Comment by JoshTriplett

Note that Python, in 2022, adopted a "target tier policy" based on the one I wrote for Rust in 2019. (See https://peps.python.org/pep-0011/ , history at https://peps.python.org/pep-0011/#discussions ; original Rust version at https://doc.rust-lang.org/nightly/rustc/target-tier-policy.h... .)

It's important for projects to distinguish between "this might or might not work but it's nobody's job to support it" versus "people depend on this and will keep it working", so that people don't build on the former and think it's the latter. The latter requires active support and widespread user/developer interest, as well as comparable support from the upstream projects you build on (something shouldn't be tier 1 for you if it's tier 3 for one of your critical dependencies).

See https://news.ycombinator.com/item?id=43673439 for a comment from last time this came up.

s390x is currently supported by Rust, at tier 2, including host tools: https://doc.rust-lang.org/nightly/rustc/platform-support/s39... .