bootstrap blog post should document how `x check` has changed and what to do instead

[jyn514]

Location

https://blog.rust-lang.org/inside-rust/2025/05/29/redesigning-the-initial-bootstrap-sequence/

Summary

x check has changed behavior since #119899. my understanding of the new behavior, as explained to me by @onur-ozkan, is:

• x check --stage 0 std is a noop (✅ this is already in the post).
• x check --stage 0 is the same as before, except it skips x check --stage 0 std. (❌ not in the post)
• x check without arguments is equivalent to x check --stage 0 && x check --stage 1 library. this is very surprising to me and i wish it did not imply the latter command. but if it does imply the latter, we should document that instead of silently changing behavior. (❌ not in the post)

we should also say what people should switch to. i do not think the default behavior (x check without arguments) is good for all use cases; see in particular #141955 and #t-infra/bootstrap > Stage 0 std redesign support thread @ 💬).

i suggest the following docs:

• if you have modified only std, use x check --stage 1 std --set rust.download-rustc=true
• if you have modified only compiler, use x check --stage 0 compiler
• if you have modified neither and are on a tier 1 platform, we recommend only x check --stage 0 compiler. it is very unlikely for checking std to break for a tier 1 platform when the compiler has not been modified.
• in all other cases, we recommend running both commands in sequence: x check --stage 0 compiler && x check --stage 1 std --set rust.download-rustc=true

cc #t-infra/bootstrap > Stage 0 std redesign support thread @ 💬

@rustbot label T-bootstrap A-bootstrap-stages