One of our supported use cases is for when people want to put their build tree somewhere other than the root of the rust checkout.

I tried this code:

% git clone git@github.com:rust-lang/rust.git rust
[...]
% mkdir rust-build
% cd rust-build
% ../rust/x setup
[...]
Welcome to the Rust project! What do you want to do with x.py?
a) library: Contribute to the standard library
b) compiler: Contribute to the compiler itself
c) codegen: Contribute to the compiler, and also modify LLVM or codegen
d) tools: Contribute to tools which depend on the compiler, but do not modify it directly (e.g. rustdoc, clippy, miri)
e) user: Install Rust from source
Please choose one (a/b/c/d/e): b
[...]
Would you like to install the git hook?: [y/N]
Ok, skipping installation!

To get started, try one of the following commands:
- `x.py check`
- `x.py build`
- `x.py test`
For more suggestions, see https://rustc-dev-guide.rust-lang.org/building/suggested.html
Build completed successfully in 0:01:05

I expected to see this happen: A config.toml generated in the local directory that reflects the profile I selected.

Instead, this happened: x.py setup generated a config.toml into the root of the rust.git checkout. As a user, I have no indication that I can locally copy that config.toml into my current working directory in order to impose settings that are local to this build in this working directory.

(also, it was a little surprising to me, though understandable, that I was able to subsequently do a build in this local directory, even though the config.toml was sitting in the root of the source tree instead. We may want to issue a note at the beginning of the x.py output when it does that, i.e. something saying "local config.toml not found; using instead confg file found at ")

((i mistakenly thought that the situation was worse in that the script would even overwrite pre-existing config.toml files, but I no longer believe that to be the case; I have now seen useful errors when you try to setup a new config file but one already exists at the target location.))