Fix `clippy::result-large-err` in the config crate

[TheJanzap]

I run clippy in my CI configured with --workspace --all-targets -- -D warnings and on a newly created gerust app, Clippy reports that figment::error is too big.

    Checking test-config v0.0.1 (/home/me/git/gerust/test/config)
error: the `Err`-variant returned from this closure is very large
   --> config/src/lib.rs:231:36
    |
231 |           figment::Jail::expect_with(|jail| {
    |  ____________________________________^
232 | |             let config_dir = jail.create_dir("config")?;
233 | |             jail.create_file(
234 | |                 config_dir.join("app.toml"),
...   |
269 | |             Ok(())
270 | |         });
    | |_________^ the `Err`-variant is at least 208 bytes
    |
    = help: try reducing the size of `figment::Error`, for example by boxing large elements or replacing it with `Box<figment::Error>`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.94.0/index.html#result_large_err
    = note: `-D clippy::result-large-err` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::result_large_err)]`

error: the `Err`-variant returned from this closure is very large
   --> config/src/lib.rs:275:36
    |
275 |           figment::Jail::expect_with(|jail| {
    |  ____________________________________^
276 | |             let config_dir = jail.create_dir("config")?;
277 | |             jail.create_file(
278 | |                 config_dir.join("app.toml"),
...   |
313 | |             Ok(())
314 | |         });
    | |_________^ the `Err`-variant is at least 208 bytes
    |
    = help: try reducing the size of `figment::Error`, for example by boxing large elements or replacing it with `Box<figment::Error>`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.94.0/index.html#result_large_err

error: the `Err`-variant returned from this closure is very large
   --> config/src/lib.rs:319:36
    |
319 |           figment::Jail::expect_with(|jail| {
    |  ____________________________________^
320 | |             let config_dir = jail.create_dir("config")?;
321 | |             jail.create_file(
322 | |                 config_dir.join("app.toml"),
...   |
357 | |             Ok(())
358 | |         });
    | |_________^ the `Err`-variant is at least 208 bytes
    |
    = help: try reducing the size of `figment::Error`, for example by boxing large elements or replacing it with `Box<figment::Error>`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.94.0/index.html#result_large_err

error: could not compile `test-config` (lib test) due to 3 previous errors
warning: build failed, waiting for other jobs to finish...

The quick fix is to manually apply #[expect(clippy::result_large_err)], but I would like to see this fixed upstream.

[marcoow]

I assume that would need to be fixed in figment though?

[TheJanzap]

Using Box<figment::Error> as the Err type should be enough. Even though the size remains the same, it is now allocated on the heap and the stack element can be smaller, solving the problem.