Rebuild llvm spuriously less frequently #138333
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I noticed that `x check` was rebuilding rustc_llvm basically every time I modified
a source file. I tracked this down to the following env variable change:
```
cargo::core::compiler::fingerprint: dirty: EnvVarChanged { name: "REAL_LIBRARY_PATH", old_value: Some("/home/jyn/.local/lib/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib"), new_value: None }
```
The problem was that I had installed rust-analyzer from rustup, not as a
standalone tool. As a result, rustup sets
`LD_LIBRARY_PATH=$(rustc --print target-libdir)` in the environment
under the assumption that rust-analyzer needs it to link to
rustc_private crates. This is not in fact the case; RA does not link to
rustc_private. But rustup does not know this. Ideally we would make
rustup smarter, but that takes a while because rustup has infrequent
releases.
In the meantime, as a workaround, be a little more selective about when we forward
LD_LIBRARY_PATH. See the new comment for more details.
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I noticed that
x checkwas rebuilding rustc_llvm basically every time I modified a source file. I tracked this down to the following env variable change:The problem was that I had installed rust-analyzer from rustup, not as a standalone tool. As a result, rustup sets
LD_LIBRARY_PATH=$(rustc --print target-libdir)in the environment under the assumption that rust-analyzer needs it to link to rustc_private crates. This is not in fact the case; RA does not link to rustc_private. But rustup does not know this. Ideally we would make rustup smarter, but that takes a while because rustup has infrequent releases.In the meantime, as a workaround, be a little more selective about when we forward LD_LIBRARY_PATH. See the new comment for more details.
Note that there are still cases when we will spuriously rebuild; in particular when setting
llvm-config = /path/to/llvm-configand using rust-analyzer installed from rustup. but both of those are individually rare, and the combination seems very unlikely to me.