I'm working on adding slang as an optional parser for prettier-plugin-solidity.

I discovered this syntax that is a bit obscure since there weren't many examples in the documentation even when it was allowed.

In versions prior to 0.5.0, stack assignment existed and the following code did compile.

contract AssemblyStackAssignment {
    function f() public returns(uint y) {
        assembly {
            4 =: y
        }
    }
}