using ~/.cargo/config.toml to enable cranelift results in a confusing error when bootstrapping rustc

[lolbinarycat]

the offending config:

# .cargo/config.toml
unstable.codegen-backend = true
# commenting out the following line will allow rustc to bootstrap successfully
profile.dev.codegen-backend = "cranelift"

the errror:

error: failed to find a `codegen-backends` folder in the sysroot candidates:
       * /home/binarycat/src/rs/rust/build/x86_64-unknown-linux-gnu/stage0
       * /home/binarycat/src/rs/rust/build/x86_64-unknown-linux-gnu/stage0

reproduction steps:

1. enable download-rustc
2. remove all of build/
3. ./x doc library/core (probably any build command works)

there's a lot of workarounds for this, but i was only able to diagnose it on a hunch, so at the very least the error message should be more clear.

[onur-ozkan]

reproduction steps:

enable download-rustc
remove all of build/
./x doc library/core (probably any build command works)

x check without 1 and 2 should be enough for reproduction.

The failure happens during bootstrap compilation itself, which is the first thing we compile on bootstrapping. In order to give a friendly message, what we have to do is:

1. Parse .cargo/config.toml in the bootstrap python script (which will be migrated to rust eventually)
2. Check whether sysroot contains the related artifacts for the backend used in .cargo/config.toml
3. Give a nice error message.

I strongly believe this is too much to give a error message just for a slightly better error message than the current one (I even think the current one is quite clear already).

Even with a nice error message, there is no way to use different backends in .cargo/config.toml if you are using the default stage0 compiler. So, the path for setting different codegen on cargo is requires custom configurations and custom compiler toolchain. If we want to switch codegen backend, that should be done in the bootstrap way rather than doing it with cargo directly.

I am going to close the issue as this isn't something we can consider doing it.