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Document rustc internal env vars a little bit (#513)

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Who? Me?! 2019-12-21 07:00:09 -06:00 committed by Mark Rousskov
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src/building/bootstrapping.md
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This `stage2/bin/rustc` compiler is shipped to end-users, along with the
`stage 1 {std,rustc}` artifacts.
## Environment Variables
During bootstrapping, there are a bunch of compiler-internal environment
variables that are used. If you are trying to run an intermediate version of
`rustc`, sometimes you may need to set some of these environment variables
manually. Otherwise, you get an error like the following:
```text
thread 'main' panicked at 'RUSTC_STAGE was not set: NotPresent', src/libcore/result.rs:1165:5
```
If `./stageN/bin/rustc` gives an error about environment variables, that
usually means something is quite wrong -- or you're trying to compile e.g.
`librustc` or `libstd` or something that depends on environment variables. In
the unlikely case that you actually need to invoke rustc in such a situation,
you can find the environment variable values by adding the following flag to
your `x.py` command: `--on-fail=print-env`.