emacs-sourcekit-lsp is a client for sourcekit-lsp, a Swift/C/C++/Objective-C language server created by Apple.
Uses lsp-mode, but it's also open to be extended with additions outside of the LSP protocol, like semantic highlighting, if the server supports that.
You need to download sourcekit-lsp and follow their repo instructions to build it using Swift Package Manager, for example.
After that, you need to download and install a custom development Swift toolchain from Swift.org website (in the future, Apple plans to make sourcekit-lsp work with Swift release toolchains).
Finally, add the following code to your init.el:
(require 'sourcekit-lsp)
(setenv "SOURCEKIT_TOOLCHAIN_PATH" "/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2018-11-01-a.xctoolchain")
(setq sourcekit-lsp-executable (expand-file-name "<path_to_sourcekit-lsp_executable>"))You can add sourcekit-lsp-swift-enable to your swift-mode hook and it will load sourcekit-lsp whenever you visit a .swift file.
This package is not available on MELPA yet (but it's planned).