`ignore` is overwritten in nested rustfmt.toml layouts

[pyrrho]

Given a layout like,

my-crate
├── rusfmt.toml
└── src
    ├── lib.rs
    └── sub
        └── dir
            ├── file-a.rs
            ├── file-b.rs
            └── rustfmt.toml

where my-crate/rustfmt.toml include ignore = ["src/sub/dir/file-a.rs"] and my-crate/src/sub/dir/rustfmt.toml includes ignore = [], running --print-config current for files in src/sub/dir shows an empty ignore array.

$ rustfmt +nightly --print-config current ./src/sub/dir/file-b.rs | grep ignore
ignore = []

I initially expected the returned config to include "src/sub/dir/file-a.rs".

This was somewhat surprising to me, as the rest of rustfmt.toml "feels" additive to me. I put "feels" in quotations, as child rustfmt.toml files really just overwrite the single-value configurations of their parents. As that's exactly what's going on, this may be correct behavior (if somewhat limiting in atypical repo layouts).

Looking back over the configuration list, it seems ignore is the only configuration that takes a list, and one of two that expects a path (the other being license_template_path).

I would consider it an improvement if,

1. ignore arrays were union'd together as rustfmt assembled it's 'current' configuration
2. --print-config current emitted absolute-resolved paths (in addition to, or in place of relative paths)

I'm pairing these two features together specifically because I think it would be necessary to resolve paths into an absolute form in order to union them.

[topecongiro]

ignore arrays were union'd together as rustfmt assembled it's 'current' configuration

This is the expected behavior, though currently not executed properly.

[pyrrho]

To flesh out my thoughts regarding the absolute paths, here's the kind of pathological case I was considering when I was typing up this issue,

my-crate
├── Cargo.toml
├── my-crate-sys
│   ├── Cargo.toml
│   ├── rustfmt.toml # ignore = ["src/lib.rs"]
│   └── src
│       ├── data.rs
│       └── lib.rs
├── rustfmt.toml # ignore = ["src/data.rs"]
└── src
    ├── data.rs
    └── lib.rs

The intention there being src/lib.rs and my-crate-sys/src/data.rs should be formatted. Depending on how naive the union and path-matching operations are, I could see all four .rs files being ignored.

This is also why I would like to see --print-config (at least optionally) emit absolute paths. If the values of ignore are merged, but relative paths are emitted, it would be very difficult for tooling to look at the output of rustfmt +nightly --print-config current my-crate-sys/src/data.rs and not conclude that my-crate-sys/src/data.rs should be ignored.