bootstrap: Remove the distinction between compiler and compiler_for

[jyn514]

I am not sure whether compiler/compiler_for is a meaningful + worthwhile distinction; I think the best way to find out is probably to try to unify and see what goes wrong. I suspect the intent behind the addition was largely the typical "it seems like this one place wants something slightly different" that perhaps was more of a patchwork fix than done with long-term correctness, so it's unlikely to be super principled I expect in terms of where it's used and such.

I tried changing this and it went ... not well. Removing compiler altogether gives 71 build errors, and changing it to alias compiler_for(stage, host, host) fails a bunch of the tests, including some that look like real issues:

---- builder::tests::dist::dist_with_targets_and_hosts stdout ----
thread 'builder::tests::dist::dist_with_targets_and_hosts' panicked at 'assertion failed: `(left == right)`

Diff < left / right > :
 [
     Rustc {
         compiler: Compiler {
<            stage: 1,
>            stage: 2,
             host: A,
>        },
>    },
>    Rustc {
>        compiler: Compiler {
>            stage: 2,
>            host: B,
         },
     },
 ]

', src/bootstrap/builder/tests.rs:314:9

Looking at the call sites, they usually look like this:

            fn make_run(run: RunConfig<'_>) {
                run.builder.ensure($name {
                    compiler: run.builder.compiler(run.builder.top_stage, run.builder.config.build),
                    target: run.target,
                    extra_features: Vec::new(),
                });
            }

so I think the "proper" fix is to pass run.target into compiler_for in most of these cases; at very least, we can't assume it's always host.

In some cases it may not be clear what target is appropriate - feel free to ask on Zulip.

@rustbot label +A-rustbuild +E-mentor +E-medium

Originally posted by @jyn514 in #96000 (comment)

[ohno418]

I'd like to work on this.

@rustbot claim

[jyn514]

Hi @ohno418, have you had time to work on this? Happy to help out if you're running into trouble :)

[ohno418]

Currently, I'm looking into the bootstrapping process. I'll ask you on Zulip about the details that I don't understand, maybe within a week or so.
Thanks so much for your concern, @jyn514 !

[jyn514]

@ohno418 here are some existing resources on how bootstrapping works: https://discord.com/channels/442252698964721669/487245758739906560/964730715222732800
I will try to write this up into a proper README at some point.

[ohno418]

Thank you! I'll check it.

[jyn514]

@ohno418 I worry this is harder than I expected it to be originally :( don't feel obligated to keep working on it if you're stuck, I can find other useful work for you to do :)

@rustbot label +E-hard -E-medium

[ohno418]

I've been working on this for a while but found some difficulties. Or maybe I just don't understand much.

For future tips, leaving the part that I got stuck, and some advice that @jyn514 gave me:
https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/bootstrap.3A.20compiler.28.29.20and.20compiler_for.28.29/near/285070882

@rustbot release-assignment