Add ability to extend an existing config with missing defaults

[axos88]

Having an old configuration file, I would like the ability to add any new, missing configuration values after upgrading rustfmt - probably at the end -, to be reviewed and configure according to our style. I would expect this to be be a "maximal" configuration containing all supported configuration values.

Currently rustfmt print overwrites the file with the default config, and needs to be merged by hand - also I'm not entirely sure if the "default" configuration is exhaustive or not.

[ytmimi]

running rustfmt --print-config default is exhaustive, and even includes the default for nightly only options that you can't set on stable. I'm not sure what you mean about overwriting the default. The configs are just output to your terminal.

Could you not do something like this:

rustfmt --print-config default > rustfmt.toml.new
git diff --no-index rustfmt.toml rustfmt.toml.new

And then check which new defaults were added. It's very rare that default values change.

[axos88]

Sure, but manually merging the current config and the default config is tedious, especially since - in my case - a few options have been renamed, or removed.

One approach would be to give rustfmt print out its active configuration in an exhaustive way (handling renames and defaulting missing values).