Description

This PR leverages derive_setters and merge to reduce the amount of changes required when adding a field to the configuration options.

The manual setup is error prone as there are 4 places that need to be updated and forgetting the final place will result in code compiling, but the field never getting layered properly.

Testing Instructions

Existing unit & integration tests, but also looking what the proc macros are generating (outputs shown are produced using rust-analyzer's Expand macro feature:

Output of #[derive(Setters)]

impl TurborepoConfigBuilder {
    pub fn with_api_url(mut self, value: Option<String>) -> Self {
        self.override_config.api_url = value;
        self
    }
...

Which exactly matches what create_builder would produce:

pub fn with_api_url(mut self, value: Option<String>) -> Self {
    self.override_config.api_url = value;
    self
}

#derive(Merge) produces

impl ::merge::Merge for ConfigurationOptions {
    fn merge(&mut self, other: Self) {
        ::merge::Merge::merge(&mut self.api_url, other.api_url);
...

and Merge is defined for Option as :

impl<T> Merge for Option<T> {
    fn merge(&mut self, mut other: Self) {
        if !self.is_some() {
            *self = other.take();
        }
    }
}

source

So self.api_url will only be set to other.api_url if there isn't a value present which is the behavior we want since we merge configs from highest to lowest precedence.