doc: fix instruction about running Rustfmt from source code (#5838)

Update docs to include an example of running `rustfmt` built from src,
and show how users can set the `RUSTFMT` environment variable to test
`cargo-fmt` using the `rustfmt` they built from src.
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xxchan 2023-07-19 20:22:24 +02:00 committed by GitHub
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@ -95,10 +95,18 @@ wish there weren't. You can leave `FIXME`s, preferably with an issue number.
You may want to run a version of rustfmt from source code as part of a test or You may want to run a version of rustfmt from source code as part of a test or
hacking on the rustfmt codebase. It's strongly discouraged to install a version hacking on the rustfmt codebase. It's strongly discouraged to install a version
of rustfmt from source. Instead, run it using `cargo run`, and `--manifest-path`. of rustfmt from source.
To run `rustfmt` on a file:
``` ```
cargo run --bin cargo-fmt -- --manifest-path path/to/project/you/want2test/Cargo.toml cargo run --bin rustfmt -- path/to/file.rs
```
If you want to test modified `cargo-fmt`, or run `rustfmt` on the whole project (You may need to build rustfmt first):
```
RUSTFMT="./target/debug/rustfmt" cargo run --bin cargo-fmt -- --manifest-path path/to/project/you/want2test/Cargo.toml
``` ```
### Version-gate formatting changes ### Version-gate formatting changes