Cargo runs rustc from the workspace root

Bug? Inconvenience? Who knows, but it confused me greatly.

Problem

I ran cargo build --lib -- -Z self-profile from src/librustdoc in a clone of https://github.com/rust-lang/rust/. I expected the output file to be in src/librustdoc/rustdoc-24097.mm_profdata, since that's where it would be if I ran rustc directly. Instead they showed up in the workspace root: rustdoc-24097.mm_profdata.

Steps

1. git clone https://github.com/rust-lang/rust/
2. cd rust/src/librustdoc
3. git checkout 500b0f3c1ba651f31abc551ccbcf222f48e355a2
4. cargo build --lib -- -Z self-profile

(but I think this is true in any workspace)

Possible Solution(s)

Run rustc from the src/librustdoc folder instead, which will put the file in the expected place.

Notes

Output of cargo version: cargo 1.50.0-nightly (bfca1cd22 2020-11-24)

I don't think this is a bug per-se, but it would be nice to document, maybe.

Reproduce the behavior from src/librustdoc with cargo build --lib -- -Z self-profile, and compare the generated file location with a direct rustc invocation. Review the Cargo documentation for workspace build paths and self-profile output. Done means the workspace-root behavior is clearly documented, including the expected output location.