c64d6bf5af
The "CI" environment var isn't universal (for example, I think Azure uses TF_BUILD). However, we are mostly concerned with rust-lang/rust's own CI which currently is GitHub Actions which does set "CI". And I think most other providers use "CI" as well.
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.