required for cargo expand to pretty-print the expanded code)¹.

The creators of cargo expand anticipated this exact situation, so they allow running the command in a mode that doesn’t depend on rustfmt: cargo expand --ugly will skip the pretty-printing, so it should run on any version of nightly that supports cargo expand². But what happens if you really want your expanded macro code to be pretty-printed? Or, as was the case for my project, what happens if your project itself depends on rustfmt?

What I really wanted was to install the latest version of nightly that supported rustfmt. I didn’t particularly care if I was a few days behind the most recent nightly, since I wasn’t using any nightly features in my project itself. Fortunately, rustup supports exactly this use case:

rustup toolchain install nightly --allow-downgrade -c rustfmt

The -c rustfmt option tells rustup that we want to download the rustfmt component along with the rest of the nightly toolchain. The --allow-downgrade option tells rustup to install the latest version of nightly that supports all of the components we asked for³.

My initial thought had been that this would be managed via rustup component add (my mental model was that my goal was to add the rustfmt component to nightly, rather than downgrade nightly until it supported rustfmt). It turns out that there was an attempt to add support for --allow-downgrade to rustup component add but it turned out to be trickier than expected (there’s an open issue if you want to help out). Fortunately, the rustup toolchain install approach does what we need.

1. The fact that rustfmt (or any given component) wasn’t available on the version of nightly that I was running isn’t particularly surprising. nightly updates frequently and doesn’t have the same stability guarantees as, well, stable, so even heavily-used components like rustfmt will often not work on some versions of nightly. From what I can tell, rustfmt tends to catch up within a few days so it’s usually possible to find a nightly version that supports rustfmt and isn’t too far out of date. ↩

2. As a side note, this is a really great user experience from a cargo command that must be run on nightly. If you know that your command can only run on nightly and that it depends on components that may not be present on any given version of nightly, it’s really nice to provide users a way to toggle off those dependencies. ↩

3. Here’s the PR that added –allow-downgrade if you want to see the discussion ↩