Support (hidden) `.rust-toolchain.toml`

Problem you are trying to solve

In the Linux kernel, we would have liked to take advantage of rustup's rust-toolchain.toml feature to simplify our Quick Start instructions (see https://github.com/Rust-for-Linux/linux/issues/953) back when we needed to pin our compiler version.

However, it appears that there is no support for a hidden version of that configuration file, i.e. .rust-toolchain.toml, which would mean it is very visible when listing the root directory of the kernel sources.

However, most kernel developers did not use Rust, and even for those that did, the file is not really relevant: it would have very rarely changed and if they needed to change the toolchain version, they should have used a directory override in any case, rather than modifying the file.

Thus it is really not needed to show it all the time in listings, and so a hidden .rust-toolchain.toml file would be better. I guess for other projects that may be the case as well.

This would follow other files like .clang-format, .gitignore, .gitattributes, .mailmap, .rustfmt.toml, etc. where configuration is not directly related to the project (say, a Kconfig file) but to tooling.

Solution you'd like

Support for the .rust-toolchain.toml filename.

Start by locating rustup's handling of the existing rust-toolchain.toml filename and any related tests; the issue also identifies analogous hidden configuration files such as .clang-format and .rustfmt.toml. Done means rustup accepts .rust-toolchain.toml as an equivalent configuration filename, with coverage for the new name.