I tried building the kernel module for the Linux 6.1 kernel and these were the changes that were needed. I'm not sure if it makes sense to merge this, since it is probably due to changes in https://github.com/Rust-for-Linux/linux which were not yet up-streamed, but if anyone tries something similar this diff might be helpful.

Using the linux/scripts/generate_rust_analyzer.py script as inspiration to add a make rust-analyzer target for an out-of-tree module.

Process:

• Make the $KDIR rust-project by issuing make rust-analyzer
• Extend the crates config with the out-of-tree module
• Write to rust-project.json in the module directory

Here's an example of the added module crate config:

$ more rust-project.json| tail -n 20
        {
            "cfg": [],
            "deps": [
                {
                    "crate": 6,
                    "name": "kernel"
                }
            ],
            "display_name": "rust_out_of_tree",
            "edition": "2021",
            "env": {
                "RUST_MODFILE": "This is only for rust-analyzer"
            },
            "is_proc_macro": false,
            "is_workspace_member": false,
            "root_module": "rust_out_of_tree.rs"
        }
    ],
    "sysroot_src": "/home/mat/.rustup/toolchains/1.62.0-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library"
}

And the updated kernel crate config (pointing at my KDIR of ../rust_for_linux):

more rust-project.json| jq 'del(.crates[].cfg) | .crates[]| select(.display_name=="kernel") '
{
  "deps": [
    {
      "crate": 0,
      "name": "core"
    },
    {
      "crate": 2,
      "name": "alloc"
    },
    {
      "crate": 3,
      "name": "macros"
    },
    {
      "crate": 4,
      "name": "build_error"
    },
    {
      "crate": 5,
      "name": "bindings"
    }
  ],
  "display_name": "kernel",
  "edition": "2021",
  "env": {
    "RUST_MODFILE": "This is only for rust-analyzer"
  },
  "is_proc_macro": false,
  "is_workspace_member": true,
  "root_module": "../rust_for_linux/rust/kernel/lib.rs",
  "source": {
    "exclude_dirs": [],
    "include_dirs": [
      "rust/kernel",
      "rust"
    ]
  }
}

===

One janky thing I'm not currently a fan of is getting the root_module and rust source file name. This script currently only supports one file/module. I'd prefer to get it from the Kbuild file by reading obj-m but I'm not sure how to do that, any suggestions here are welcome.