[swift-dev] Support for per-project code styles in SourceKit

[Daniel Martín]

Hi, all:

I've been planning the implementation of per-project code style support 
for Swift. Hopefully, if implemented correctly, this tweet would not be 
true anymore: https://twitter.com/CodaFi_/status/821142027077238784

As I understand it, right now we have two possible kind of clients for 
formatting Swift code (both are exercising code inside the swiftIDE 
library):

- Xcode: IDELanguageSupportUI, which is a plugin installed by default 
with Xcode, is responsible for sending a message to SourceKit with the 
"format.usetabs", "format.indentwidth", "format.tabwidth" parameters, 
among others. SwiftIDE simply formats the region and updates the buffer. 
I suppose other third party editors also use this mechanism.

- Swift-format: This little internal tool only uses a single function 
from Formatting.cpp, passing the desired format options as a parameter.

In order to support different code styles per project, we need to make, 
at least, the following changes:

- Create some factory methods for the CodeFormatOptions struct, one for 
each supported style (initially, only "Default", or "Apple", ie. no 
tabs, 4 spaces, no case label indentation in switch statements).

- Add support for reading an optional .swift-format file inside the 
current directory. This file would be a simple YAML file with key-value 
pairs like this:

---- .swift-format ----

Style: Apple

IndentWidth: 2

Does it make sense? I'll try to maintain the same SourceKit interface so 
that Xcode and other editors don't break and at the same time it's easy 
for them to opt-in this feature.

Opinions?

Thanks,

Daniel Martín