imports_granularity: implement One/Single variant

[calebcartwright]

Add a new variant and associated formatting to imports_granularity that reformats all imports into a single use statement. The variant name should be One or Single, with One being the leading contender (refer here for One and here for Single for respective community votes/feedback)

use {
    futures::{
        ready,
        stream::{Stream, StreamExt},
    },
    std::pin::{Pin, Unpin},
};

Background

imports_granularity is a configuration option that allows users to control the granularity of their use declarations (https://github.com/rust-lang/rustfmt/blob/master/Configurations.md#imports_granularity)

By default rustfmt will not modify the granularity and will preserve the developers original structure, but the imports_granularity option provides several different variants that users can leverage to instruct rustfmt to reformat their use statements with a specific level of granularity.

Several different styles/granularity levels have been identified (summarized here, and already implemented, with the exception of the one/single granularity variant.

Implementation

There will be 3 high level tasks required here

• Updating the configuration option information
• Updating the imports merging logic to support the new granularity style
• Adding tests

For the configuration side:

• Documentation will need to be extended with the new variant details
• Adding the new variant

For implementation, refer to relevant sections in src/formatting/imports.rs and src/formatting/reorder.rs, some suggested starting points:

rustfmt/src/formatting/imports.rs

Lines 872 to 875 in 79c3696

	pub(crate) enum SharedPrefix {
	Crate,
	Module,
	}

rustfmt/src/formatting/imports.rs

Lines 550 to 565 in 79c3696

	fn share_prefix(&self, other: &UseTree, shared_prefix: SharedPrefix) -> bool {
	if self.path.is_empty()
	|| other.path.is_empty()
	|| self.attrs.is_some()
	|| !self.same_visibility(other)
	{
	false
	} else {
	match shared_prefix {
	SharedPrefix::Crate => self.path[0] == other.path[0],
	SharedPrefix::Module => {
	self.path[..self.path.len() - 1] == other.path[..other.path.len() - 1]
	}
	}
	}
	}

rustfmt/src/formatting/reorder.rs

Lines 231 to 238 in 79c3696

	normalized_items = match context.config.imports_granularity() {
	ImportGranularity::Crate => merge_use_trees(normalized_items, SharedPrefix::Crate),
	ImportGranularity::Module => {
	merge_use_trees(normalized_items, SharedPrefix::Module)
	}
	ImportGranularity::Item => flatten_use_trees(normalized_items),
	ImportGranularity::Preserve => normalized_items,
	};

For tests (more information on writing tests here):
Some unit tests for the other variants can be found here
https://github.com/rust-lang/rustfmt/blob/master/src/formatting/imports.rs#L1032-L1087

Additional files for the system/idempotence tests can be found under tests/{src,target}/imports_granularity_{crate,item,module}.rs for reference

[magurotuna]

I'm interested in this issue. Can I work on it?

[calebcartwright]

I'm interested in this issue. Can I work on it?

Absolutely! Let us know if you have any questions