cargo run doesn't rebuild bootstrap binaries

[jyn514]

Currently, cargo run has a big footgun: if you modify src/bootstrap/bin/rustc.rs (or rustdoc.rs), it won't be rebuilt until you manually run cargo build --bins. I tried avoiding this in #94828 by running that automatically in bootstrap, but it discards various linker arguments and environment variables that prevent people from configuring the build.

A better solution is to introduce a binary that dispatches to different main functions, like rustup. If called as bootstrap it would run the main builder; if called as rustc or rustdoc it would run the logic currently in rustc.rs / rustdoc.rs; if called by any other name it should give a hard error.

cc #94828 (comment), #94829

I am happy to help mentor this work.

@rustbot label: +A-rustbuild +C-enhancement +E-mentor +E-help-wanted

[fee1-dead]

@rustbot claim

[bstrie]

Coming from #94828, does the solution to this involve leveraging artifact dependencies somehow? I'd be very impressed if that worked.

[jyn514]

No, artifact dependencies are still unstable, so bootstrap can't use them yet.

[jyn514]

(of course, if anyone is interested in stabilizing artifact dependencies, this would still be a fun experiment to try out ...)

[bstrie]

Ah, I was hopeful that something like this could help to exercise artifact dependencies to get them to stable sooner. :) Is the problem that artifact dependencies are unstable, or is the problem just that bootstrap is using a version of Rust that doesn't contain support for them? I'm not generally aware of unstable features being forbidden in the compiler.

[jyn514]

Not in the compiler, but in bootstrap itself: #92260 (comment)
(let's please move further discussion about stable vs unstable to zulip)

[jyn514]

I am planning to add a shim binary that should make this unnecessary (see https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/bootstrap.20user.20survey/near/283897117). The shim will use a pre-downloaded bootstrap from CI, which makes the approach here unnecessary. Until I implement downloadable bootstrap, I'll just live with the broken caching locally.