x build lives blissfully assuming llvm does not exist

[BoxyUwU]

When running x b I get the following output:
Warning: no codegen-backends config matched the requested path to build a codegen backend. Help: add backend to codegen-backends in config.toml.
after building stage1 library artifacts.
for reference here is my config.toml:

# Includes one of the default files in src/bootstrap/defaults
profile = "compiler"
changelog-seen = 2

[rust]
debug = true
deny-warnings = false
incremental = false

cc @jyn514

[jyn514]

cc #109610, #109642, @lenko-d. I am not sure the fix in #109642 was correct, but if you want to keep that approach, you should count LLVM as one of the possible codegen backends.

[KittyBorgX]

I'd be happy to work on this but does #109642 have to be reverted before working on this? I'm just waiting on that one thing, else I can work on this along with those changes

[jyn514]

so, there are two parts to this issue. one is that codegen-backends acts as a hard-off switch: #109610
whereas i'd prefer it were a default, the same way that build.docs works. because it's a hard switch, the warning is necessary to tell people to enable it in config.toml.

the second part is that the warning is wrong for LLVM, which is built by default and not handled by codegen-backends; Steps which call ensure(CodegenBackends) or the default x build shouldn't give a warning, only explicitly calling x build rustc-codegen-cranelift from the command line. you could use something like the existing is_explicitly_invoked function to determine if that's the case; i think i might have deleted it recently, but you can dig it out of the git history.

i'd prefer to change the option from a hard off to a default, but i'd be okay with just fixing the warning too.