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Dec 21, 2019
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Document rustc internal env vars a little bit #513

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document rustc internal env vars
mark-i-m Nov 12, 2019
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add note from Mark-Simulacrum
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17 changes: 17 additions & 0 deletions src/building/bootstrapping.md
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Expand Up @@ -159,3 +159,20 @@ links against.
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`.